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					Furnace Atmospheres
No. 3
Gas Nitriding and Nitrocarburising.


This booklet is part of a series on heat treatment, brazing and soldering
process application technology and expertise available from Linde Gas.
The booklet focuses on the use of furnace atmospheres; however, a brief
introduction to each process is also provided. In addition to this work on
nitriding and nitrocarburising, the series includes:

3                             Gas Carburising and Carbonitriding
    Furnace Atmospheres No. 1 –
3                             Neutral Hadening and Annealing
    Furnace Atmospheres No. 2 –
3                             Nitriding and Nitrocarburising
    Furnace Atmospheres No. 3 –
3                             Brazing of Metals
    Furnace Atmospheres No. 4 –
3                             Sub-zero treatment of steels
    Furnace Atmospheres No. 5 –
3                             Low pressure carburising and high
    Furnace Atmospheres No. 6 –
                              pressure gas quenching
3 Furnace Atmospheres No. 7 – Tube Annealing

The author gratefully acknowledges the support of the following com-
panies and persons in providing information and images: Ipsen GmbH,
ABB AB, Nitrex Metal Inc, Sulzer Metaplas GmbH, GE Sensing & Inspec-
tion Technologies AB, Bodycote Värmebehandling AB, Professor Brigitte
Haase, Bremerhaven University and Professor Marcel Somers, Technical
University of Denmark.

 Table of contents

I. Introduction                                                  4   D. Layer Growth Determination                     26
                                                                     E. Guidelines for Regulating the Atmosphere       27
II. Process Selection                                            7      1. Nitriding                                   27
                                                                        2. Nitrocarburising                            27
   The Process Steps in Nitriding and Nitrocarburising
III.                                                             9      3. Post Oxidation Control                      29
A. Prior Heat Treatment Condition                                9
B. Cleaning                                                      9   VI. Compound Layer and Diffusion Zone Formation   30
C. Preheating and Pre-Oxidation                                 10   A. Nitriding                                      30
D. Nitriding                                                    10   B. Nitrocarburising                               31
E. Nitrocarburising                                             11       1. Furnace Interior Influence                 33
F. Vacuum, High Pressure and Plasma Nitriding and                        2. Influence of Amount of Active Gas          34
    Nitrocarburising                                            12       3. Steel Alloy Content Effect                 35
G. Post Oxidation                                               13   C. Pore Formation                                 35
H. Austenitic Nitrocarburising                                  13
I. Combined Processes                                           14   VII. The NITROFLEX® Solution                      36
J. Cooling/Quenching and Post Treatment                         14   A. Gas Supply                                     36
                                                                          1. Nitriding                                 39
IV.    Properties of Nitrided and Nitrocarburised Steels        16        2. Nitrocarburising                          39
A.     Hardness and Wear Resistance                             16   B. NITROFLEX® Process Recipes                     40
B.     Static and Fatigue Strength                              18   C. Case Studies                                   40
C.     Selection of Nitriding/Nitrocarburising Case Depth       18        1. Nitrocarburising                          40
D.     Corrosion Resistance and Surface Appearance              18        2. Austenitic Nitrocarburising               41
E.     Dimensional Changes                                      19        3. Nitriding                                 42
F.     Properties of Nitrided/Nitrocarburised Stainless Steel   20
                                                                     VIII. Safety                                      43
V.     Atmosphere – Surface Interaction                         22   A. Toxicity and Asphyxiation                      43
A.     The Atmosphere Nitriding Potential                       23   B. Flammability                                   43
B.     The Atmosphere Carbon Potential                          24
C.     Atmosphere Analysis                                      24   IX. Concluding Remarks                            45
       1. Ammonia and Hydrogen Analysis                         24
       2. Oxygen Probe Analysis                                 25   X. References                                     46
       3. FTIR Gas Analysis of Nitrocarburising Atmospheres     26
4            Introduction

I. Introduction

Nitriding and nitrocarburising of steel parts give unique improve-                  shafts, camshafts and parts in sliding contact such as cylinders and
ments in wear and corrosion resistance that cannot be obtained by                   pistons where good tribological properties are needed. An elegant
carburising or carbonitriding. Increased fatigue strength is also                   demonstration of the benefits of nitrocarburising was given in a
obtained. These improvements can be understood when examining                       paper by Dawes and Tranter [1] in which the application of a screen
the surface microstructure and hardness after treatment illustrated                 wiper linkage was described. The strength increase associated with
in Figure 1. The outermost compound layer of a nitrided or nitrocar-                nitrocarburising resulted in a weight decrease of 62%. The very
burised steel is 2–30 mm thick and consists of the iron/nitrogen/car-               good tribological properties made it possible to eliminate the bronze
bon e-phase with variable compositions of carbon and nitrogen and                   bearings previously used. The corrosion resistance was very good
the g’-phase with the virtually stoichiometric chemical formula Fe4N.               and corresponded to a neutral salt corrosion resistance of 250 hours.
This layer is called the compound layer, sometimes also referred                    Finally, the parts were made aesthetically attractive with a black sur-
to as the white layer or the ceramic layer. Corrosion resistance and                face produced by a post oxidation treatment. High alloy steels used
tribological properties (friction and wear) are mainly determined by                for forging and extrusion tools are other examples that benefit from
the compound layer’s properties, which differ notably from those of                 being nitrided or nitrocarburised.
the base material.
                                                                                    Atmosphere composition and control are of crucial importance for
Under the compound layer there is a “diffusion zone”, which goes                    the nitriding/nitrocarburising result with respect to final properties
deeper into the steel, typically 0.1– 0.5 mm. Load bearing capacity,                such as wear and fatigue resistance. This is exemplified in Figure 46,
and static and fatigue strength are largely determined by the hard-                 showing four very different compound layer morphologies, merely as
ness and depth of the diffusion zone.                                               the result of the use of different gas compositions during the process
                                                                                    (See section V.E.2, page 27, for explanation).
Because of the benefits of a shortened production cycle, limited dis-
tortion, elimination of post grinding and attractive aesthetic surface              Nitriding and nitrocarburising are thermochemical processes, as
appearance, a great number of parts subjected to wear and fatigue                   are carburising and carbonitriding, see Table 1. In these processes,
are nitrocarburised today instead of being carburised or carboni-                   nitrogen and/or carbon are transferred from the process medium,
trided, as was previously the case. Examples of applications for the                normally gas, to the surface of the treated steel. An elevated temper-
nitriding and nitrocarburising of low alloy steels are gears, crank-                ature is required in order to ensure fast transfer of carbon/nitrogen

                                                             e- and/or g’-phase compound layer

                                                                     Hardness                                                                   Hardness


Figure 1. Micrographs showing compound layer and diffusion zone on nitro-           b) 5 % Cr tool steel with tempered martensite core microstructure. The diffusion
carburised: a) Low carbon steel with ferritic-pearlitic core microstructure.        zone is the dark etched region below the compound zone.
                                                                                                                              Introduction                      5

Table 1. Characteristics of thermochemical processes involving nitrogen and/or carbon.

Process                 Temperature                    Typical            Element              Case depth          Sur face          Distortion
                        °C (°F)                        process time       transferred          mm                  hardness HV
Carburising          850-950 (1562-1742)                  4-10 h             C                  0.2-1.5            750-850

                                                                                                                                         Increased distortion
Carbonitriding       750-900 (1382-1652)                   2-5 h             C+N                0.1-0.8            750-850
nitrocarburising     600-700 (1112-1292)                   2-4 h             N+C                0.1-0.5            750-850
Nitrocarburising     560-580 (1040-1076)                   2-4 h             N+C                0.05-0.2           450-1200
Nitriding            500-510 (932-950)                   5-100 h             N                  0.05-0.8           450-1200

from the gas to the steel surface and also to allow carbon/nitrogen       Classical gas nitriding was developed for the purpose of increasing
to diffuse into the steel at an appreciable rate.                         fatigue strength and load bearing capacity without involving sig-
                                                                          nificant distortion of treated components. Specially alloyed nitriding
An important feature of nitriding and nitrocarburising is that they are   steels are used in order to achieve a high hardness. Very long nitrid-
“low temperature methods” whereas carburising and carbonitriding          ing times, from ten to hundreds of hours, have been and are used to
are “high temperature methods”. Here low temperature refers to a          obtain sufficient case depths.
temperature below that where phase transformation to austenite
starts (A1), and high temperature is above said temperature. A valu-      Nitrocarburising began to grow drastically with the development
able consequence is notably reduced distortion of treated parts.          of the salt bath process Tenifer (Tufftride) and the gaseous process
This can often save time and costs by eliminating the need for post
grinding to meet dimensional tolerance requirements. The production
cycle of a part therefore becomes faster and cheaper. A limitation        Table 2. An example of energy requirements for two process
caused by the lower temperature is that the diffusion rate for nitro-     routes, one being nitrocarburising [2].
gen and carbon is modest, which sets limits on the case depths that
can be obtained.                                                          Process step                            Energy requirement, KWh
                                                                                                           Harden, temper        Nitrocarburising
Carburising and carbonitriding give a surface hardness in the range                                        and nickel plate
of 750-850 HV that is largely independent of the steel type, whereas      Heat to processing                    265                    163
nitriding and nitrocarburising give a wide possible range of surface      temperature
hardness determined by the steel selection.
                                                                          Hold at processing                      27                     66
Austenitic nitrocarburising is a process that has characteristics in
between the high temperature methods of carburising and carboni-          Quench                                   5
triding and the low temperature processes of nitrocarburising and         Heat to tempering                       94
nitriding.                                                                temperature
                                                                          Hold at tempering                       44
A consequence of the low process temperature, the short process           temperature
time and the elimination of productions steps is low energy con-
                                                                          Electroplating                          20
sumption. Table 2 shows an example in which the energy saving was
about 50 % when the process route was changed to nitrocarburising.        Total                                 455                    229
6           Introduction

Nitemper in the sixties. Compared to classical nitriding, nitrocarbu-      Table 3. Nitriding and nitrocarburising features and
rising is a short-time process, typically lasting 30 minutes to 4 hours,   process names.
and is performed at a higher temperature, about 570 °C (1058 °F)
                                                                                              Temperature   Time    Name or             Media
compared to 500-510 °C (932-950 °F) for gas nitriding. It is typically
                                                                                              °C (°F)        h      trade name
applied to low alloy steels but also to unalloyed steels and cast
irons as well as to tool steels.                                                              500-510       5-100   Gas nitriding       NH3/N2/H2
                                                                                              (932-950)             Oxynitriding        NH3/Air
There are numerous names on the market to describe nitriding and                                                                        NH3/ H2O

nitrocarburising. Some of them are given in Table 3. NITROFLEX® is
                                                                                                                                        NH3/ N2O
Linde’s trademark for nitriding and nitrocarburising processes.
                                                                                                                    Plasma nitriding    N2/H2

The process medium can be salt, gas or plasma. The salt bath proc-                                                  High pressure       N2, NH3
esses are losing market to atmospheric gas pressure processes due                                                   nitriding
to the environmental problems with salts, which contain cyanide.
                                                                                              560-580       0.5-5   NITROFLEX®          NH3/N2/CO2/
The use of plasma processes has steadily increased in recent dec-
                                                                                              (1040-1076)                               (H2/CO/C3H8)
ades although the number of installations is still limited in compari-
                                                                                                                    Nitemper            NH3/Endogas

son with atmospheric pressure processes.
                                                                                                                    Nitrotec            NH3/Endogas/air
                                                                                                                    Nitroc              NH3/Exogas
                                                                                                                    Tenifer/Tufftride   Salt
                                                                                                                    Plasma              N2/H2/CH4
                                                                                                                    Post oxidation      Air, H2O, N2O
                                                                                                                                              Process Selection        7

II. Process Selection

The functional properties of a part, such as fatigue and static                             The steel alloy and carbon content and the type of prior heat treat-
strength, or wear and corrosion resistance, are the basis for                               ment determine the core strength of the part and will also influence
specifying the proper process and steel as illustrated in Figure 2.                         the development of the compound layer structure, case depth and
                                                                                            hardness (see Figures 17 and 19). Surface fatigue strength and wear
                                                                                            resistance are therefore to a great extent dependent on the steel
                         Required functional properties                                     specified and will improve with increased alloy content.
                          Fatigue, wear, corrosion resistance
                                                                                            The functional part properties that essentially depend on the com-
                                                                                            pound layer are wear resistance, tribological properties, corrosion
 Compound layer                  Diffusion zone                      Steel
 Structure, chemistry,            Hardness, depth           Alloy, content, heat treat
                                                                                            resistance and general surface appearance. Both abrasive and
   hardness, depth                                          condition, core hardness        adhesive wear resistance increase with hardness and with minimised
                                                                                            porosity of the compound layer. Porosity can be positive in lubricated
                                                                                            machinery parts as the pores act as lubricant reservoirs. The
                             Process specification
                            Temperature, time nitridning                                    compound layer depth has to be deep enough not to be worn away.
                             potential, gas composition                                     The diffusion layer (depth, hardness and residual stress) determines
                                                                                            surface fatigue resistance and resistance to surface contact loads.
Figure 2.
The steps to process specification starting from required part properties.
                                                                                            Cost is a factor that limits the number of options. Variable costs
                                                                                            increase proportionally to increased time for nitriding/nitrocarbu-
                                                                                            rising. Increased treatment temperature will also increase running
                                                                                            costs due to higher energy losses and wear of equipment (See Table
The interdependence between required functional properties,                                 2). Capital costs are highest for vacuum and plasma equipment, but
the selected steel and nitriding/nitrocarburising specifications is                         the variable cost for utilities (electricity and gas) is much lower than
illustrated in Table 4.                                                                     for conventional atmospheric furnaces.

     Table 4. Relation between part properties, steel selection and nitriding/nitrocarburising parameters.

                                                                                           Part properties

                                     Static and fatigue strength                     Contact load        Abrasive wear       Adhesive wear         Corrosion
                                                                                       fatigue             resistance          resistance         resistance
                                        High            Moderate
       Steel                     High alloy and Low alloy steels, cast            High and low alloy     High and low alloy steels, cast irons, sintered steels
                                 nitriding steels irons, sintered steels                steels

       Compound layer                       Minor influence                                  High hardness, high e-content                          Dense,
       Diffusion layer            High hardness      Moderate hardness           High hardness and                        Minor influence
                                    and depth           and depth                      depth
       Process                    Gas nitridning       Nitrocarburising                      Gas nitridning and nitrocarburising               Nitrocarburising +
8           Process Selection

Part geometry and dimensions may limit the number of possible              4. Dimensional scraps: The low temperature nitriding/nitrocarbu-
alternatives. Very long objects such as shafts or axles cannot be             rising processes reduce distortion, compared to high temperature
treated in standard box or sealed quench furnaces, for instance,              processes.
but tend to be treated in cylindrical pit furnaces. Requirements for
dimensional tolerances may also limit the number of alternatives           5. Elimination of post grinding: It is possible in certain situations
with respect to treatment temperature as increased treatment                  to predict the dimensional change after treatment. The limited
temperature leads to increased dimensional changes. Cooling options           distortion makes it possible to eliminate tolerance adjustment
are also limited as increased cooling severity will increase distortion.      operations such as post grinding.

Nitriding and nitrocarburising have significant cost benefits that         6. Additional processes: With gaseous treatments there is no need
include:                                                                      to remove salt residues from the surface after treatment, thus
                                                                              eliminating post cleaning operations. Nitrocarburising treatments
1. Cycle times: Short cycle times lead to reductions in energy and            produce a compound layer which is scuff and corrosion resistant.
   atmosphere costs. In addition, the furnace capacity and the                This layer does not need to be ground after treatment, and is
   furnace equipment life will increase. This will also increase the          more flexible than the white layer that is produced by traditional
   production capacity of the manufacturing facility.                         nitriding treatments. Processes also produce an excellent surface
                                                                              finish that requires no post operation and that can be used
2. Materials: Many engineering components are made from                       directly on the assembly.
   expensive highly alloyed steels, not because the properties they
   offer are required throughout the material, but because they are
   required at or near the surface. The use of lower cost materials
   combined with nitriding/nitrocarburising can reduce costs.

3. Energy costs: There are a number of possibilities to decrease
   energy costs. These include decreasing cycle times, as described
   above, and decreasing process temperature. Nitriding/
   nitrocarburising processes operate at lower temperatures than
   conventional methods, resulting in energy savings.
                                                                                                                   The Process Steps in Nitriding and Nitrocarburising             9

III. The Process Steps in
Nitriding and Nitrocarburising

 A nitriding/nitrocarburising cycle has three major steps: 1) heating                        There may also be pollution from manufacturing machinery in the
 to temperature, 2) holding at temperature for a sufficient time to                          form of hydraulic fluids, tool wear debris, chips, turnings, blasting
 reach the required nitriding depth, and 3) cooling. This is illustrated                     agents and abrasives, and, if machines are used for different metals
 in Figure 3, which also shows the optional additional steps of pre-                         such as aluminium, even residues from non-ferrous metals. Anti-cor-
 heating/pre-oxidation and post-oxidation used in nitrocarburising.                          rosives used to protect parts from rust in storage and transport may
                                                                                             be a further source. Contaminations may be in the form of surface
                                        Nitride/Nitrocarburise                               films or layers, or particles.
                                        at 500-580°C
                                        (932-1076°F) for 30      Oxidize at
                                        minutes to 100 hours     450-550°C                   Haase [3] studied the effect of different additives in cutting oils on
                              Heating                            (842-1022°F)
                                                                                             nitriding results with results shown in Figure 4. An increased amount of

              preoxidize at                                                     Cool in
                                                                                             additive reduced hardness and gave uneven and locally zero compound
                                                                                gas or oil
              (662-842°F)                                                                    layer thickness. In another study Persson and Troell [4] found that the
                                                                                             specific chemicals sulphur and phosphorous added to the cutting oil as
                                                                                             well as sodium, boron, and calcium in cutting fluids all had a negative
                                                                                             impact on compound layer formation. They also found a negative
                                                                                             influence if fluids were allowed to dry on the surface before nitrid-
 Figure 3. Nitriding/nitrocarburising cycle.
                                                                                                     a) ground                                b) milled                  10 mm

 A. Prior Heat Treatment Condition
 For parts subjected to high stress, the normal state of the steel prior
 to nitriding or nitrocarburising is hardened and tempered at a tem-
 pering temperature at least 20-30 °C higher than the nitriding/nitro-
 carburising temperature in order to prevent loss of hardness during                                                                                                     10 mm

 nitriding/nitrocarburising. If nitriding/nitrocarburising is conducted
 primarily to increase resistance to wear and scuffing, steels in
 annealed or normalized conditions can be used. Cast irons may be
 nitrided or nitrocarburised in the annealed state.

