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HEAT TREATMENT OF TOOL STEEL (PDF)

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					HEAT TREATMENT OF TOOL STEEL
                                                                                                    Contents
                                                                                                    What is tool steel? .................................... 3
                                                                                                    Hardening and tempering ......................... 3
                                                                                                    Dimensional and shape stability ............... 7
                                                                                                    Surface treatment ..................................... 8
                                                                                                    Testing of mechanical properties .............. 10
                                                                                                    Some words of advice to tool designers ... 11




This information is based on our present state of knowledge and is intended to provide general
notes on our products and their uses. It should not therefore be construed as a warranty of
specific properties of the products described or a warranty for fitness for a particular purpose.
Classified according to EU Directive 1999/45/EC
For further information see our “Material Safety Data Sheets”.

Edition 8, 05.2008
The latest revised edition of this brochure is the English version,                                                                             SS-EN ISO 9001
                                                                                                                                                SS-EN ISO 14001
which is always published on our web site www.uddeholm.com
                                                                                                                       HEAT TREATMENT


The purpose of this brochure is to pro-
vide some idea of how tool steel is heat
                                               Hardening                                        Note that the carbides are partially dis-
                                                                                                solved. This means that the matrix be-
treated and how it behaves.                    and tempering                                    comes alloyed with carbon and
   Special attention is paid to hardness,      When a tool is hardened, many factors            carbide-forming elements.
toughness and dimensional stability.           influence the result.                                When the steel is heated to the hard-
                                                                                                ening temperature (austenitizing tem-
                                               SOME THEORETICAL ASPECTS                         