 For parts that have been subject to turning, drilling or any other ma-
                                                                                                     fatty ester                                                            c)
 chining or cold forming operation, it is necessary to release internal                       mineral oil (base)
 stresses by stress-relieving annealing. After stress relieving the part                      S/P additive (1%)
 dimensions are adjusted by fine machining or grinding to meet the                              S additive (1%)
 tolerance requirements before nitriding/nitrocarburising. The tem-                             S additive (2%)
 perature for stress relieving should preferably be 20-30 °C above the                          P additive (1%)
 nitriding/nitrocarburising temperature in order to avoid stress reliev-                        P additive (2%)
 ing and concurrent distortion during nitriding/nitrocarburising.                            commercial product

                                                                                                               600                   700                   800                   900
 B. Cleaning                                                                                                                           Surface hardness HV0.5
 Cleaning is an important process step before nitriding/nitrocarburis-
                                                                                             Figure 4. Uneven nitriding results due to the effect of surface passivation. Micro-
 ing as surface contaminants disturb nitride layer formation. In manu-                       graphs showing compound layer after a) grinding and b) milling. c) Surface hard-
 facturing steps before heat treatment, contamination sources are                            ness of steel after machining with mineral oil containing different additives. Steel
 lubricants, coolants and cuttings oils used in machining and grinding.                      42CrMo4 [3].
10             The Process Steps in Nitriding and Nitrocarburising

Cleaning agent residues can lead to passive surface layers if not re-                                                50
moved by thorough rinsing. Their negative effect on surface hardness
after nitriding depends on their volatility; inorganic salts such as sili-
cates and phosphates have high melting points and do not vaporize                                                                      Pre-oxidized,
at nitriding temperature. Furthermore, they form glassy films, which                                                 20                400°C (752°F), 15 min
may completely prevent diffusion or nitrogen uptake. However, if a

                                                                                    Nitrogen concentration, atom %
cleaning installation provides rinsing stages, they can be removed

Water-based cleaning solutions have to a large extent replaced chlo-
rinated hydrocarbons that are no longer permitted for environmental
reasons. In addition to water (> 80%), the washing liquid consists of                                                10                   No pre-oxidation
active cleaning agents such as surfactants, inorganic builders, and
complex agents and anti corrosives. The surfactants remove oil films
whereas particles are removed by the inorganic builders.
The amount of the metal ions calcium and magnesium define the
water hardness. Both these ions are detrimental if left on the surface
before nitrocarburising as this may lead to spots with thin or no com-                                               0
pound layer [5]. The washing agents dissolved in the water should                                                         0     100            200        300
react with and bind the Ca and Mg ions into chemical complexes,                                                               Distance from surface, nm
thereby eliminating the deposits of free ions and the risk of weak
                                                                             Figure 5. Nitrogen concentration profiles after short-time nitriding of the cold
spots. If the water hardness is high, then the amount of washing             work steel X155CrVMo12-1 with and without pre-oxidation [3].
agent and complex building capacity may not be sufficient to bind
all Ca and Mg ions into complexes to avoid spot defects. Low water
hardness is therefore preferred to guarantee good and even nitrocar-
burising results.                                                            D. Nitriding
                                                                             Gas nitriding is typically performed in convection furnaces, either of
Plastic deformation in the surface region prior to nitrocarburising will     pit type as in Figure 6 or a box furnace, at a temperature in the range
also reduce compound layer growth. A 25 µm plastic deformation               of 500-520 °C (932-968 °F) and in an ammonia atmosphere. The am-
zone has been found to reduce the layer thickness by 30% [4].                monia may be diluted with nitrogen or hydrogen. The parts to be ni-
                                                                             trided are placed on fixtures or in “baskets”, which are transferred to
                                                                             and loaded into the furnace. The furnace cover or door is then closed.
C. Preheating and Pre-Oxidation                                              To ensure precision with regard to compound layer and diffusion zone
Preheating in air at a temperature in the range of 350-450 °C (662-          thickness, it is important with enough good furnace temperature
842 °F) for 30-60 minutes is a standard procedure before nitrocarbu-         uniformity, typically ± 5 °C (9 °F).
rising for a number of reasons:
                                                                             1 Sours of process gas
                                                                               (tanks, process gas generator)
     – The process time in the nitrocarburising furnace, which is more
                                                                             2 Nitrex Gas Nitriding Process Control System
       costly than the preheat furnace, is reduced.                          3 Nitrex Furnace
                                                                             4 Turbo coling system
     – Heating in air leads to surface oxidation that is found to acceler-   5 Closed-loop water cooling system
       ate compound layer nucleation and growth during nitrocarburis-        6 Racking
                                                                             7 Exhaust gas neutralizing equipment
       ing, see Figure 5.

     – Residues on the part surfaces are oxidized and vaporized, result-
       ing in cleaner parts and improved nitriding ability.

     – Safety is ensured for salt bath nitriding/nitrocarburising by re-
       moving any water that has adhered to the parts.

One possible reason for the positive effect of pre-oxidation is that
oxide formation results in a notable increase in the surface area,
which facilitates nitrogen uptake and the nucleation and growth of
the nitride compound layer [6]. Other possible explanations are that
nucleation and growth of e-carbonitride is facilitated and/or that the
reverse process of nitriding, i.e. desorption of nitrogen, is retarded by
oxidation [7].                                                               Figure 6.
                                                                             Example of a nitriding furnace installation [courtesy of Nitrex Metal Inc].
                                                                                                               The Process Steps in Nitriding and Nitrocarburising           11

A nitriding cycle consists of the steps illustrated in Figure 7. Nitrogen                   and put into a cooling station. In furnaces without a retort, cooling
purging of the furnace is conducted to remove air before ammonia is                         takes place in the furnace.
let into the furnace. This purge is carried out to eliminate the risk of
explosion, as ammonia and oxygen form an explosive mixture within                           Nitriding furnaces have to be tight for safety reasons and also be-
a certain concentration range. For this reason it is also advantageous                      cause of the odour resulting from possible leakage of ammonia gas.
to perform heating to nitriding temperature in nitrogen. When the
nitriding temperature is reached, ammonia is let into the furnace. In
the beginning a high flow rate is used to increase the efficiency of                        E. Nitrocarburising
nitrogen transfer to the steel surface.                                                     In principle, the same type of furnaces can be used in nitrocarburis-
                                                                                            ing as in gas nitriding; however, one special feature of nitrocarburis-
                                                                                            ing is that the final cooling is usually fast. Brick-lined sealed quench
                   N2      NH3       H2                                                     furnaces with an oil quenching capability of the same type as for
                                                                                            carburising are therefore used (see Figure 8a). Other common solu-
                                             Temperature                                    tions are box-type atmosphere furnaces, often with fibre lining, and
                                                                                            batch furnaces with a vacuum pumping option for quick atmosphere
Gas flow rates

                                                                                            conditioning and with integrated gas cooling (Figure 8b), as well as
                                                                                            metallic retort furnaces of the type shown in Figure 6. Table 5 gives
                                                                                            specific advantages and disadvantages of each type of furnace.

                                                                                            Table 5. Specific features of each furnace type.
                 Purging   Conditioning      Processing      Cool and Purge
                                                                                            Furnace type              Advantage                             Disadvantage
                                                                                            Brick-lined               Slow ammonia                          Slow change
                                          Time                                              atmosphere furnace        dissociation                          in atmosphere
Figure 7. Nitriding steps and connected gas consumption.                                                              Modular construction
                                                                                            Metallic retort           Fast change in                        Fast ammonia
                                                                                            furnace                   atmosphere                            dissociation

As soon as a compound layer is formed, the nitriding rate is control-                                                 Low nitriding potential               Lifetime of retort
                                                                                                                      without hydrogen addition
led by diffusion from the layer into the steel. The ammonia flow rate
may then be lowered just to give a nitrogen transfer rate from gas to                                                 Evacuation possible
surface which keeps up with the diffusion rate [8]. Nitriding is contin-
ued until the desired nitriding depth is reached.
                                                                                            After the parts have been cleaned, they are loaded into baskets or
After finishing the nitriding step, the furnace interior is purged with                     fixtures and transferred to the furnace for heating (preceded by pre-
nitrogen to remove the ammonia gas in order to ensure safety.                               heating) to process temperature, 570-580°C (1058-1076°F), and kept
                                                                                            at that temperature for a time that yields the desired compound layer
Cooling should continue in nitrogen to avoid discoloration by oxida-                        and diffusion depth. As in the case of nitriding, close temperature
tion. In pit furnaces the retort is normally lifted out of the furnace                      uniformity, typically ±5 °C (9 °F), is required. In addition to containing

                                 a                                                             b                                                      c

Figure 8. Furnaces for gaseous nitrocarburising: a) Sealed quench furnace with              furnace with integrated gas cooling also showing the gas system and c) Cross
integrated oil quench bath. b) Side view of one chamber vacuum/atmosphere                   section of the same furnace as in b) [Courtesy of Ipsen International GmbH].
12                      The Process Steps in Nitriding and Nitrocarburising

ammonia, the furnace atmosphere should also contain carbon mon-                                                         nitriding media. It is necessary to use a higher relative amount of
oxide and hydrogen in order to transfer both carbon and nitrogen to                                                     ammonia than for atmospheric pressure nitriding. The major benefits
the steel surface.                                                                                                      of the vacuum nitriding process are low consumption of gases, almost
                                                                                                                        no effluents, a pure atmosphere, clean surfaces and fast change of
                                                                                                                        nitriding parameters [9]. The disadvantages are relatively high equip-
                   N2         NH3                       H2   CO2                                                        ment costs and problems with uniformity in the nitriding result for
                                                                                                                        parts with deep narrow bores.
                                                                                                                        High pressure nitriding is a very different process. It is carried out in
Gas flow rates

                                                                                                                        nitrogen, which at normal ambient pressure is neutral with respect to

                                                                                                                        nitriding ability, but which at very high pressure up to 1000 bar has
                                                                                                                        a nitriding effect. Its benefits are the use of environmentally friendly
                                                                                                                        nitrogen gas and the possibility to treat steels that are difficult to
                                                                                                                        nitride. The major disadvantage is very high equipment costs, which
                                                                                                                        has been a barrier to its use outside research laboratories.

                 Purging       Conditioning                        Processing            Cool and Purge                 The fourth state of matter, plasma, is characterized by the fact that it
                                                                                                                        consists of free charged particles, ions and electrons. In a DC plasma
                                                              Time                                                      nitriding furnace (see Figure 10a) an electrical voltage is applied
                                                                                                                        between workload (the cathode) and the furnace vessel (the anode).
Figure 9. Nitrocarburising steps and connected gas consumption.
                                                                                                                        A vacuum of the order of a few mbar is maintained in the vessel,
                                                                                                                        which contains nitrogen gas. In the near vicinity of the load the elec-
The process cycle diagram shown in Figure 9 is similar to that shown                                                    trical potential drops and a plasma with nitrogen ions is obtained.
in Figure 6 with the main difference that CO2 (carbon dioxide) is                                                       The positively charged nitrogen ions are accelerated by the electrical
also an added gas. There is sometimes a post oxidation step after                                                       voltage towards the load. The nitrogen ion bombardment results in
finalised nitrocarburising. Final cooling is carried out in gas or oil.                                                 the nitriding of the steel as well as the heating of the part. Hydrogen
The fast cooling in oil reduces the process time compared to when                                                       is added to obtain reducing conditions and to control the nitriding
slow gas cooling is used. It also results in better properties as regards                                               potential. Argon is sometimes used as a cleaning agent before actual
the hardness and residual stress pattern of the nitrocarburised steel.                                                  nitriding. The argon ions are heavy and therefore efficient in cleaning
Milder cooling has the benefit of minimising distortion. One cost                                                       the surface by so-called sputtering, which is the removal of surface
benefit of gas cooling is that it can be conducted in a one-chamber                                                     layer atoms by ion bombardment.
furnace which is less costly than a sealed quench furnace with an
integrated oil quench.                                                                                                  The DC plasma technology has weaknesses with respect to tempera-
                                                                                                                        ture uniformity and the risk of damage from arching. The availability
                                                                                                                        of pulse plasma technology with multiple heating and cooling options
F.  Vacuum, High Pressure and Plasma Nitriding                                                                          minimize these drawbacks explicitly (see Figure 14).An ongoing de-
    and Nitrocarburising                                                                                                velopment that also eliminates these drawbacks is active screen plas-
A low pressure nitriding process starts with the evacuation of the                                                      ma. In this case the plasma is created in a separate chamber, and a
furnace chamber followed by refilling it with nitrogen to atmos-                                                        metal screen surrounding the load is used as the cathode, Figure 10b.
pheric pressure to enable fast heating by convection. When the
process temperature is reached, vacuum pumping to a pressure of                                                         The plasma technique offers similar benefits to those of vacuum ni-
150-400 mbar is performed. Ammonia and hydrogen are added as                                                            triding including very low consumption of gases, the environmental

                                                                                                                                                       Mesh cathode
                                                                                                                                  Furnace wall anode
                               Furnace wall anode

                                                              cathode                          DC Power                                                 Sample

                                                    Gas in                         Gas out

                                                                         Figure 10. Cross sections of a) DC and b) active screen plasma furnaces [10].
                                                                                                              The Process Steps in Nitriding and Nitrocarburising             13

advantage of almost no effluents (see Table 6), the possibility to
tailor the phase constituency in the compound layer to be pure g’
or pure e, and the possibility to nitride to only yield a diffusion zone                                                                                Oxide, ( ≈ 3 mm)

without the compound layer. Plasma nitriding can be used at very                                                                                        Compound zone

low process temperatures, down to 400-450 °C (752-842 °F), which
means that hardened and tempered steels can maintain their hard-                                                                                        Core microstructure

ness. Lower distortion is a connected benefit. Because of the ion
bombardment, surfaces are activated to make it also possible to ni-
tride stainless steels and even the nonferrous metals aluminium and             Figure 12. Compound layer with oxide layer at top.

Table 6. Comparison of effluent emission data for gaseous and                   H. Austenitic Nitrocarburising
plasma nitrocarburising [10].                                                   Austenitic nitrocarburising is developed in order to create thicker
                                                                                cases that can sustain greater surface loads or bending stresses. It
                                   Plasma       Gaseous        Reduction in %   is performed at a temperature above the temperature for the partial
Amount of gas used
                                                                                transformation of the steel to austenite. At the process temperature
m3/h (100ft3/h)                   0.6 (0.21)     6.0 (2.12)          90 (32)
                                                                                austenite enriched with carbon and nitrogen is formed beneath the
                                                                                compound layer. Upon cooling after finalised nitrocarburising some
Total carbon emission via
                                                                                of this austenite will remain as retained austenite and some will
CO/CO2 , mg/m3                       504           137 253           99.63
                                                                                transform into bainite, pearlite or martensite. A subzero treatment
Total amount of NOX gas,                                                        will transform the retained austenite further into martensite with a
mg/m3                                1.2               664           99.82      hardness in the range of 750 to 900 HV. Alternatively, a tempering
Output of residual carbon                                                       operation can be carried out to transform the retained austenite into
bearing gas, mg/h                    302           823 518           99.96      bainite/martensite. This will also raise the hardness both in the diffu-
Output of residual NOX gas           0.72             3 984          99.98      sion zone and in the compound layer as shown in Figure 13.

The combination of plasma nitriding with atmosphere nitrocarburising
has been shown to give excellent surface appearance, wear and cor-                                                               Austenitic nitrocarburised
                                                                                                   800                           at 700°C (1292°F)
rosion resistance. An example is the Corr-I Dur process, see Figure 11.
                                                                                                                                 Austenitic nitrocarburised
                                                                                                                                 at 700°C (1292°F) and tempered
                                                                                                   700                           at 350°C (662°F) for 2 hours
                                                      Oxide layer 1-3 mm
                                                      Compound layer 5-25 mm

                                                      Diffusion 100-500 mm
                                                                                Hardness, HV 0.1

                                                                                                   500                                        CORE
Figure 11. Micrograph cross section of a steel specimen treated in
           the CORR-I-DUR® process [11].


G. Post Oxidation
A remarkable improvement in corrosion resistance is obtained if                                                                               225 HV 0.5
nitrocarburising is followed by a short oxidation in the temperature
range 450-550°C (842-1022°F). The aim is to create a Fe3O4 oxide                                             γ/α’
                                                                                                   200       layer
layer with a thickness of about 1 µm formed on top of the compound
layer, as shown in Figure 12. Fe2O3 must not be formed because it                                                                            200HV 0.5
deteriorates both the aesthetic surface appearance and corrosion
resistance. If done properly, the oxidation treatment gives the proc-                              100

essed parts an aesthetically attractive black colour with high surface                                       ε/Fe3C
corrosion resistance.                                                                                         layer
                                                                                                         0           0.1           0.2            0.3               0.4
The first gaseous post oxidation process was developed by Lucas,
England, and is called Nitrotec [1]. It is based on the Nitemper proc-                                                     Distance from surface, mm

ess with an added oxidation step in air. Other oxidation methods                 Figure 13. The influence of tempering at 350°C (662 °F) for 2 hours on the
using water vapour [12] or nitrous oxide (N2O) [13] have later been              hardness profile across an austenitic nitrocarburised case produced at 700 °C
successfully developed and found to yield good properties.                       (1292 °F) in a mixture of 14 % ammonia and 86% endogas [14].
14                        The Process Steps in Nitriding and Nitrocarburising

I. Combined Processes                                                                         the hardness in the diffusion zone. This effect is pronounced only in
PVD (Physical Vapour Deposition) and CVD (Chemical Vapour Deposi-                             unalloyed or low-alloy steels. For steels containing high concentra-
tion) are processes that produce coatings such as TiN or CrN with very                        tions of nitride forming alloying elements such as chromium and alu-
high hardness. These coatings are thin and their load bearing capac-                          minium, the precipitation hardening effect of nitrides dominates the
ity is limited, which makes the coating susceptible to failure because                        hardness contribution and the solution hardening effect is negligible.
of cracking or flaking. Performing nitriding or nitrocarburising as a
preceding operation before the PVD or CVD coating improves bearing
capacity. Combined processes have therefore been developed. Figure                                                                                                  Compound
14 shows a cross section of equipment capable of performing both
plasma nitriding and PVD coating. An example of the resulting hard-                                                                                                 Nitride
ness profile after nitriding + PVD/TiN coating is also shown in the                                                                                                 precipitates

     Vacuum measurement                     Process gas                                       Figure 15. Microstructure showing precipitates of needle-like nitrides in the
     and control systems                                                  Vacuum
                                                                          pumpset             diffusion zone.


                                                                                              An increased cooling rate increases the compressive residual stress in
                     + –
                                                                                              the case. Both hardness and compressive residual stresses contribute
supplies                                                                                      to fatigue hardness, which means that the best fatigue hardness is
                     + –                                                        Infrared-     obtained with high cooling rates. Increasing part dimension leads to
                                                                                measurement   a decreased cooling rate for a given cooling setup. This leads to low-
                                                                                              ered hardness as seen in Figure 16.
                   Coating                                          holder

                                  Pulsed DC                                                                  900
                                  generator for                                                              800
                                                                                                                                  10 mm
                                                                                              Hardness. HV

                                                                                                             400        40 mm
                   2000                                                                                      300
     Hardness HV

                   1500                          Plasma                                                      100
                               PVD              nitriding
                                                                                                                    0      0.1    0.2           0.3           0.4              0.5
                                                                                                                                        Depth, mm

                                                                                              Figure 16. Hardness of bars with two diameters after gaseous nitrocarburising
                          0          1        10       100       1000                         followed by oil quenching. Steel approx. corresponding to 16MnCr5. [15].

                                                     Distance, mm

Figure 14. a) Cross section of a unit for both plasma nitriding and PVD coating.
                                                                                              If high fatigue strength or surface contact load bearing capacity is
b) Result of plasma nitriding followed by PVD-TiN coating. Note logarithmic
distance scale. [Courtesy of Sulzer Metaplas GmbH].
                                                                                              required, a fast cooling should be applied. If wear and/or corrosion
                                                                                              resistance is the main objective, the properties of the compound
                                                                                              layer are decisive. Compound layer properties are to a lesser extent
                                                                                              dependent on the cooling rate. Therefore the cooling rate is unimpor-
                                                                                              tant in such cases. Productivity is of course increased with increased
                                                                                              cooling rate.
J. Cooling/Quenching and Post Treatment
Cooling after nitriding is usually performed in nitrogen gas with no or                       The number of available cooling options depends on the type of fur-
moderate convection, whereas the cooling step after nitrocarburis-                            nace used as illustrated in Table 7. Water, the fastest type of quench
ing can be accelerated by quenching in oil or by forced gas cooling                           medium (not included in the table), is used only in connection with
(water quenching is an alternative after salt bath processes). If the                         salt bath nitriding/nitrocarburising. Oil quenching, the second fast-
cooling rate is low, it will lead to the precipitation of needle-like iron                    est, is common as it is the standard quench method in sealed quench
nitrides in the diffusion zone when unalloyed steels are treated as                           furnaces for which gaseous nitrocarburising was primarily developed,
shown in Figure 15. With increased cooling intensity the amount of                            such as in the Ipsen Nitemper process. Gas quenching is growing in
nitrogen, kept in solid solution in ferrite, is increased. This increases                     importance for several reasons: Gas quenching is the only method
                                                                                           The Process Steps in Nitriding and Nitrocarburising   15

that can be integrated into single chamber furnaces of both vacuum
and atmosphere types. Gas quenching has environmental advantages
compared to oil quenching, both for the external and work environ-
ment. Gas quenching also has the advantage that post cleaning can
be eliminated. It is possible to increase and vary the cooling rate by
increasing and varying the gas pressure and velocity.

Table 7. Cooling options for different types of nitriding/nitrocarburising furnaces.

                                  Furnace type                                                 Possible cooling options

                                  Sealed quench furnace with integrated oil quench             Oil
                                                                                               Gas, slow convection
                                  Box furnace                                                  Gas, slow convection
 Atmosphere furnaces
                                  Box furnace with integrated high speed gas circulation       Gas, high convection
                                  Pit retort furnace                                           Gas, slow convection
                                  Furnace line with integrated high pressure gas cooling       Gas, high pressure and high convection

                                  Single chamber                                               Gas, slow convection
 Vacuum furnaces
 (including plasma furnaces)      Single chamber/high pressure                                 High pressure, high convection
                                  Furnace line with integrated high pressure gas cooling       High pressure, high convection
16                      Properties of Nitrided and Nitrocarburised Steels

IV. Properties of Nitrided and Nitrocarburised Steels

Nitriding and nitrocarburising of parts is carried out to improve wear                   ficient time for the precipitation of iron or alloying element nitrides
resistance and strength, in particular fatigue strength, and corrosion                   (Figure 15), which reduces the hardening effect from solid solution
resistance. Sometimes improved aesthetic surface appearance is also                      hardening, see Figure 18. This is why low alloy steels are normally
a desired property.                                                                      quenched in water or oil after nitrocarburising.
A. Hardness and Wear Resistance
The case on nitrided/nitrocarburised steel is divided into the com-                                     270                                         water
pound zone, typically 5-20 µm thick, and the diffusion zone, with a
thickness typically of several tenths of a millimetre. The hardness of                                                                     oil
                                                                                         Hardness, HV

the compound zone is about constant through its thickness with the                                      230
exception of the outer porous zone, where hardness is reduced due
                                                                                                        210                       air
to porosity. It is higher than the diffusion zone hardness, which con-
tinuously decreases from the surface into the steel interior.                                           190

Compound layer hardness after nitrocarburising is about 700 HV for
low alloy steels and hardness increases with increasing alloy con-                                      150
tent in the steel as shown in Figure 17. The measured hardness falls                                       0.00   0.05   0.10       0.15         0.20   0.25   0.30   0.35

as the degree of porosity in the outermost compound layer surface                                                               Depth below surface, mm
increases. Generally porosity is greater for low alloy steels than for
high alloy steels.                                                                       Figure 18. Influence of quenching on diffusion zone hardness after salt bath
                                                                                         nitrocarburising of steel C15 [17].

                                                       13% Cr stainless steel
               1100                                                                      The second hardening effect is precipitation hardening. This harden-
                            4% Cr                                           12% Cr       ing process is predominant with alloyed steels. A dramatic conse-
                                                     5% hot work
                                                                            tool steel   quence of this mechanism is that a hardness range as wide as 300-
Hardness, HV

               900                                   tool steel
                                                                                         1300 Vickers is obtained depending on which steel has been nitrided
                            1.5% Cr                                                      or nitrocarburised, see Figure 19.
               700          1% Cr
               600                                                                       The hardness of the compound layer will determine wear resistance.
               500                                                                       Increased hardness in the compound layer gives increased resist-
                 0.00     2.00      4.00      6.00    8,00     10.00    12.00   14.00
                                                                                         ance to abrasive wear, in which abrasive particles such as sand wear
                                           Alloy content, weight %
                                                                                         a surface. An important point is that compound zone hardness after
Figure 17. Approximate relation between hardness of the compound layer and               nitriding or nitrocarburising can give rise to “low level” abrasive wear
alloying content (Sum of Cr+Mo+V+W) of the treated steel [16].                           situations. This means a very low wear rate when the surface hard-
                                                                                         ness is higher than the hardness of the abrasive particles. The limited
                                                                                         depth of the compound layer is a drawback and thus nitriding or
There are two mechanisms which determine diffusion zone hardness.                        nitrocarburising can only be used successfully in mild abrasive wear
First, solid solution hardening is a mechanism that is of high impor-                    situations.
tance for low alloy steels. The process temperature determines the
degree of solid solution of nitrogen, carbon and alloying elements.                      Abrasive wear resistance is proportional to hardness and for nitrocar-
Quench rate from the process temperature determines how much                             burised or nitrided steels is therefore dependent on the type of steel
can be maintained in solid solution. A slow cooling rate allows suf-                     used (See Figure 19). For alloyed steels hardness and thus abrasive
                                                                                                                                                               Properties of Nitrided and Nitrocarburised Steels          17

                                                                 Nitriding 510°C (950°F)/24 h                                                          Nitrocarburising 570°C (11058°F)/2 h

                                                                          5 % Cr hot work tool steel

                                              1200                                                                                        1200
                                                                                   Al – alloyed steel                                                        5 % Cr hot work tool steel

                                              1000                                                                                        1000
                                                                                     1,5 % Cr
                                                                                     air hardening steel
                                                                                                                                                                      1,5 % Cr air hardening steel
                                               800                                                                                        800
                               Hardness HV

                                                                                                                   Hardness HV
                                                                                                                                                                                           1 % Cr oil
                                               600                                                                                        600                                              hardening steel

                                               400                                                                                        400

                                                                                                                                                                                                  low alloyed
                                               200       low                         case                                                 200                                                     steel
                                                         alloyed steel               hardening steel
                                                                                                                                                                                    mild steel

                                                 0                                                                                          0
                                                     0           0.1         0.2       0.3         0.4                                                0.01     0.02          0.04    0.1    0.2      0.4    Logscale
                                                                           Depth, mm                                                                                         Depth, mm

                                             Figure 19. Typical hardness after nitriding and nitrocarburising. Please note the logarithmic length scale in the right hand figure [13].

   wear resistance is superior to that of carburised steels. However,                                              an increased ratio e/g’ in the compound layer [18] and is highest for
   abrasive wear will increase drastically when the thin case of a nitro-                                          100% e-phase layers [19]. The initial lubricated wear rate of nitrocar-
   carburised steel part is worn off. The thicker case of a carburised part                                        burised parts is higher than the steady state wear rate, probably due
   will result in longer resistance to wear.                                                                       to porosity. In certain situations this can be an advantage as running-
                                                                                                                   in wear will smoothen the contact surfaces, which leads to a low
   Figure 20 shows that both the dry and lubricated adhesive wear                                                  steady state wear rate.
   resistance of nitrocarburised steels is superior to that of carburised
   steels, and is higher than could be expected from hardness alone.                                               Nitriding or nitrocarburising improves the service life of hot and cold
   The compound layer, which is a ceramic, gives low friction and a                                                work tools. The adhesive wear resistance of the compound layer
   low tendency to “weld” opposing steel surfaces. The oil retention                                               contributes to this effect, as does the fact that the hardness of the
   properties of the porous outer zone of the compound layer offer                                                 diffusion zone of hot work tool steels is maintained at temperature
   self lubrication properties similar to those of sintered non-ferrous                                            levels of the order of 500 °C (932 °F). Aluminium extrusion dies is an
   bearing materials. These properties act together to give excellent                                              application in which nitriding and nitrocarburising have proven to
   adhesive wear resistance. Adhesive wear resistance improves with                                                yield improved wear resistance and endurance [21].

                   0.009                                                                                                                  0.090
                   0.008                                                                                                                  0.080
                   0.007                                                                                                                  0.070
                                                                                                                       Wear volume, mm3
Wear volume, mm3

                   0.006                                                                                                                  0.060
                   0.005                                                                          Carburised                              0.050
                   0.004                                                                                                                  0.040
                   0,003                                                                                                                  0.030
                   0.002                                                                  Nitrocarburised                                 0.020                                     Nitrocarburised
                   0.001                                                                                                                  0.010
                   0.000                                                                                                                  0.000
                           0                   426        1096         3059      5044      7096     9118 10110                                    0          200       400       500        600       700       1200   1400
                                                                       Sliding distance, m                                                                                     Sliding distance, m

    Figure 20. Adhesive wear resistance measured by a pin/disc test: a. With lubrication. b) Dry (without lubrication). [20]
 18                                 Properties of Nitrided and Nitrocarburised Steels

 B. Static and Fatigue Strength                                                                            (often called white layer after nitriding). This is possible by proper
 Static strength increases with increased surface layer hardness and                                       adjustment of the nitriding potential of the nitriding atmosphere (see
 relative case depth (= case depth divided by part thickness). Related                                     section VI, page 30). In rare cases the white layer is removed after
 to nitriding process parameters this means that static strength in-                                       nitriding to obtain the best fatigue properties.
 creases with increased nitriding/nitrocarburising time and also with
 increased cooling rate after nitrocarburising. In order to achieve the                                    C. Selection of Nitriding/Nitrocarburising Case Depth
 best effect from solution and precipitation hardening, it is essential                                    The case depth is determined by process time and temperature and
 that cooling starts from the nitrocarburising temperature before the                                      by the type of steel nitrided. After nitriding/nitrocarburising there
 temperature drops. A drop in temperature from 550 (1022) to 450°C                                         are two depths that are of interest: 1) the compound zone thickness
 (842°F) can reduce static strength by more than 20 % [1].                                                 and 2) the diffusion zone depth. Which depth is important depends
                                                                                                           on the application involved, as outlined in Table 9.
 The improvement in fatigue strength is greatest for specimens with
 notches that act as stress raisers, see Table 8. This is because the                                      Table 9.
 hard case will withstand high stress much better than the core mate-                                      Selection of surface properties after nitriding/nitrocarburising.
 Table 8.                                                                                                  Type of application       Primary objective           Depth
 Bending fatigue strength improvement after nitriding [22].                                                Abrasive wear             Compound zone hardness      Enough for wear life
                                                                                                           Adhesive wear             Compound zone hardness,     Enough for wear life
 Type of steel                                            Bending fatigue strength increase,                                         porosity and phase
                                                              Nitrided/Not nitrided, %                                               constituency
                                                          Smooth test bars         Notched test bars       Corrosive protection      Compound layer porosity     Enough to ascertain a
 Nitriding                               Cr-Mo-Al                                                                                    and phase constituency      dense layer
 steels                                  Cr-V                      28-41                      64-130       Contact & bending         Diffusion zone hardness     Enough to incorporate
                                         Cr-Mo-V                                                           fatigue                   and residual stress state   maximum point of stress

 Oil hardening                           Cr-Mo-Mn
 steels                                  C-steel                   43-55                      7-96
                                                                                                           D. Corrosion Resistance and Surface Appearance
                                                                                                           Compared to other thermal or thermochemical surface hardening
 The fatigue strength is improved by a state of compressive residual                                       methods on steels, nitriding and nitrocarburising are unique in that
 stresses in the surface layer after nitriding/nitrocarburising. This is                                   corrosion resistance is improved because of the superior electro-
 also the case for the diffusion zone as illustrated in Figure 21. There                                   chemical properties of the “ceramic surface layer” consisting of e
 is also a state of compressive stress in the compound zone if it con-                                     or e/g’ carbonitride. Nitrocarburised parts have excellent humidity
 sists of a g’-layer, whereas tensile surface stress has been predicted                                    corrosion resistance that is better than that of mild steel. Although
 but not clearly confirmed for e-layers [23]. The highest compressive                                      better than that of steels, salt water corrosion resistance is modest.
 stresses are obtained when nitrocarburising is completed with a fast                                      Corrosion resistance is greatly improved by a post oxidation step that
 quench in water or oil. As distortion will increase with increased                                        creates a thin oxide layer on top of the compound layer. As shown
 quench severity, it is necessary to find the quench rate that gives the                                   in Table 10, nitrocarburising can replace chromium plating or other
 best combination of strength and minimised distortion.                                                    surface treatments for salt spray corrosion resistance. By additional

 Fatigue crack initiation in the compound zone has been observed,                                          Table 10. Corrosion data [1, 2, 25].
 especially after gas nitriding. Therefore there is sometimes a need
 to perform gas nitriding without the formation of the compound layer                                      Type of finish                              Salt spray corrosion resistance
                                                                                                                                                       (ASTM B117), hours to failure
                                                                                                           Phosphate/Oiled                                           64
                      –100                                                                                 Passivated zinc plating                                   64
                                                                                                           Electroless nickel plating                                64
Residul stress, MPa

                      –200                                 Nitrided
                                                                                                           Passivated cadmium plating
                                                                                                           (10/12 µm)                                               348
                                                                                                           Passivated cadmium plating
                      –400                                                                                 (25/35 µm)                                              3800
                                                                                                           Nitrocarburised                                           17
                                                                                                           Nitrocarburising + post oxidation
                      –600                                                                                 (10/20 µm)                                               645
                             0             0.2           0.4           0.6              0.8            1
                                                                                                           Nitrocarburising + post oxidation
                                                     Depth below surface, mm
                                                                                                           (25/35 µm)                                              3800

 Figure 21. Residual stress profiles after nitriding and nitrocarburising [24].                            Hard chrome plating                                      120
                                                                                                                                          Properties of Nitrided and Nitrocarburised Steels                          19

oiling, waxing or painting it is possible to increase the corrosion re-                nitriding/nitrocarburising. The inner diameter of cylinders will shrink
sistance of nitrocarburised parts even more [1, 2, and 25].                            for large wall thicknesses but will grow for thin thicknesses as shown
                                                                                       in Figures 24-25. For a real part with a complex shape the dimensional
Nitriding and nitrocarburising gives the surface a light grey surface                  change is more difficult to predict, especially for a non-uniform part.
appearance. By post-oxidizing, possibly combined with oiling or wax-
ing, a dark black, shiny surface appearance can be obtained, see                                                  300.0

Figure 22.                                                                                                        250.0

                                                                                         Diameter change, mm

                                                                                                                  150.0                              Outside diameter


                                                                                                                                      Inside diameter

                                                                                                                            1         2          3        4       5         6       7         8        9        10
                                                                                                                                                              Wall thickness, mm

                                                                                       Figure 24. Diameter change as a function of the wall thickness of hollow cylin-
                                                                                       ders (70 mm diameter) nitrided for 72 hours [27].

Figure 22. Example of surface appearances after nitrocarburising.                                                  35
                                                                                            Diameter change, mm

Nitriding or nitrocarburising of stainless steels will deteriorate corro-                                                                                                        Outside diameter
sion resistance because the passive chromium oxide layer that pro-                                                 15
vides the corrosion protection of stainless steels will be destroyed.                                              10
(Certain plasma nitriding techniques explained in IV.F are exceptions                                               5
                                                                                                                                Inside diameter
to this).                                                                                                           0
E. Dimensional Changes                                                                                                  0         2          4        6           8        10      12         14      16        18
Compound layer growth results in dimensional growth. A solid bar
                                                                                                                                                              Wall thickness, mm
will therefore grow in diameter during nitriding/nitrocarburising. Fig-
ure 23 shows that dimensional growth is proportional to compound                       Figure 25. Diameter change after nitrocarburising (2h) followed by oil quench-
layer thickness. A guideline is that diameter growth is 30-50 % of                     ing for cylinders with varying wall thickness [15].

the compound layer thickness. Thick walled (wall thickness > 25mm)
hollow cylindrical parts will decrease on the inner and increase on
the outer diameter by about the same factor as the growth of a solid                   Figure 26 shows that both the inner and outer diameter changes
bar.                                                                                   are positive in the range from 30 to 100 µm for thin-walled long
                                                                                       cylinders. The figure also shows that the least diameter change is
                                                                                       obtained after salt bath treatment (Tenifer) with water quench but
                                                                          Compound     that the scatter of measured diameter changes is highest for that
                                                       C45                zone         treatment.
Diameter growth mm

                     14                                                   thickness
                     10                                                                                                                                               Water (Tenifer)   Oil        Atmosphere
                      8                                   C45                                                                     Outer diameter
                      6                                                                                           100                                                           Inner diameter
                                                                                         Diameter change, mm

                      4                                      16MnCr5      growth
                      0                                                                                            60
                          0   30   60          90        120       150   180     210
                                        Nitrocarburising time, min                                                 40

Figure 23. Growth of the compound layer and of the diameter of cylindrical
parts made from l6MnCr5 and C45 steel, depending on the nitrocarburising time                                       0
[26].                                                                                                                                                         Cooling method

                                                                                       Figure 26. Measured diameter changes after nitrocarburising of 300 mm long
                                                                                       cylinders with outer diameter 108 mm and wall thickness 4 mm. Nitrocarburis-
The dimensional change is determined not only by compound layer                        ing was conducted by Tenifer with water quench and Nitemper with both oil and
growth but also and primarily by the thermal stress history during                     atmosphere cooling [15].
20                                     Properties of Nitrided and Nitrocarburised Steels

F. Properties of Nitrided/Nitrocarburised Stainless Steel                                                  Somers [28] has developed a method to perform gaseous nitriding
It is not possible to carry out standard gas nitriding or nitrocarburis-                                   of stainless steel without plasma. The problem of nitriding inhibition
ing of stainless steels in a reproducible way without deteriorating                                        due to the chromium oxide surface is overcome by using a special
corrosion resistance. The first reason is that the thin chromium oxide                                     coating applied to the steel surface before nitriding. Examples of
covering the surface of stainless steels that gives them such good                                         results achieved with this process are given in Figure 29.
corrosion resistance acts as a barrier to nitriding. A second reason is
that the nitrogen that actually enters into the steel will form chro-
mium nitrides and thereby deplete the chromium concentration in the
matrix. Although high hardness can be achieved, the nitriding will
result in reduced corrosion resistance.

It is possible to partly eliminate these drawbacks with plasma nitrid-
ing. The plasma activates the surface, enabling nitrogen to be trans-
ferred and diffused into the steel. It has additionally been shown
to be possible to avoid chromium nitride formation by performing                                                                           445°C 22H                            445°C 22H
nitriding at a low temperature. The nitriding will then result in a
surface layer consisting of austenite with a highly supersaturated
concentration of nitrogen, called the S-phase. The hardness of the
supersaturated S-phase is very high, as shown in Figure 27. Corrosion                                                                                          AISI 316
resistance is maintained, as shown in Figure 28. The highly super-                                                                                             T=718K (445°C, 851°F); 22h
                                                                                                                            1200                                 KN = 0.293
saturated S-phase is not thermochemically stable but will decompose                                                                                              KN = 2.49
at temperatures above approximately 700°C (1292°F).                                                                                                            T=780K (507°C, 944°F); 6h
                                                                                                                            1000                                 KC = inf
                                                                                                            Hardness , HV

                                                                                400°C (752°F)/15h
                                                                                450°C (842°F)/15h
 Hardness, HK

                         1200                                                                                               400

                          800                                                                                               200
                                                                                                                                   0   5       10         15        20         25           30
                                                                                                                                                       Depth, µm

                                                                                                           Figure 29. Surface microstructure and hardness gradients obtained after gas
                           0                                                                               nitriding at two nitriding potentials of stainless steel, type AISI 316 [28].
                                   0             10          20           30               40        50
                                                        Distance from surface, µm
                                                                                                           Examples of S-phase nitriding and carburising processes that have
Figure 27. Hardness profiles of low temperature plasma nitrided 18/8 type
stainless steel [10].                                                                                      been developed are given in Table 11.

                                                                  550°C (1022°F)

                                                                                     500°C (932°F)
   Corrosion depth, µm



                                                                                     450°C (842°F)
                               0            20         40           60          80          100      120
                                                              Test time (min)

Figure 28. Corrosion resistance of stainless steel AISI 316 plasma nitrided at 450
(842), 500 (932) and 550°C (1022°F) compared to untreated steel. Dipping test
in 50%HCL+25%HNO3+25%H2O [10].
                                                                                       Properties of Nitrided and Nitrocarburised Steels   21

Table 11. Different processes for S-phase nitriding and carburising of stainless steel [10].

Company                        Process                       Element          Temperature, °C (°F)             Medium
Birmingham                     LTPN                          N                <450 (842)                       Plasma
University, UK                 LTPC                          C                <550 (1022)                      Plasma
Nitruvid, France               Nivox2                        N                <400 (752)                       Plasma
                               Nivox4 & NivoxLH              C                <460 (860)                       Plasma
Nihon Parkerizing, Japan       Palsonite                     N+C              450-490 (842-914)                Cyanide salt bath
Airwater Ltd, Japan            NV Super Nitriding            N                300-400 (572-752)                Gas + Fluorine
                               NV Pionite Process            C                <500 (932)                       Gas + Fluorine
Bodycote, UK                   Kolsterising                  C                -                                -
DTU, Denmark                   Gas nitriding                 N                <450 (842)                       Gas
22             Atmosphere – Surface Interaction

V. Atmosphere – Surface Interaction

During nitriding, ammonia (NH3) in the furnace atmosphere decom-
poses into hydrogen and nitrogen at the surface, enabling nitrogen               CN (gas)
atoms to be adsorbed at the steel surface and to diffuse further into
the steel as illustrated in Figure 30a. In nitrocarburising it is addition-
                                                                                              CN (surface)
ally necessary to have a carbonaceous gas transferring carbon to the
steel surface. This is illustrated in Figure 30b by the carbon depositing         CC (gas)
surface reaction with carbon monoxide, CO. A compound layer con-                                       dcN /dx
sisting of e and g’ nitride is formed when a sufficiently high surface                        CC (surface)
nitrogen (and carbon) concentration forms on the steel surface.
                                                                                                                         dcN /dx

There are three major stages of nitrogen/carbon transfer:                                              dcC /dx

     1.      From the gas to the steel surface

     2.      Diffusion through the compound layer                                                                       dcC /dx

     3.      Diffusion into the diffusion zone

(Additional stages not considered here are transport within the gas                Gas               Compound layer             Diffusion zone
boundary layer and carbon diffusion from the steel matrix into the
compound zone.) The concentration gradients driving the transport
of nitrogen and carbon are shown in Figure 31.                                  Figure 31. Concentrations and concentration gradients of nitrogen and carbon.

                         2NH3                                2N + 3H2                                   CO + H2                              C + H2O

                                                                   ¿                                                                             ¿
                                          a                                                                               b

Figure 30. Schematic illustration of the nitriding and carburising processes.
                                                                                                                         Atmosphere – Surface Interaction         23

The flux of nitrogen and carbon from the gas to the steel surface is         and hydrogen gas when the ammonia hits the hot furnace interior.
proportional to the concentration differences between the gas and            The part of the ammonia which does not dissociate is called residual
the surface:                                                                 ammonia, and is what causes nitriding according to reaction 4.1. In
                                                                             the expression for the nitrogen potential,
          dmN /dt(surface) = k1 [ cN (gas) – cN (surf) ]
                                                                                  rN= PNH /PH 3/2
                                                                                                 3    2
          dmC /dt(surface) = k2 [ cC (gas) – cC (surf) ]
                                                                             is PNH the residual ammonia partial pressure and PH is the partial
                                                                                   3                                            2
Here m denotes mass, t time, c concentration per volume unit and k1          pressure of hydrogen, formed by the ammonia dissociation (in addi-
and k2 are reaction rate coefficients.                                       tion possibly with separately added hydrogen).

The transfer of nitrogen and carbon from the surface further into the        The ammonia dissociation may be described by the reaction,
steel is controlled by diffusion. Diffusion rates follow Fick’s first law,
which for the compound layer and diffusion zone are respectively:                 1NH3 → (1 − a)NH3 (residual) +3/2aH2 +1/2aN2

          dm/dt(comp layer) = – DComp dc/dx                                  where a is the degree of ammonia dissociated. The total number of
                                                                             moles after dissociation is
          dm/dt(diff zone) = – DDiff dc/dx
                                                                                  (1 − a) + 3/2a + 1/2a = 1+ a
Balance of mass requires that all three mass transfer rates are equal:
                                                                             For a nitriding atmosphere produced from 100% ammonia, the partial
     dm/dt(surface) = dm/dt(comp layer) = dm/dt(diff zone)                   pressures can therefore be expressed as

The slowest of the three stages controls the nitrogen and carbon                  PNH = (1 − a)/(1 + a)
transfer rates. For a compound layer consisting of alternating e-g’-e
layers, the rate will be determined by the phase with the slowest                 PH           = 3/2a/(1 + a)
diffusion properties.
                                                                                  PN           = 1/2a/(1 + a)                                               4.3

                                                                             and the nitriding potential as
A. The Atmosphere Nitriding Potential
                                                                                  rN = (1 – a)(1 + a)1/2/(3/2a)3/2                                          4.4
The nitrogen activity in the gas is determined by the equilibrium,

     NH3 = N + 3/2H2                                           4.1           (This method of calculating the nitriding potential is only fully
                                                                             accurate if the amount of ammonia consumed in the nitriding reac-
with the chemical equilibrium constant                                       tion 2NH3 → 2N+3H2 can be neglected in comparison to the available
                                                                             amount of ammonia.)
     K = aN · PH 3/2 / PNH
                  2           3
                                                                             The nitriding potential can alternatively be expressed as a function of
where PH and PNH are the partial pressures of hydrogen and am-               the residual ammonia or the hydrogen partial pressure according to
         2        3
monia respectively. Accordingly the nitrogen activity, aN, can be            the equations
expressed as
                                                                                  rN = 8/√3 × PNH /(1– PNH)3/2
     aN = K PNH / PH 3/2
                  3       2
                                                                                  rN = (1 − 4/3PH )/PH
                                                                                                          2       2
where aN is proportional to the concentration, cN. The ratio
                                                                             As each dissociated ammonia molecule produces two gaseous mol-
     rN = PNH / PH 3/2                                         4.2           ecules (1/2N2 + 3/2H2), the furnace pressure will increase as a result
              3       2
                                                                             of the volume expansion. A measure of the furnace pressure is there-
is commonly called the nitriding potential.                                  fore an indication of the degree of ammonia dissociation and accord-
                                                                             ingly also of the nitriding potential [29, 30].
If 100% NH3 (ammonia) is added to the furnace, which is fairly com-
monplace in gas nitriding, some ammonia dissociates into nitrogen
24                                      Atmosphere – Surface Interaction

B. The Atmosphere Carbon Potential                                                                      Table 12. Calculated quasi equilibrium compositions of various
Similar to nitriding, the carburising reaction                                                          nitrocarburising atmospheres at 580°C (1076°F).

                                 CO+H2 → C + H2O                                                        Added gas           Quasi equilibrium composition (vol-%)                          Activities1)
                                                                                                        mixture               N2       H2      CO     H2O       CO2      NH2         aN          aC       aO
has an equilibrium constant expressed by
                                                                                                        50% Endogas 24.1 25.9 9.0                     0.76      0.68 38.4          1620        22.4 0.072
                                                                                                        50% NH3
                                 K = aC × PH     / PCO · PH
                                            2O              2                                           40% Endogas
                                                                                                        10% Air     29.2 24.3 6.7                     2.46      1.77 35.4          1650        4.81       0.25
where P stands for the partial pressure, and the carbon activity, aC , is
                                                                                                        50% NH3
proportional to the carbon concentration,
                                                                                                        5% CO2
                                                                                                        60% N2              58.8 15.5 2.9             2.98      1.45 18.4          1680        1.09       0.48
                                 aC = const.× cC
                                                                                                        35% NH3
                                                                                                        5% CO2
The atmosphere carbon potential can alternatively be expressed by
                                                                                                        5% CO
the ratios
                                                                                                        55% N2              54.4 15.9 6.4             2.22      2.31 18.6          1640        3.32       0.34
                                                                                                        35% NH3
                                 rC1 = PCO × PH /PH
                                                 2    2O                                                5% CO2
                                                                                                        20% H2
                                 rC2 = PCO2/PCO
                                                 2                                                      45% N2              45.4 26.6 3.4             3.50      1.16 19.9            806       1.88       0.33

Endogas or exogas which both contain carbon monoxide have been                                          1)
                                                                                                          The reference state for the activity values are nitrogen gas at 1 atm, graphite and wustite (FeO), for
and are used as CO sources. Pure CO is an alternative carbon source                                     nitrogen, carbon and oxygen respectively.

but the high cost is a barrier to its use. The alternative developed
with NITROFLEX® technology is to use carbon dioxide, CO2 , which by
reacting with the hydrogen inside the furnace produces CO according
to the reaction,                                                                                        C. Atmosphere Analysis
                                                                                                        The nitriding potential is determined by gas analysis. In nitrocarbu-
                                 H2 + CO2 = CO + H2O                                                    rising the gas analysis aims to determine both the nitriding and the
                                                                                                        carbon potentials. The analysis of nitrocarburising atmospheres is
The hydrogen needed for this reaction originates from dissociated                                       difficult due to the complex gas composition (NH3 , H2 , CO , CO2 and
ammonia or from a separate addition of hydrogen. Figure 32 shows                                        H2O) with fairly high water vapour content, which may result in the
that CO forms according to this reaction. Furnace walls, retort and                                     precipitation of ammonium carbonate, which clogs the analyser and
load catalyse the reaction, which means that the reaction rate                                          analysing sample gas pipes. The use of a heated sample gas system
shown will be different for different furnaces.                                                         is one way to inhibit condensation.

                             6                                                                          1. Ammonia and Hydrogen Analysis
                                                                                                        The simplest method of determining the nitriding potential is ammo-
Carbon monoxide content, %

                                                                                                        nia analysis with the ammonia burette technique shown in Figure 33.
                                                                                                        The analysing principle is based upon that ammonia gas dissolves in
                                                                                                        water. The valves at the top and bottom of the graduated container
                             3                                                                          in the figure are first placed in purging position. Closing the upper

                             2                                                Furnace, gas flow rate
                                                                                 IPSEN RTQ – 1,4 m3/h
                             1                                                   IPSEN RTQ – 1,2 m3/h
                                                                                 UTAB OMG, 4 m3/h
                                  0        2.5          5         7.5        10        12.5        15

                                                       Carbon dioxide addition, %

Figure 32.
Relation between the added amount of CO2 and produced amount of CO.

The atmosphere carbon activities of atmospheres based on blends
between ammonia/endogas and ammonia/exogas are excessively
high and practice has shown that soot deposits may be a problem
in furnaces running with these atmospheres for a long time. An at-
mosphere based on a blend between ammonia/CO2 has much lower
carbon activity, see Table 12.                                                                                                   Figure 33. Ammonia analyser
                                                                                                                                        Atmosphere – Surface Interaction             25

valve leads to a gas sample being locked into the container. Dur-                            the resistor and thus a resistance change, which can be measured as
ing analysis the bottom valve is opened to allow water from the left                         a signal. Figure 35 shows the principle of analysis.
hand container into the measuring container. The un-dissociated am-
monia will thereby dissolve in the water, causing the water to rise in                       Hydrogen can also be analyzed with a sensor with a similar appear-
the graduated container. The more un-dissociated ammonia there is,                           ance to an oxygen probe as shown in Figure 36. It uses a measuring
the higher the water level will rise.                                                        tube material with the ability to sensor the difference in the hydro-
                                                                                             gen partial pressure between a reference gas inside and the actual
On-line analysis can be performed with infrared analysis of ammonia                          furnace atmosphere outside the tube. The analysis signal is a direct
or hydrogen analysis with a thermal conductivity analyser or a com-                          value of the hydrogen concentration in the furnace atmosphere.
bination of both. The principle of IR analysis is based on the ability of
multi-atomic gas molecules to absorb an IR wavelength that is spe-                                                       Furnace Wall

cific to each gas molecule. The principle is illustrated in Figure 34.                                                                                   Head
A beam of infrared light is split into two separate beams, one of                                                        Protection Tube

which passes through a cell containing the sample gas, the other
through a reference cell, which in the figure is filled with nitrogen.
A rapidly alternating splitter (on the left of the figure) separates the                                 Quarz Tube              Tube                              Pressure sensor
radiation reaching the detector on the right of the figure. A measure
of the amount of ammonia is obtained by comparing the detector
                                                                                                  Furnace atmosphere
signals for the two beams.
                                                                                                                                           Measurment Tube

Light source Shutter    Chopperwheel      Sample cell          Detector
                                                                                                                                                       pa (H2) = pi (H2)
                                                                                               pa (H2)      pi (H2)               H2
                            Sample gas outlet        Sample gas inlet                                                                                 Pressure Balance

                                                                                 NH3                                                       a.

                                                 Reference cell           Calibration cell

                                                                                             Figure 36. a) Principle and b) appearance of the hydrogen sensor [31] [Courtesy
Figure 34. Principle of an infrared NH3 analyser [Courtesy of ABB AB].                       of Ipsen International GmbH]

Hydrogen gas, being a one-atom gas, cannot be analyzed by IR anal-                           In cases where nitriding is performed with a variable unknown am-
ysis. However, due to the very high thermal conductivity of hydrogen                         monia/nitrogen mixture, two hydrogen analysers can be used to
gas, analysis based on thermal conductivity can be used. A classic                           accurately establish the nitriding potential. The analysers are posi-
thermal conductivity detector design utilizes a Wheatstone bridge in                         tioned so that one analyses hydrogen in the furnace atmosphere,
which a resistor is in contact with the gas to be analyzed. Changes                          PH , and the second the hydrogen concentration after the complete
in the hydrogen concentration will result in a temperature change in                         dissociation of ammonia, P’H . From these two measurements the
                                                                                             nitriding potential is expressed by

  Bridge                                        Temperature                                              rN = (P’H – PH )/[(3/2 – P’H ) PH ]
                       Heater                                                                                        2        2              2    2
                                                                                             An alternative atmosphere analysis is hydrogen analysis combined
                                                                                             with oxygen probe readings.

                                                                                             2. Oxygen Probe Analysis
                                                                                             Oxygen probe analysis can be used to gain a measure of the nitriding
                                                                                             potential even though oxygen is not active in the nitriding process.
                                                                                             The atmosphere oxygen partial pressure is proportional to the water/
                                                                                             hydrogen ratio and thus the oxygen probe signal A can be expressed
                                                                                             by the following equation

                                                                                                         A= PH O /PH
                                                                                                                 2        2

                                                     Reference gas      Sample gas
                                                                                             The principle of the oxygen probe method is based on the fact that
                                                                                             the inlet gas composition is known, as is its water content, which
Figure 35. Principle of a thermal conductivity H2 analyser [Courtesy of ABB AB].             means that PH O is also known. The hydrogen concentration, PH ,
                                                                                                             2                                                                 2
26           Atmosphere – Surface Interaction

can be expressed as a function of the degree of ammonia dissocia-        absorption at one specific IR wavelength as in conventional IR spec-
tion (see equation 4.3) and the nitriding potential from equation 4.4.   troscopy. The principle setup of such a system is shown in Figure 38
A is accordingly an indirect measure of the nitriding potential.         and is connected to a furnace as in Figure 39, which also includes a
                                                                         hydrogen analyser.
With two separate oxygen probes the nitriding potential can be de-
termined without any knowledge of the ingoing gas composition.                                        Computer
The first probe measures the oxygen potential A in the actual furnace                                  Monitor
atmosphere, whereas the second measures the oxygen potential
after passing the atmosphere through an ammonia cracker, which                       Plotter
completely dissociates all ammonia (a = 1). The second oxygen                                          station
probe measures the signal:

                         Diss       Diss                                 Figure 38. Basic components of a computerized FTIR spectrometer system.
          B = PH                / PH
                  2O               2

where the superscript “Diss” refers to the atmosphere composition
after the ammonia cracker. If the nitriding atmosphere consists only
of ammonia, it can be shown that the ratio between the two oxygen
probe signals is a measure of the degree of ammonia dissociation
inside the furnace according to the equation [32]

          D = A /B = 1/a

One development of the oxygen probe to determine the nitriding                                                                              sample
potential is the type of sensor shown in Figure 37. The outer elec-                                                                         line

trode is in direct contact with the furnace atmosphere. The furnace                                                                         Exhaust
atmosphere is also led into the inner side of the ceramic tube but
passes through a catalyser that promotes complete dissociation of
                                                                                                    Data     FTIR and hydrogen Control
all residual ammonia before it comes into contact with the inner
                                                                                                  handling        analyzers     unit
electrode. The resulting voltage DU is a measure of the difference
in oxygen potentials between the actual furnace atmosphere and           Figure 39. Analyser set up including FTIR and hydrogen analysis.
the atmosphere of completely dissociated ammonia. This voltage is
a direct indication of the degree of ammonia dissociation, a, in the
furnace atmosphere expressed by [32, 33].                                This method also suffers from the clogging and corrosion problems
                                                                         caused by condensation, but for short time analysis it is a method of
     DU = 0.0992 T log a                                                 obtaining a complete gas analysis “footprint” not only from NH3 and
                                                                         H2 but also from CO, CO2, H2O, N2O etc., as indicated in Figure 40. It is
            Gas                                                          used for calibrating individual furnaces.
          entrance          Electrolyte                 Catalyst

                   Inner electrode
                                                     Protection tube
               Outer electrode

Figure 37. Oxygen probe as a nitrogen sensor [34].

An oxygen probe signal will also be a measure of the atmosphere
carbon activity, aC, according to the relations
                                                                         Figure 40. Example of FTIR analysis result.

aC = K1 × PCO /(PO )1/2 = K2 × (PCO )2/PCO = K3 × PCO × PH /PH
                     2                     2            2     2O

where K1, K2, K3 are constants.
                                                                         D. Layer Growth Determination
                                                                         In addition to analysing the atmosphere, it is possible to register the
3. FTIR Gas Analysis of Nitrocarburising Atmospheres                     compound layer thickness and its growth with a sensor that utilises
FTIR (Fourier Transform Infrared) analysis gives as a result a wave-     electromagnetic principles. It is inserted into the furnace as shown in
length-dependent interference pattern called an interferogram that       Figure 41.
makes it possible to quantitatively determine the concentrations of
all multi-atomic gas species in a gas sample, in contrast to detecting
                                                                                                                                                                Atmosphere – Surface Interaction   27

                                                  Frequency                                            the ammonia flow rate. A control system incorporating a hydrogen
                                                  generator                               Measuring    sensor and using the combination of ammonia and hydrogen is
                                                                                          4–20 mA      shown in Figure 42. Hydrogen could alternatively be supplied from
                                                                                                       cracked ammonia.
  KiNit-Sensor                Coil                                                Slide
                                                                                                       It is also possible to dilute ammonia with nitrogen, thereby affecting
                                                                                                       the nitriding potential. The controllability range, however, is then
                                                                                                       limited, but from the viewpoints of safety and economy it may be
      Furnace atmosphere
                                                                             Transport rod

                                               Isolation           Furnace wall

Figure 41. Schematic principle of the installation of an electromagnetic sensor
for registration of compound layer growth. [35].

                                                                                                                                                                        H2                  NH3
E. Guidelines for Regulating the Atmosphere
1. Nitriding                                                                                                                Furnace
Controlled nitriding is realized by a control system that determines
the actual nitriding potential and adjusts the atmosphere composition
                                                                                                                            HydroNit                                                Controller
to the chosen set point value. This can be achieved either by manual                                                        Sensor
analysis and flow adjustments or by automatic closed loop control.

                                                                                                       Figure 42. Nitriding potential control (Courtesy Ipsen International GmbH).
At a high flow rate most ammonia supplied for nitriding to a furnace
remains un-dissociated, but at low flow rates it dissociates more
easily into nitrogen and hydrogen as the residence time in the fur-
nace is sufficiently long. One way to perform controlled nitriding is                                  2. Nitrocarburising
therefore to start the process with a high ammonia flow rate (= high                                   The atmosphere for nitrocarburising consists of 20-50 % ammonia,
residual ammonia = high nitriding potential) in order to build up the                                  2-20 % carbon dioxide and the balance nitrogen, the specific com-
compound layer as fast as possible. Later on the ammonia flow rate                                     position depending on which furnace equipment is used and which
is decreased to typically give a residual ammonia concentration of                                     properties are desired. Experiments have shown that an addition of
about 60 vol-percent [8].                                                                              about 5 vol-percent CO2 is often sufficient (see Figure 32).

An alternative method of controlling the nitrogen activity is by hydro-                                Figure 43 gives examples of analysis results for residual NH3 in a
gen addition. In such cases the atmosphere nitrogen activity can be                                    nitrocarburising process where 50% NH3 + 50 % endogas were intro-
varied over a much wider range than is possible by simply changing                                     duced into a sealed quench ceramic lined furnace. The figure shows a

                              5                                                                                         5

                              4                                                                                         4

                              3                                                                                         3
            Process time, h

                                                                                                      Process time, h

                              2                                                                                         2

                                           Loading into
                              1            furnace                                                                      1                   Loading into
                                      Loading into                                                                                          furnace
                                      prechamber                                                                                 Loading into
                              0                                                                                         0
                                  0              15           30            45                 60                            0             15              30           45            60
                                             Residual ammonia concentration, vol%                                                      Residual ammonia concentration, vol%

                                                              a                                                                                            b

   Figure 43. Examples of residual ammonia concentration variations during                             The “b” cycle was the same as the “a” cycle with the exception that the
   nitrocarburising cycles at 570°C (1058°F) in a sealed quench furnace.                               furnace was preconditioned with an active atmosphere in the “b” cycle.
28                Atmosphere – Surface Interaction

large variation in residual ammonia content both during one process                Figures 44-45 show how adjusting the atmosphere concentrations
cycle and between two different cycles. The same gas flow ratios and               of CO2 and NH3 can change the atmosphere carbon and nitrogen ac-
mixing ratios were used. Obtained depths on the compound layer and                 tivities. This possibility is of great value when optimising compound
diffusion zone correlated positively with residual ammonia analysis                layer properties for different steels.
                                                                                   With the NITROFLEX® system it is possible to use a “boost” technique
The shortest possible cycle for a certain compound layer thickness is              with high nitrogen activity in the first stage of the process and a low-
achieved if the atmosphere composition is changed during the cycle,                er activity in a second stage. There is an upper limit for the nitrogen
with high nitrogen activity and a carbon activity promoting e-nuclea-              activity corresponding to excessive porosity formation. In this way
tion and growth in the first part of the process.                                  it is possible to control the degree of porosity and compound layer
For medium and high carbon steels, carbon is donated to the com-
pound layer by the steel. The atmosphere carbon activity is then                   Examples of the influence of atmosphere composition on compound
less important, which explains why carbon-free atmospheres work                    layer microstructure are shown in Figure 46. For high nitrogen activi-
in such cases even though such atmospheres are “pure nitriding”                    ties entailing a high ratio for PNH /PH 3/2, the compound layer is thick
                                                                                                                      3   2
and not “nitrocarburising” atmospheres. With low carbon steels, on                 and has extensive porosity, as illustrated in the photographs on the
the other hand, it is necessary that the atmosphere has a balanced                 left, whereas a lower nitrogen activity gives a dense layer, as illus-
carbon activity to ensure a good compound structure with mainly                    trated in the photographs on the right. By increasing the atmosphere
– e-phase.                                                                         carbon activity, the porosity decreases and the amount of e-phase in
                                                                                   the compound layer increases, see the lower right photograph.

 1.6                                                                      1600
 1.4                                                                      1500

 1.2                                                                      1400

 1.0                                                                      1300

 0.8                                                                      1200                       a                                      b

 0.6                                                                      1100

 0.4                                                                      1000

           5 10      20        30    40   50   60    70        80   90   % NH3

Figure 44. Theoretically calculated carbon and nitrogen activities at 580 °C
(1076 °F) for different ammonia additions to an atmosphere based on                                  c                                      d
N2+ 5%CO2.                                                                         Figure 46. Four examples of compound layer depth and morphology as the
                                                                                   result of different gas recipes. Treatment temperature and time were the same
                                                                                   in all four cases. Unalloyed 0.1% C steel. Gas atmospheres:
                                                                                   a) 35% NH 3, 5% CO2 , balance N2, aC = 0.93, aN = 2390,
     aC                                                                  aN
                                                                                   b) 33% NH3 , 5% CO2 , 5% CO, balance N2, aC = 2.95, aN = 2320,
                                                                                   c) 28% NH 3, 4% CO2, balance N2, aC = 1.54, aN = 1300,
     1.2                                                                           d) 60% NH 3 , balance endogas, aC = 23.5, aN = 1690 [16, 36].
     1.0                                                                  1700
                                                                                   In order to lower the nitrogen activity during the later part of the
                    aN                                                             treatment, the addition of carbon dioxide could be lowered or com-
                                                                                   pletely shut off. It is also possible to lower the ammonia addition in
                                                                                   order to lower the nitrogen activity; however, the carbon activity will
                                                                                   drastically decrease as well.

                                                                                   In order to attain high carbon concentrations, one alternative is to
           2.5 5          10         15   20              30             % CO2     add propane to the atmosphere. This has given positive results for
                                                                                   the obtained hardness and wear resistance of treated parts. This is
Figure 45. Theoretically calculated carbon and nitrogen activities at 580°C        because the carbon provided by the propane addition favours nu-
(1076°F) for different carbon dioxide additions to an atmosphere based on          cleation and growth of the e-phase. This is illustrated in Table 13,
N2+35%NH3.                                                                         which also shows wear results. Care must be taken not to create soot
                                                                                                                                                       Atmosphere – Surface Interaction            29

Table 13. Wear test results for crankshafts in steel C45 nitrocarburised at 570°C (1058°F) [18].

Process gas mixture,                     Compound layer,                        Nitriding depth,                     Relation                              Wear rate                Ductility
                %                                 µm                                      mm                                            g’:e
55N2/43NH3/3CO2                                   20                                      0.95                                          1:4                   High                    Worst
92NH3/5CO2/3C3H8                                  18                                      1.0                                           1:11                  Low                     Best

with the propane addition. Other hydrocarbons such as ethylene and                               If H2O is used as an oxidant, it is necessary to purge effectively with
propylene are also commonly used to provide carbon [37].                                         nitrogen to bring down the H2 concentration before H2O addition, in
                                                                                                 order to achieve sufficient oxidizing power (PH O /PH ). Alternatively
                                                                                                                                                  2     2
3. Post Oxidation Control                                                                        a larger amount of water may be added.
To obtain a result with an adhering iron oxide that provides corrosion
protection it is necessary that only the Fe3O4 oxide is formed. If Fe2O3                         Corrosion test results indicate that a thin oxide with a thickness of
forms, the result will be poor surface appearance with varying colour                            approx. 1 mm is the best for yielding good corrosion results. The
and flaking of the oxide.                                                                        oxidation time must therefore be short. In order to achieve sufficient
                                                                                                 oxidation to yield a black surface within this short time, there must
In an atmosphere with oxygen partial pressure higher than that                                   be enough oxidizing gas inside the furnace. Typically 10 vol% of
required for the formation of Fe2O3 , the equilibrium conditions are                             the input gas (the remainder being nitrogen) is used when N2O is
expected to allow all three iron oxides to form. However, FeO will                               the oxidant and the oxidation time is of the order of 10-15 minutes.
only form at temperatures above 570°C (1058°F). The post oxidation                               Results with respect to corrosion resistance are shown in Figure 48.
temperature should therefore be below that for formation of FeO,
although FeO formation is very slow up to 700°C (1292°F) [38].                                   Pitting corrosion resistance tests of test specimens post oxidised with
                                                                                                 water vapour have given almost double resistance for post oxidised
Figure 47 shows the equilibrium oxygen pressures over Fe2O3 and                                  variants compared to purely nitrocarburised specimens. The best
Fe3O4 plotted versus inversed temperature (1/K) as straight lines. It                            results were obtained after post oxidation at 450°C (842°F) [13].
can be seen that the equilibrium oxygen pressure stability range for
Fe3O4 is approximately 10-31 to 10-30 atm at 450°C (842°F). Table                                    With rust protecting liquid.
14 indicates that, in order to avoid Fe2O3 but to form Fe3O4 at 580°C                                Salt spray test, one week
(1076°F), the following condition must be fulfilled:                                                 Oxidation + Dinitrol 115
2.4 ×10–26 < PO < 3.7 × 10 –25 or 0.30 < PH O /PH <1.18 or 0.69 <
               2                           2     2
PCO /PCO < 2.71. It is possible to avoid Fe2O3 formation and thus only                               No
    2                                                                                                corrosion
to form Fe3O4 by adjusting the oxygen partial pressure in accordance
                                                                                                                 Corrosion Resistance

with the guidelines given in the table and figure.
                                              1/T, K –1
                       0.00135      0.00130            0.00125        0.00120          0.00115

          –26                                                                                        Severe
                                 Fe2O3                                                               corrosion                             1-3     4   5      6      7        8       9       10
Log PO2

                                         Fe 3O4                                                                                                                      Test Oxidising parameters
                                                   Fe                                                                                                                 1-3. 100% CO2 /450°
          –30                                                                                                                                                            4. 100% CO2 /350°
                                                                                                     Without rust protecting liquid.
                                                                                                     Salt spray test, one week                                           5.       5 % N2 O/350°
          –32                                                                                        Oxidation, no inhibitor                                             6.       5 % N2 O/450°
                450                490                    530             570                                                                                            7.   50% Air/350°
                                          Temperature, °C                                            No                                                                  8.   50% Air/450°
Figure 47. Iron oxide equilibrium as a function of temperature.                                                                                                          9.   40% H2 O/350°
                                                                                                                 Corrosion Resistance

                                                                                                                                                                       10.    40% H2 O/450°

Table 14. Equilibrium oxygen partial pressures over iron oxides
at 580ºC (1076ºF).

Oxide           Equilibrium partial pressure ,atm
                PO 2             PH2O/ PH2              PN 2O/ PN 2             PCO2 /PCO            corrosion                                 2   4   5      6      7        8       9       10
FeO             3.4 ·10–27       0.11                   7.1 ·10–23              0.26
                                                                                                 Figure 48. Corrosion after one week 5% NaCl test. Specimens were post oxi-
Fe3O4           2.4 ·10–26       0.30                   1.9 ·10–22              0.69
                                                                                                 dised at different temperatures and in different gas atmospheres as indicated by
Fe2O3           3.7 ·10–25       1.18                   7.4 ·10–22              2.71             the test 1-10 description.
30                      Compound Layer and Diffusion Zone Formation

VI. Compound Layer and Diffusion Zone Formation

A. Nitriding                                                                                                                                                                             Compound
                                                                                                                                                                 Test no: PNH3 /PH23/2
From the Fe-N phase diagram in Figure 49, it follows that as the                                                                                                                         layer mm
nitriding potential, PNH3 /PH23/2, is increased, the compound layer                                                                                                 1          15000        20
                                                                                                                                                                    2               7        6
is expected to start to form by g’-nucleation, illustrated at 510°C                                                  900
                                                                                                                                                                    3            1.01        4
by point A in the diagram. Later e is expected to form on top of g’                                                                                                              0.16        0
                                                                                                                     800                                            4
when the surface nitrogen concentration exceeds the solubility limit                                                                                                5            1.79        0
                                                                                             Hardness HV

in g’ (point B). If the nitrogen activity in the gas is kept below the                                               700                                            6             0.3        0
maximum nitrogen solubility limit in g’ (point B), then a monophase
g’- layer will be the end result. If the nitrogen activity is kept below
the maximum nitrogen solubility in a (point A), then no compound                                                     500
layer is formed but nitrogen uptake is fully contained in a. In order
for g’ to be formed at 500°C the nitriding potential, PNH3 /PH23/2, has                                                             6
                                                                                                                                                      5   4    3                    2         1
to be above approximately 0.25 and above approximately 3 for e to                                                                35 %              55 % 63 % 70 %                 80 %     100 %
be formed.
                                                                                                                           0                0.1           0.2               0.3            0.4
                      750                                                                                                                                  Depth mm

                                                                                           Figure 50. Changes in diffusion zone hardness gradient and compound layer
                                                                                           thickness after nitriding of an Al-alloyed steel at 510°C (950°F) over 24 h. The
                                                                                           NH3 content of the ingoing gas mixture as indicated by the number at each curve
                      650                                                                  was achieved by addition of hydrogen [40].
     Temperature °C

                      600                                                        ε         The diffusion zone thickness increases with increased nitrogen po-
                                                                                           tential, Figure 50, and with time in a parabolic manner, Figure 51. As
                      550          α                                                       seen in Figure 51, nitriding depth at a certain time and temperature is
                                                       A                     B             lower for high alloy steels. This can be understood from the fact that
                      500                                                                  alloying elements like chromium trap nitrogen when forming nitrides.
                      450                                                                                                      Gasnitrided 520 °C (968 °F)
                            10–2               10–1               100                102                                                32AlCrMo4
                                                PNH /PH 3/2,    atm   –1/2
                                                                                                                                        34 CrAl6
                                                   3   2                                                             0.6
                                                                                              Nitriding depth, mm

Figure 49. The Fe-N phase diagram plotted as a function of temperature                                                                  34 CrAl6 500°C (932°F)
and of the nitriding potential, PNH / PH 3/2 [39].
                                                  3    2

The g’ -phase is an almost stoichiometric compound, Fe4N, with
limited concentration variation. The growth rate of the g’-compound                                                  0.2
layer is therefore relatively low since the difference cN (surface) –
cN (nitride/a) is low (see Figure 31). On the other hand e-nitride has
a high solubility range for nitrogen, which leads to higher growth                                                     0
                                                                                                                           0      1 2      4 6 8 10       20        30    40      50 60 70 80 90100
rate. This is illustrated in Figure 50, which shows that the compound
                                                                                                                                                         Process time, h
layer thickness increases markedly when the nitriding potential is
very much higher than the lower solubility limit for e to form, ex-                        Figure 51. Relation between nitriding depth and treatment time for different
pressed by the ratio PNH /PH 3/2 (test no. 1 in the figure).                               steels. [41].
                                           3      2
                                                                                                                                 Compound Layer and Diffusion Zone Formation       31

More nitrogen atoms thus have to diffuse into the steel to reach a                     The nitrogen take-up and thus also the layer growth rate increases
certain depth compared to when no alloy elements trap nitrogen.                        with increased surface area (surface roughness) as shown in Figure
The nitriding rate increases if small concentrations of oxygen are
present in the nitriding atmosphere as shown in Table 15. The table
also shows that oxygen addition favours the formation of e-phase.                                                     12
                                                                                       Compound layer thickness, mm

Table 15. Influence of oxygen addition on compound layer
thickness and phase composition in gas nitriding at 30 min-                                                            8

utes at 550 °C (1022 °F). Steel DIN C10 [41].                                                                          6

Nitriding atmosphere                         Compound layer,      Phase composition,
                                                  mm                   vol%
                                                                    e        g’                                        2
No oxygen                                        1.5-2.0            34       47
No oxygen but pre-oxidised                       2.9-3.5            67       25                                        0
                                                                                                                           0.1        1.0                    10.0              100.0
Oxygen added during nitriding                    2.5-3.5            67       25
                                                                                                                                        Surface roughness Rz , mm

                                                                                       Figure 53. Dependence of the thickness of the compound layer on the mean
                                                                                       surface roughness Rz. 0.45%C carbon steel, nitriding temperature 570°C
                                                                                       (1058°F), nitriding time 3 hours [43]
The addition of CO2 and NO2 has also been found to increase the
nitriding rate. Small additions of water, however, seem to retard
nitriding, see Figure 52.

       1.4E–01             40/60 +
       1.3E–01             300 ppm O2                                                  B. Nitrocarburising
       1.2E–01       CrN+Fe2N1-z                                                       In nitrocarburising the compound layer starts to form by nucleation
       1.1E–01                                                                         of cementite even if the carbon activity of the gas is lower than that
       1.0E+01       CrN+Fe4N1-x
       9.0E–02                                    38/62                                of cementite [44, 45]. A possible explanation is that the gas/surface
       8.0E–02                                                                         reaction delivering carbon to the surface (the heterogeneous water

       7.0E–02                           38/62 +
                                         300 ppm H2O                                   gas reaction) is faster and kinetically favoured compared with the
       5.0E–02       CrN                                                               nitriding (ammonia decomposition) reaction during heating before
       4.0E–02                     38/62 +                                             reaching the nitrocarburising temperature. There is additionally
       3.0E–02                     100 ppm H2O                                         evidence that cementite formation at moderate atmosphere
       1.0E–02                                                                         carbon activity is favoured by the presence of ammonia in the
       0.0E+00                                                                         atmosphere. [46]. Within the order of minutes after reaching the
                 0           10         20        30         40       50      60
                                                                                       nitrocarburising temperature, e phase is nucleated on the primary
                                              Time (103 s)
                                                                                       formed cementite. e is favoured because its crystal structure is similar
Figure 52. Effect of oxygen and water vapour on weight gain during nitriding           to that of cementite. The e phase layer then grows at the expense
in a thermo-balance at 550°C (1022°F) of Fe20Cr powder. The ratio NH3/H2 is            of cementite, which is consumed by transformation to e phase,
indicated at each graph [42].                                                          leading to an almost homogeneous e phase layer. Later g’ forms at
32                           Compound Layer and Diffusion Zone Formation

the interface between the substrate and the e-layer. Redistribution                                                                30
of nitrogen and carbon at the g’/substrate interface will eventually                                                                                  experiment
create a second e phase layer between the g’-layer and the substrate                                                                                  absorption isotherm
a-phase. The compound layer will therefore ultimately consist of                                                                   29                 local equilibrium at ε/γ'
three alternating layers of e/g’/e. This is indirectly shown by the N

                                                                                      Nitrogen concentration, atomic %
and C concentration profiles shown in Figure 54.

The nitrogen surface concentration increases with increased process
time and increased nitriding potential (Figure 55), whereas the
carbon surface concentration decreases (Figure 54). The total
amount of nitrogen in the compound layer increases, whereas the
total amount of carbon is constant or decreases with increased                                                                     26
treatment time. Carbon is redistributed as shown in Figure 54 with a
depletion of carbon in the intermediate g’-layer, an accumulation of                                                                                                                           Interface
carbon in the e phase adjacent to the core ferrite/cementite matrix                                                                25
and a positive carbon concentration gradient in the outer e phase
compound layer. For nitrocarburising of medium and high carbon
steels, carbon originating from the steel matrix is incorporated in the                                                            24
                                                                                                                                                10         15            20         25         30            35         40
compound layer, resulting in carbon enrichment of the e phase, (see
the lower part of Figure 54).                                                                                                                                      Nitridning potential, Pa–1/2 × 10–3

                                                                                     Figure 55. Surface and e/g’ interface nitrogen concentration plotted as a func-
                                                                                     tion of nitriding potential for gaseous nitriding of pure iron at 500 °C (932 °F)
                                                                           120 min
                                                                           240 min

                                                                                     The contact between the e- and the a-phases observed at the
N-content (at.-%)

                    15                         γ’                                    interface compound layer/matrix is consistent with the phase
                                                                                     diagram evaluated by Du [39] and shown in Figure 56. (It does
                                                                                     not, however, correspond to the phase diagram from Naumann and
                                                                                     Langenscheid [48] that is often referred to.)


                         0                 5                 10              15                                                         2.5
                                                 Depth, µm

                                                                                                                                                                      cem + ε
                                                                                                                         Carbon, wt.%

                    8                                                                                                                   1.5                                                              ε
                                                                           120 min
                                                                           240 min
C-content (at.-%)

                                                                                                                                        0.5                                         γ'+ ε
                                                    γ’                                                                                                                        γ'
                                                                                                                                                                α + γ'
                                                                                                                                              4.0    4.5     4.0      5.5     6.0        6.5    7.0      7.5      8.0
                                                                                                                                                                         Nitrogen, wt.%

                                                                                     Figure 56. Isothermal section at 580°C (1076°F) of the ternary Fe-N-C phase
                                                                                     diagram [39].
                         0                 5                 10              15
                                                 Depth, µm

Figure 54. Nitrogen and carbon concentration profiles after nitrocarburising at      Figure 57 shows an interpretation of compound layer growth data:
575°C (1067°F) [45].
                                                                                     The growth rate is initially low (step I) when cementite growth is
                                                                                     dominating. After cementite has transformed to e-phase the growth
                                                                                     rate is high (step II). This high growth rate is lowered in step III after
                                                                                     g’ has formed as an interlayer between e and the substrate.
                                                                                                                                                                          Compound Layer and Diffusion Zone Formation                  33

                                 18                                                                       ceramic interior and 2) a pit furnace with a steel retort (UTAB). For
                                              ac = 0.6–1.4; aN = 750                                      reference the figure shows the line of no ammonia decomposition.
                                              ac = 1.2–2.2; aN = 1220                                     The figure illustrates both the effect of gas residence time and of the
                                              ac = 1.1–1.5; aN = 670                                      catalysing effect of the furnace interior material. A high flow rate, in
Compound layer thickness, mm

                                              ac = 3.0;     aN = 750                                      this case 4m3/h (1.41 100ft3/h), corresponds to a low gas residence
                                 12                                                                       time, which results in higher residual ammonia concentrations (lower
                                                                        III                               dissociation of the inlet ammonia). Residual ammonia concentrations
                                 10                                                                       are lower for the metallic retort furnace because the metallic retort
                                                                                                          catalyses ammonia dissociation to higher degree than the ceramic
                                                                                                          interior of the sealed quench furnace.
                                                   II                                                                                               40


                                                                                                                      Residual ammonia content, %
                                                             I                                                                                      30
                                      0       20        40       60           80        100   120   140

                                                                   time (s–1/2 )


Figure 57. Dependence of compound layer thickness on the square root of nitro-

carburising time showing different growth stages depending on the atmosphere

nitriding and carbon activities [45].

                                                                                                                                                                 L in
                                                                                                                                                    10                                                 Furnace, gas flow rate
                                                                                                                                                                                                          IPSEN RTQ – 1, 4 m3/h
                                                                                                                                                                                                          IPSEN RTQ – 1, 2 m3/h
                                                                                                                                                                                                          UTAB OMG437, 4 m3/h
The influence of atmosphere nitrogen and carbon activities on
compound layer morphology, especially porosity and depth,                                                                                           0
                                                                                                                                                         0           10              20          30           40         50       80
was illustrated in Figure 46. In a study by Bell and Wells [49]
                                                                                                                                                                                           Ammonia addition, %
the atmosphere carbon activity was shown to affect the phase
constituency of the compound as follows:                                                                  Figure 58. Relation between residual ammonia concentration and inlet ammo-
                                                                                                          nia addition at 570 °C (1058 °F) in a small sealed quench furnace (Ipsen RTQ-1)
                               - At low carbon activities there is a g’-layer positioned in between       for two gas flow rates and a pit metal retort furnace (UTAB OMG) for only one
                                                                                                          flow rate [16].
                                 the e-layer and the core ferrite/cementite structure;
                               - At intermediate carbon activities a g’-layer forms as a band
                                 inside the e-layer.
                               - At high carbon activity no g’ at all was observed but the                The ammonia dissociation is also catalysed by the surface of the steel
                                 compound layer was e-monophase.                                          parts loaded into the furnace. An increase in surface area increases
                               - At very high carbon activity a duplex thin layer was formed              the degree of dissociation. An example from a laboratory test is
                                 consisting of e + cementite.                                             shown in Figure 59.

The higher temperature for nitrocarburising compared to that for                                                                           1.0
nitriding means that the diffusion coefficients for nitrogen and carbon                                                                                                                                         25% NH3
                                                                                                                                                         Theoretical                                            50% NH3
are higher. Austenitic nitrocarburising utilises this to create thicker                                                                                                                                         75% NH3
cases (diffusion + compound layer). The thermodynamic stability of                                                                         0.8                                                                 100% NH3
                                                                                                          Degree of dissociation

the e-phase is favoured by increased temperature. For these two
reasons the compound layer growth rate is higher in nitrocarburising                                                                        0.6          100% NH3
compared to nitriding.                                                                                                                                                                         results
                                                                                                                                                                                                               630 mm2

The treatment time in normal nitrocarburising processes for low alloy,
low carbon steels must not be so long that the porosity and thickness                                                                                                     10% NH3
of the compound layer reach excessively high values. Poor adherence                                                                         0.2                                                                      40 mm2
and low hardness will be the result. In view of this, practice has
shown that a treatment time of 1.5 to 3 hours is optimum.
                                                                                                                                                    300        400        500            600    700     800        900    1000    1100
1. Furnace Interior Influence                                                                                                                                                              Temperature, K
Gas reaction rates depend on the catalysing effect from available
surfaces inside the furnace and will therefore vary between furnaces,
                                                                                                          Figure 59. Degree of ammonia dissociation plotted against temperature: cal-
depending on whether the type of furnace interior materials are
                                                                                                          culated curves shown for thermal dissociation for two volume fractions of NH3
metals or ceramics. Figure 58 shows experimental results regarding                                        in the NH3-H2 gas mixture entering the furnace (bold and dashed curves to the
the relation between residual ammonia and added ammonia for two                                           left). Experimental data presented for dissociation over catalytically active iron
types of furnaces: 1) a sealed quench furnace (Ipsen RTQ-1) with a                                        surfaces of 40 and 630 mm2 [47].
34                    Compound Layer and Diffusion Zone Formation

In a study tests were made in different types of furnaces where                       shows that the atmosphere is far from the state of equilibrium.
identical nitrocarburising atmosphere recipes were applied (constant                  It was found that furnaces that start the nitrocarburising process
gas mixture and flow rate settings), the compound layer thicknesses                   from room temperature have the poorest results with respect to
obtained on steel samples varied from 11 to 26 µm. To interpret the                   compound layer thickness. (A possible explanation could be that
differing results the FTIR gas analysis results from these trials are                 formation of g’ during heating up slows down layer growth). The
plotted as a function of nitriding potential (PNH /PH 3/2 ) and carbon                highest thicknesses were obtained for cases when the start value
                                                  3    2
potential (PCO2 /PCO ) in Figure 60. The direction of the arrows in the               of the atmosphere nitrogen activity was high. A low start value for
figure indicates analysing results with progressing process time.                     the nitrogen potential was found not to be compensated by that the
                                                                                      activity value later increased to a high value.
                                                                                      In Figure 61 the measured compound layer depths are compared with
             45                                                                       the results predicted by Hong Du (see Figure 57). An interpretation of
                                                                                      this comparison is that the slow compound layer growth for the steel
             40                                                                       retort furnace corresponds to g’ growth rate control and the high
                                                                                      compound layer growth to e growth rate control. The low compound
                                                                                      layer growth rate for the 1-chamber/fibre lined furnace can be
             30                                                                       attributed to the low nitriding potential start value (Figure 60).

             20                                                                                                 3-chamber/brick lining: aN = 4.5
                                                                                                       25       2-chamber/fibre lining: aN = 3.9
             15                                                                                                 2-chamber/brick lining: aN = 2.0
                      γ'                                                                                        1-chamber/steel retort: aN = 1.0
             10                                                                                        20       1-chamber/fibre lining: aN = 2.0
                  α                                                                                             Hong DU – ε growth
                                                                                       Thickness, µm

             5                                                                                                  Hong DU – γ’ growth
                           γ                                                                           15
                  0            1        2         3          4      5         6
                                                       3/2                                             10

Figure 60. Results of continuous FTIR gas analysis plotted in an isothermal
section of the Fe-C-N phase diagram. Results are expressed by the nitriding
potential, PNH3 /PH23/2, and the carbon potential, PCO2/PCO2. Nitrocarburising tem-
perature 580°C (1076°F). The directions of the arrows indicate the direction of                             0   20           40             60     80          100
change in gas analysis during the 3hr process time. Indications of furnace inte-
                                                                                                                               time1/2 (s1/2 )
riors: dark blue line = 3 chamber/brick lining, light blue line = 2 chamber/brick
lining, orange line = 1 chamber/ stainless steel retort, yellow line =1 chamber/
fibre lining, purple line =2 chamber/ fibre lining.                                   Figure 61. Compound layer thickness as a function of the square root of the
                                                                                      process time for five different furnaces (identical to Figure 60) operated with
                                                                                      identical process cycles as regards temperature (580°C (°F)), time (3 hr) and
                                                                                      input gas mixture. Atmosphere nitrogen activities (aN) expressed by the ratio
                                                                                      PNH3 /PH23/2 were determined 15 minutes into the process cycle. Hong Du’s data
                                                                                      from Figure 57 are shown for reference [50].
The figure shows large differences between furnaces both with
respect to absolute values of the potentials as to the direction of
change in these potentials during the nitrocarburising process.
The two fibre insulated furnaces showed a greater variation in
atmosphere activities during the process compared to the brick
walled furnaces. This may be connected to the fact that the water                     2. Influence of Amount of Active Gas
content was initially very high in the fibre lining furnaces. The                     The fact that the surface nitrogen concentration increases with time
steel retort furnace is the extreme and exhibits the lowest nitrogen                  is evidence of the fact that the gas/surface nitrogen transfer is a rate
activities close to the borderline g’/e in the phase diagram, but also                limiting step. This was confirmed in a study of the influence of the
the highest carbon activities. This is explained by the catalysing                    amount of active gas species (NH3 , H 2, CO, CO2) [51] on compound
effect of the steel retort on ammonia dissociation and on the reaction                layer growth. A comparison of the nitrocarburising result was made
between hydrogen and CO2 to form CO respectively. Logically, the                      between high and low active gas percentages, being atmospheres
lowest compound layer thickness was measured for the steel retort                     with 40 vol% and 20 vol % of active gas respectively. Figure 62
furnace.                                                                              illustrates that compound layer thickness as well as total case depth
                                                                                      showed a significant dependence on the amount of active gas for
The carbon activities calculated from FTIR gas analysis could not                     low alloy steel. However, no significant effect was determined for
be related to compound layer thickness results. Carbon activities                     a high alloy steel. Increased nitriding potential led to an increase in
determined from CO, H2, and H2O were found to be an order of                          the compound layer thickness, whereas no significant effect of the
magnitude lower than those calculated from CO and CO2, which                          carbon potential was determined.
                                                                                                                                                             Compound Layer and Diffusion Zone Formation        35

                                          22                                                                                             The depth of the diffusion zone also decreases with increased alloy
                                                                                 40 vol% active gas                                      content principally in the same way as in nitriding (See Figure 51).

Thickness, mm

                                                                                     20 vol% active gas                                  Pore Formation
                                                                                                                                         The depth of porosity in the outer part of the compound layer on un-
                                                                                                                                         and low-alloy steels is of the order 30-40 % of the total compound
                                          12                                                                                             layer depth, as illustrated in Figure 64a. The degree of porosity
                                                                                                                                         increases with increased process time and increased nitrogen
                                                                                                                                         potential. For high nitrogen activities the porosity depth may exceed
                                          8                                                                                              50 % of the total compound layer depth. High alloy steels are
                                               0           0.5        1.0         1.5           2.0        2.5        3.0          3.5   somewhat less prone to porosity formation, as illustrated in Figure
                                                                                  Carbon activity                                        64b, probably due to the lowering effect of alloying elements on
                                                                                                                                         nitrogen activity.
Figure 62. Compound layer thickness as a function of the amount of active gas
(NH3+H2+CO+CO2) and carbon activity. Low alloy steel. [51].
                                                                                                                                         Pores are formed at discontinuities (grain boundaries, slag inclusions)
                                                                                                                                         because of the denitriding step:

3. Steel Alloy Content Effect                                                                                                                     2 N → N2
As in the case of nitriding, increasing the alloy content of the steel
leads to decreased compound layer thickness, Figure 63.                                                                                  The equilibrium nitrogen gas pressure is high enough to create pores
                                                                                                                                         in the compound layer. The probability of pore formation increases
                                           30                                                                                            drastically above nitrogen activities over 650-750.

                                           25          Unalloyed steel                                                                   The pores will grow together, forming channels in which effusion of
                                                                                                                                         nitrogen out to the surface may take place. Carbonaceous gas can
         Thickness of compound layer mm

                                                                                                                                         enter the channels and cause carbon uptake within the layer.
                                           20              1 % Cr steel
                                                            1,5 % Cr steel

                                           15                             4 % Cr steel
                                                                                                                      12 % Cr
                                                                                                                      tool steel
                                           10                                            5 % Cr
                                                                                         Hot work
                                                                                         tool steel                    13 % Cr
                                           15                                                                          steel

                                                   0        2         4          6          8         10         12         14
                                                                            Total alloying content, %

Figure 63. The thickness of the compound layer decreases with increasing alloy-
ing content of treated steels for a given treatment time [16].

                                                                  Figure 64. Depth measurements of compound (blue text) and porous (red text) layer on a) carbon steel (St50-2)
                                                                  and b) alloy steel (34NiCrMo5). Nitrocarburising process recipes were identical for both steels [51].
36                                        The NITROFLEX Solution

VII. The NITROFLEX® Solution

When Linde and BOC merged in 2006, a substantial concentration                                    Post oxidation with air, water or N2O gives aesthetically attractive
of expertise on nitriding and nitrocarburising was created by adding                              black surfaces (Figure 22) with markedly improved corrosion resist-
together the Epsilon and the NITROFLEX® packages of BOC and Linde                                 ance (Table 10).
respectively. This merged solution package has been maintained, fur-
ther developed and marketed under the NITROFLEX® trademark. The
package covers the atmosphere supply solutions, flow control units,                               A. Gas Supply
related know-how (the content of this booklet) and specifications of                              An atmosphere supply system consists of three major parts: media
the complete details of process cycles and of safety instructions.                                storage, mixing and intake to the furnace.

The NITROFLEX® atmosphere system has the inherent advantages of                                   There are five major supply forms for nitrogen:
flow rate and mixing (composition) flexibility typical of synthetic in                               a. Gaseous nitrogen in cylinders
situ generated atmospheres. It provides an opportunity to optimise                                   b. Liquid nitrogen supplied by truck to the customer container
the gas composition in relation to the type of furnace, steel and to                                 c. Nitrogen produced on-site with cryogenic technology
the different stages in the process with the objective of obtaining                                     (CRYOSS®)
the required final part properties of wear resistance, fatigue resist-                               d. Nitrogen from adsorption (PSA) units installed on-site at the
ance or corrosion resistance. If used efficiently, these advantages                                     facility (ADSOSS®)
lead to minimized costs and high quality. Safety and the potential                                   e. Nitrogen from on-site membrane units installed on-site at the
for increased productivity are additional benefits.                                                     facility (MEMOSS®)

One feature of NITROFLEX® atmospheres is a carbon activity that                                   Nitrogen supplied in liquid form has high purity, with typical contami-
is much lower than that in systems using endogas together with a                                  nation levels of O2 + H2O of 5 ppm. Liquid supply is common for a flow
higher oxygen activity (see Table 11). Due to this fact practice has                              from 10 to 100-200 m3/h. The liquid nitrogen is supplied by truck to a
shown that a faster growth rate of the compound layer can be                                      liquid nitrogen storage tank at the manufacturing plant, as illustrated
obtained. It has been demonstrated that alloyed steels such as hot                                in Figure 66. The liquid nitrogen supply form has the advantage that
work tool steels obtain a thicker and more even compound layer,
see Figure 65. The balanced carbon activity means that the driving
force for soot deposits in furnaces is low.

                                                                           Gas flow rate
                                       50% ENDO + 50% NH3                           4 m3/h
     Compound layer thickness mm

                                                                                    2 m3/h

                                       40% ENDO + 50% NH3 + 10% AIR

                                       35% NH3 + 5% CO2+ 60% N2

                                   0            5           10        15       20            25

Figure 65. Compound layer thickness for different atmospheres showing faster
growth for the NITROFLEX® system [13].                                                            Figure 66. Filling of a storage tank for liquid nitrogen
                                                                                                                         The NITROFLEX Solution   37

                                                                    ®                            ®
               Figure 67. a) PSA (Pressure Swing Adsorption), ADSOSS , and b) Membrane, MEMOSS , unit for nitrogen production on site.

the amount of nitrogen supplied to the furnaces can be varied within
wide limits. The customer only takes the amount needed at any time.

ECOVAR® is a family of on-site production units supplied by Linde
Gas. Nitrogen produced using the PSA (Pressure Swing Adsorption)
technique, Figure 67a, has a purity of 99 to 99.99 %. Flow rates from
10 to 1500-2000 m3/h can be accommodated. Nitrogen produced at
the customer site using the membrane technique, Figure 67b, has a
purity of 90-99 %. Flow rates from 5 to 1000 m3/h can be accommo-
dated. The purity requirement of membrane nitrogen is normally not
sufficiently high for nitriding and nitrocarburising.

Cryogenic on-site production, Figure 68, yields high purity, typically
5 ppm oxygen and moisture content. It is relevant for flow rates from
250 to 1500-2000 m3/h.

The on-site production methods are normally combined with a liquid
nitrogen tank supply or a gas cylinder supply. This extra supply is for
backup purposes and to meet instant needs of higher flow rates than
is possible with the OSS unit.                                                 Figure 68. Cryogenic nitrogen production with CRYOSS®
38           The NITROFLEX Solution

                                         a                                                                              b

                                         c                                                                              d
Figure 69. Different supply forms for hydrogen. a) Cylinder bundle with hydrogen, b) Hydrogen tube trailer,
c) Container with on-site hydrogen electrolyser, d) Liquid hydrogen truck with production facility in the background.

Hydrogen is alternatively supplied by:                                               Ammonia is delivered as a liquid in large containers for high con-
                                                                                     sumption, in semi-containers for moderate consumption or in cylin-
     a) Gaseous delivery from cylinders, cylinder bundles or a tube                  ders for moderate consumption.
     b) On-site production by electrolysis of water, steam reformation               Carbon dioxide is also delivered as a liquid for large-scale customers
        of natural gas, ammonia dissociation or methanol dissociation.               and as gas in cylinders for moderate or smaller customer.
     c) Liquid hydrogen supply.
                                                                                     Laughing gas for post oxidation is delivered in cylinders.
Examples of different supply methods for hydrogen are shown in
Figure 69.                                                                                                                  H2 O

                                  N2     N2



                                  NH3   NH3                          PI

                                                 Figure 70. Ammonia cylinder supply setup for nitriding.
                                                                                                                                    The NITROFLEX Solution                                     39

1. Nitriding
Figure 70 shows the schematic setup of a cylinder ammonia installa-                     Hydrogen used for nitrogen potential control is commonly supplied in
tion suitable for moderate consumption. When cylinders are used,                        cylinders.
it is an advantage to have two separate cylinders or cylinder bundles.
One cylinder is used and the other is on standby to be automatically                    2. Nitrocarburising
connected when the pressure from the first falls below required sup-                    Nitrocarburising gas supply has the special feature that carbon dio-
ply pressure. This ensures continuous and safe gas supply. For liquid                   xide is added in addition to the nitrogen/ammonia/hydrogen supply
bulk ammonia supply, it is necessary to have a vaporizer connected.                     used in nitriding. Figure 71 shows the schematic setup of a gas sup-
When cylinder supply is used, it is necessary either to ensure that the                 ply system with a liquid nitrogen tank and gas cylinders for ammo-
temperature of the cylinders is sufficiently high for vaporisation or to                nia and carbon dioxide respectively. Figure 72 shows a gas storage
have a vaporiser installed.                                                             installation also incorporating propane and methanol for carburising.
                                                                                        From storage, gases are led via pipes to the mixing panel. Here the
Nitrogen required for purging at heating, upon cooling and for diluti-                  flow rates are controlled. Figure 73a shows a flow mixing panel
on during nitriding is supplied in cylinders for moderate consumption                   assembled from cassettes for each gas and flow rate range. In advan-
and in a liquid tank for higher amounts. If nitriding is only one of                    ced setups, the flow rate and temperature control can be built into a
several heat treatment operations in a heat treatment plant, the gas                    closed loop control system as shown in Figure 73b.
supply is dimensioned to cover the whole installation, which normal-
ly means that liquid nitrogen supply is best.

                                                                            N2 NH3




       CO2                                                                                                                             Quality assurance system

                                                                                                          Control cabinet
Figure 71. Gas supply system.
                                                                                                                                          Gas Mixing and Flow
                                                                                                                    Temperature              Control Panel

                                                                                                                                                                Carbon dioxide

                                                                                                                                                                                 Other gases


Figure 72. Gas installation designed for heat treatment: The high tank in the
centre of the picture is for liquid nitrogen, the tank to the left is for oxygen. The   Figure 73. a) Gas flow mixing panel assembled from a modular cassette system.
containers to the right are for carbon dioxide and hydrogen storage. The red            From left to right there are cassettes for nitrogen, ammonia, carbon dioxide, hy-
container to the left is for ammonia storage. The horizontal tanks to the left are      drogen, carbon monoxide and nitrous oxide (laughing gas) b) Schematic setup
for propane and methanol.                                                               of a control system.
40            The NITROFLEX Solution

If carbon dioxide is mixed with ammonia before entering the hot            C. Case Studies
furnace there will be reactions leading to ammonium carbonate
formation, which deposits in the line, leading to clogging and flow        1. Nitrocarburising
disturbances. Ammonia gas and nitrogen, or carbon dioxide gas and          The cases described here are taken from reference [52].
nitrogen can be mixed before feeding to the furnace. Therefore there
are two alternatives:                                                      Case Study 1: Conveyor roller in a metal spraying shop
                                                                           Experience has shown that cast iron conveyor rollers operating in a
      1. Nitrogen + ammonia in one mixer and from one inlet,               metal spraying shop are subjected to very aggressive wear and cor-
         carbon dioxide from another pipe work and inlet                   rosion in operation, see Figure 75a. Because no form of classical heat
                                                                           treatment could solve both the wear and corrosion attack problems
      2. Nitrogen + carbon dioxide in one mixer and from one               on its own, the NITROFLEX® nitrocarburising treatment adapted to a
         inlet, ammonia from another pipe work and inlet.                  sealed quench furnace was tried. Figure 75b illustrates the condition
                                                                           of a conveyor roller treated using the process (left) compared to an
Figure 74 shows the alternative with ammonia mixed with nitrogen           untreated and worn roller (right). One of the rollers is worn flat whe-
and a separate line for carbon dioxide. Gases enter the furnace            reas the treated one is intact.
through a specially designed injector formed by two concentric
tubes. The nitrogen/ammonia mixture passes through the outer wi-
der tube. Carbon dioxide flows through the inner tube, which extends
into the furnace. All three gases do not mix until they reach the hot
furnace interior, which is essential in order to avoid the risk of ammo-
nium carbonate deposits.


                                                                           Figure 75. a. Conveyor rollers in use in a paint shop. b. Conveyor rollers

Figure 74. Gas inlet design

                                                                           Case Study 2: Forging die manufactured from L6 material
As in the case of nitriding, gas cylinders are adequate supply forms       Forging dies are generally subject to high indentation and aggressive
for hydrogen used in nitrogen potential control.                           wear. Therefore high alloy tool steel is used to increase life, but in
                                                                           order to save costs, an additional surface is required. However, the
                                                                           core hardness achieved after conventional hardening and tempering
B. NITROFLEX® Process Recipes                                              operations still needs to be preserved. A NITROFLEX® nitrocarburising
Linde engineers will support customers in setting up the necessary         treatment, performed in a sealed quench furnace with a process reci-
procedures including furnace requirements, safety arrangements,            pe adapted to alloy steel and to yield a low case depth, was used to
process specifications, gas installation and furnace control. This may     increase surface hardness without affecting core hardness. The final
involve retrofitting an existing furnace or starting up a new one.         service life was improved, and hence costs were effectively reduced.
In retrofitting, the first step is to conduct a survey to find out the     Figure 76 shows the component produced. The surface finish of the
required furnace modifications. There are a number of important            forgings was improved when the NITROFLEX® treated die was used.
requirements that must be fulfilled to ascertain safety and treatment
quality. The necessary points to be considered are as follows.

     a. Pit and bell type furnaces must be equipped with a sealable
        retort that must be checked for any defects.
     b. A circulating fan must be present within the retort of the pit
        or bell type furnace.
     c. Provisions must be made for exhaust gases to be burnt off
        outside the furnace.
     d. The furnace must have a temperature uniformity capability
        typically of ± 5 °C (± 9 °F)
     e. The furnace should preferably be capable of ramped cooling.

Once the process recipe has been adapted to the type of furnace,
steel, required microstructure and properties, it is defined and tested
with settings for temperatures and gas flows.                              Figure 76. Forging produced by the NITROFLEX treated forge die.
                                                                                                                      The NITROFLEX Solution                41

Case Study 3: Extrusion dies made of H13 material.                       Case Study 6: Clutch plates in mild steel
The time between re-polishing was significantly improved by a            Automobile clutch plates were previously carbonitrided using con-
NITROFLEX® nitrocarburising treatment, performed in a sealed quench      ventional methods. The main problem was unacceptable distortion
furnace with a process recipe adapted to the steel. A very high sur-     and even high scrap rates. Austenitic nitrocarburising, performed in
face hardness of the order of 1200HV was achieved. The appearance        a sealed quench furnace with a process recipe adapted to mild steel,
of treated dies is shown in Figure 77.                                   was used in an attempt to eliminate the distortion problem and to
                                                                         bring down costs. As a result, the scrap rate decreased by 30 % and
                                                                         the need for phosphating after carbonitriding was eliminated. This
                                                                         decreased costs by an additional 36 %. The components are shown
                                                                         in Figure 79.

Figure 77. NITROFLEX® treated H13 extrusion dies

In another case, nitrocarburising was performed on pipe welding          Figure 79. Clutch plate in mild steel treated by the austenitic nitrocarburising
clamps made of hardened and tempered H13 tool steel working in
sea water under conditions of repetitive cooling. The untreated
clamp rusted after 100 cycles in these working conditions. The
NITROFLEX® nitrocarburising treated clamps attained 300 cycles           Case Study 7: Mild steel slides
without rust.                                                            Mild steel slides used in mail sorting offices are subject to considera-
                                                                         ble wear. As a result they must be replaced regularly. Several materi-
                                                                         als and processing options were tried during their manufacture. The
2. Austenitic Nitrocarburising                                           slides are commonly made of mild steel and then carbonitrided, but
Case Study 4: Tag washers in SAE 1020                                    this method results in an excessively high level of scrap. The use of
Due to the unacceptable distortion in traditional carbonitriding, the    the austenitic nitrocarburising, performed in a sealed quench furnace
NITROFLEX® austenitic nitrocarburising treatment was performed in        and with a process recipe adapted to mild steel to yield a moderate
a sealed quench furnace and with a process recipe adapted to mild        case depth, with its lower processing temperature, solved the scrap
steel to yield a low case depth. By producing a deeper diffusion zone    problem. It also increased service life by improving wear and corrosi-
to support the compound layer, this treatment successfully solved the    on resistance. The treated components are shown in Figure 80.
problems. Figure 78 shows the appearance of the parts. Previously
carbonitrided components were also processed in a salt bath, but the
properties produced were not sufficient to prevent collapse of the
surface in service.

                                                                         Figure 80. Mild steel mail handling tracks

Figure 78. Austenitic nitrocarburised SAE 1020 tag washer
                                                                         Case Study 8: Moulds in 1020
                                                                         Most moulds or forming tools are manufactured from H13 material.
                                                                         H13 material is expensive and typically requires vacuum hardening
Case Study 5: Nitrocarburising of stainless steel                        and tempering. These moulds are now manufactured from mild steel,
Classical nitriding of stainless steels has always been somewhat         a low-cost material, and treated with extended austenitic nitrocar-
imprecise, resulting in unreliable results such as variable hardness     burising performed in a sealed quench furnace and with a process
and even a total failure to nitride. Using a special NITROFLEX® pro-     recipe adapted to yield a high case depth to achieve the properties
cess procedure it is even possible to successfully nitride and nitro-    that are required for H13 material to be treated by vacuum processes.
carburise stainless steels. The treatments are suitable for 400 series   Two examples of these moulds are shown in Figure 81. In addition
martensitic steels and 300 series austenitic steels. Treatments give     to cost savings, surface quality has also been improved because the
excellent wear and scuff resistance to stainless steels.                 compound layer eliminates pickup.
42           The NITROFLEX Solution

                                                                                   process cycle and the elimination of the post grinding process. As a
                                                                                   result, overall costs were reduced. The treated crankshafts are shown
                                                                                   in Figure 82 and a comparison between classical gas nitriding and
                                                                                   the NITROFLEX® nitrocarburising treatment is shown in Table 16.

Figure 81. a. Austenitic nitrocarburised mild steel glass bottle moulds, previ-
ously H13.
b. Austenitic nitrocarburised mild steel aluminium pan moulds, previously H13.

3.   Nitriding

                                                                                                  Figure 82. Treated crankshafts
Case Study 9: Crankshafts
Crankshafts were previously gas nitrided in a pit furnace for 105
hours to achieve the required properties. Post grinding was carried                The shorter cycle utilised by the NITROFLEX® process increased the
out for finishing. The use of the NITROFLEX® nitrocarburising treat-               production capability of the existing furnace equipment by 48 % and
ment performed in a pit furnace and with a process recipe adapted to               gave a cost saving of 1326 GBP (cost level of the year 2001) per
the alloy steel and to yield a modest case depth resulted in a shorter             furnace cycle.

Table 16. Comparison of gas nitriding and NITROFLEX nitrocarburising.
Note: Cost given in GBP with cost level of the year 2001. 1 GBP=1.25 EUR =1.76 USD [52].

Process                   Classical gas nitriding                   NITROFLEX® nitrocarburising
Furnace                   Pit nitriding furnace                     Pit nitriding furnace
Load size                 6 off × 6 cylinder crank shafts           6 off × 6 cylinder crank shafts
Operating cost            2199 GBP (105 hrs floor-to-floor)         1100 GBP (55 hrs floor-to-floor)
Process cost              350 GBP/component                         183 GBP/component
Post grinding cost        52 GBP/component (white layer)            No post-grinding (compound layer)
Total cost                404 GBP/component                         183 GBP/component
Saving                                                              221 GBP/component
                                                                                                                               Safety         43

VIII. Safety

The European Committee for Standardisation, CEN, has issued a series      uptake, and even very small CO concentrations can be dangerous.
of heat treatment safety standards, listed in references [53-58]. The     Carbon monoxide has no odour, which makes the hazard more seri-
standard covering the use of atmospheres is CEN 746-3; Industrial         ous. Carbon monoxide has the same density as air and will therefore
thermoprocessing equipment – Part 3: Safety requirements for the          not disperse naturally. Heat treatment shops should therefore ensure
generation and use of atmosphere gases [55]. In addition to the Eu-       that there is good ventilation in the work areas.
ropean Standards there are national standards and safety regulations
that have to be taken into account. The National Fire Protection Asso-    Carbon dioxide, which is normally present in atmospheric air at the
ciation (NFPA) maintains the main safety standard for heat treatment      level of approximately 300 ppm, regulates breathing; an increase in
in the USA, see reference [59]. In addition standards and regulations     concentration will increase the breathing rate. The workplace expo-
are issued by the U.S. Occupational Safety and Health Administration      sure limit is 5000 ppm (0.5 %), but changes in breathing rate may not
(OSHA), and by insurance underwriters. The Compressed Gas Asso-           be noticed until there is a concentration of 20,000 ppm (2 %), when
ciation (CGA) maintains standards for gases. National electrical codes    the rate will increase to 50 % above normal. Exposure at this level for
and local requirements of states and communities will also apply. An      several hours may cause a headache and a loss of concentration. A
overview of safety hazards and required precautions in heat treat-        concentration of 15 % soon causes unconsciousness, and may cause
ment is given in reference [60].                                          death after some hours’ exposure. Carbon dioxide is odourless, and
                                                                          thus gives no warning of its presence by any odour, except at very
The main hazards related to the use of gases in heat treatment are        high concentrations when a slight acidic pungency may be detect-
explosions/flammability, toxicity, asphyxiation, and exposure to hot      able.
objects, surfaces and flames. Specific safety issues connected to
nitriding/nitrocarburising are related to the toxicity and flammability   When the oxygen concentration in inhaled air is reduced from 21%
of the gases used and to the fact that operation temperature is be-       to 10 %, there is a serious hazard of asphyxiation. Oxygen deficiency
low the safety temperature.                                               can be caused by any asphyxiating gas, the most common being
A. Toxicity and Asphyxiation
Ammonia is a corrosive gas attacking moist skin, mucous membranes
and eyes. Severe exposure is unlikely except in confined spaces, as       B. Flammability
its characteristic smell at 20 ppm or more usually provides adequate      Both nitriding and nitrocarburising are performed at a temperature
warning. Ammonia at 100 ppm causes irritation of the eyes and nose        below the safety temperature (700 °C (1292 °F)) for igniting a flame
after a few minutes’ exposure, and at 700 ppm causes severe eye           of the combustible parts of the gas. This means that air (oxygen)
and nose irritation but no permanent effects if the exposure is lower     can enter into a furnace and mix with the combustible gas (CO, H2 ,
than half an hour. Concentrations above 1700 ppm cause serious            NH3 , CH4 etc.) without igniting a burning flame. (In high temperature
coughing, bronchial spasm, acute pulmonary oedema and asphyxia            processes such as carburising and carbonitriding, a flame occurs
and at these levels death can occur within half an hour.                  automatically in such cases.) Therefore if not controlled, it may occur
                                                                          that a large amount of a flammable ammonia/hydrogen + air mixture
Carbon monoxide is not added but formed at a low concentration            forms. In the worst case, this mixture, if ignited by a flame or spark,
level of the order of a few volume percent within the furnace room        would create a devastating explosion. This places strict requirements
when using the NITROFLEX® system. Other systems based on en-              on the use of flame curtains and safety pilot burners at all doors and
dogas/ammonia have a CO concentration of the order of 10 volume           on safe start up, shut down and operation procedures.
percent. Carbon monoxide is highly poisonous and a concentration
as low as 400 ppm is harmful. Carbon monoxide enters the blood            The NITROFLEX® system typically utilizes about 40 vol-% of ammonia
and takes the place of oxygen in haemoglobin. Carbon monoxide             and optionally up to 10vol-% of hydrogen, which are the only flam-
uptake by the body is very fast, about 250 times faster than oxygen       mable gases in the ingoing gas mixture. This can be compared to the
44          Safety

50 vol-% endogas + 50 vol-% ammonia process, which holds ap-                                                          For mixture in air at 200 °C (392 °F) and 1.0 bar (a)
proximately 80 vol-% hazardous components (H2, CO, NH3) in total.                                                     (L) Lower flammability limit in air = 5.1 vol% mixture
                                                                                                                      (U) Upper flammability limit in air = 43.5 vol% mixture
The Safety Triangle, Figure 83, shows how to operate safely. The                                                      (S) Min O2 -conc. for flammability = 4.6 vol% (Fuel = 5.1 vol %)
flammability triangle is depicted in the area L-S-U and this area                                                     (C) Start up = max 4.9 vol% oxygen
should always be avoided. This is done as follows. When starting                                                      (B) Shut down = max 6.5 vol% mixture
up a process where the furnace is partly or wholly filled with air,
ammonia must not be introduced until the oxygen concentration                                                    20        L

                                                                           Oxygen conc. in gas mixture – vol %
has been lowered to point C. This may be done by purging the fur-                                                18
nace with nitrogen. The level of required purging furnace volumes                                                16

can be taken from theoretical purging curves. The CEN standard in                                                14
reference [155] states: “The volume of inert purge gas needed to                                                 12
displace either air or a flammable gas from a furnace chamber/
enclosure to achieve 1% (V/V) or less oxygen and/or a non-flam-
                                                                                                                  6 C
mable atmosphere gas and/or 25 % of the lower flammability limit
                                                                                                                  4 S
is typically equal to five times the volume of the thermoprocessing
equipment chamber to be purged”. When point C in Figure 83 is                                                              B
reached, ammonia can be safely introduced.                                                                                     10    20      30      40       50     60         70   80      90     100
                                                                                                                                             Mixture (fuel) concentration – vol %
If nitrocarburising is performed in a furnace that will be opened to
air access after nitriding or nitrocarburising, i.e. in a pit furnace, a                                                                                                                  42 vol % H2
reversed sequence is required before closing the process and open-                                                                                                                         8 vol % CO
                                                                                                                      For mixture in air at 580 °C (1076 °F) and 1.0 bar (a)              50 vol % NH3
ing the furnace. By purging with nitrogen the ammonia concentra-
                                                                                                                      (L) Lower flammability limit in air = 0.2 vol% mixture
tion must now be lowered to point B in Figure 83 before the furnace
                                                                                                                      (U) Upper flammability limit in air = 49.8 vol% mixture
can be opened and exposed to air.
                                                                                                                      (S) Min O2 -conc. for flammability = 0.2 vol% (Fuel = 0.2 vol %)
                                                                                                                      (C) Start up = max 0.2 vol% oxygen
Vacuum pumping is an alternative to purging that is gaining in use
                                                                                                                      (B) Shut down = max 0.2 vol% mixture
by the development of suitably equipped furnaces (see Figure 8b).                                                     L
The vacuum level that is seen to be required before introducing                                                  20
                                                                           Oxygen conc. in gas mixture – vol %

flammable gas is 45 mbar according to CEN 746-3 [55].                                                            18
Sealed quench or chamber furnaces that are built for nitrocarburis-                                              14
ing have a flame curtain at the furnace door. A flame guard ensures                                              12
that the flame curtain is ignited. In such cases there is no require-                                            10
ment to purge with nitrogen before opening the door. In all cases                                                 8
double pilot burners should be installed at doors for safety reasons.                                             6
This pilot burner has the function of burning the outgoing gas                                                    4
mixture to ensure that ammonia does not cause odour and safety                                                    2
problems.                                                                                                          S
                                                                                                                               10    20      30      40       50     60         70   80      90     100
                                                                                                                                            Mixture (fuel) concentration – vol %
The outlet gas should be burnt and vented off. This is done in a
separate exhaust gas neutraliser, as was illustrated in Figure 6.                     A safety factor is not included
                                                                                      in the data given above!

                                                                              Figure 83. LINDE GAS safety system, flammability triangle.
                                                                                                                   Concluding remarks           45

IX. Concluding Remarks

Nitriding and nitrocarburising have the outstanding advantages of:      The NITROFLEX® package offers solutions that include:

  – Offering surfaces with extremely good tribological properties         – The advantages of gas flow rate and mixing flexibility connected
    (low wear and friction).                                                to the use of synthetic in situ generated atmospheres. Beneficial
                                                                            consequences are 1) safety, 2) improved process control that
  – Improved corrosion resistance.
                                                                            guarantees high quality, 3) a potential to increase productivity
  – Aesthetically attractive surface appearance.                            and 4) cost savings.

  – Good static and fatigue strength with levels that depend on the       – Options for installations as retrofits on existing furnaces or as
    steel selection.                                                        new installations.

  – Dramatically reduced distortion compared to carburising and           – Manual, semi-or full-automatic control systems.
    carbonitriding. This gives cost savings due to the elimination of
                                                                          – Recipe options to meet demands on final properties and to suit
    post grinding.
                                                                            different steels and furnaces.
  – A low temperature process that lowers energy consumption.
                                                                          – Access to the extensive expertise and the support of knowledge-
                                                                            able, experienced and skilled Linde engineers.
Compared to gas nitriding, nitrocarburising has the advantage of be-
ing a short time process that leads to cost savings.
46         References

X. References
Torsten Holm

 1. Dawes C. and Tranter D., Heat Treatment of Metals,                  24. Råhlen L., Residual Stresses after Surfaced Hardening, Masters
    Vol. 10, No. 4, (1982), 85.                                             thesis, KTH, Stockholm, 1978.
 2. Stratton P. F., Malas A. and Bennet K., Energy Efficient Manufac-   25. Dawes C. and Tranter D., Heat Treatment of Metals, No. 3,
    turing Processes (ed Anderson I., Marechaux T. and Cockrill C.),        (1985), 70.
    March 2003, 27.                                                     26. Liedtke D., Härterei Technische Mittelungen, Vol. 53 (1998), 14.
 3. Haase B., Bodycote-AGA Seminar, Lidingö, 2005.                      27. ASM Handbook No 4, Heat Treating, ASM Int, Materials Park,
 4. Troell E. and Persson U., Influence of cutting fluids and               1991.
    detergents prior to nitrocarburising on final results,              28. Christiansen T. and Somers M. A. J., Werkst Z., Wärmebeh.
    Report VBC-2007-002, IVF, Gothenburg, 2007.                             Fertigung, Vol. 60 (2005), 207.
5.   Burger W., Härterei Technische Mitteilungen, Vol. 47 (1992),       29. R. Hoffman, I. Kleffmann and H. Steinmann, Härterei Technische
     76.                                                                    Mitteilungen, Vol. 60 (2005), 178.
 6. Stiles M., Don J., Haase B., Haasner T. and Bauckhage K.,           30. Hoffman R., Kleffmann I.and Steinmann H., Härterei Technische
    Härterei Technische Mitteilungen, Vol. 53 (1998), 211.                  Mitteilungen, Vol. 60 (2005), 216.
 7. Friehling P. and Somers M. A. J., Surface Engineering, Vol. 16      31. Lohrmann M., Heat Treatment of Metals, No. 3 (2001), 53.
    (2000), 103.
                                                                        32. Hoffmann R., Mittemeijer E. J. and Somers M. A. J., Härterei
 8. Floe C. F., Metal Progress, Vol. 50 (1946), 1212.                       Technische Mitteilungen, Vol. 49 (1994), 177.
 9. Jordan D., Antes H.,Osterman V. and Jones T., Heat Treating         33. Weissohn K. H., Härterei Technische Mitteilungen, Vol. 53
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Getting ahead through innovation.

With its innovative concepts, Linde is playing a pioneering role in the global market. As a technology leader, it is our task
to constantly raise the bar. Traditionally driven by entrepreneurship, we are working steadily on new high-quality products
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If you want to keep pace with tomorrow’s competition, you need a partner by your side for whom top quality, process
optimization, and enhanced productivity are part of daily business. However, we define partnership not merely as being
there for you but being with you. After all, joint activities form the core of commercial success.

Linde – ideas become solutions.

Furnace Atmospheres publishings:
3 Furnace Atmospheres No. 1 – Gas Carburising and Carbonitriding
3 Furnace Atmospheres No. 2 – Neutral Hadening and Annealing
3 Furnace Atmospheres No. 3 – Nitriding and Nitrocarburising
3 Furnace Atmospheres No. 4 – Brazing of Metals
3 Furnace Atmospheres No. 5 – Sub-zero treatment of steels
3 Furnace Atmospheres No. 6 – Low pressure carburising and high pressure gas quenching
3 Furnace Atmospheres No. 7 – Tube Annealing
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Linde AG
Linde Gas Division, Seitnerstrasse 70, 82049 Pullach, Germany
Phone +49.89.74 46-0, Fax +49.89.74 46-12 30,,

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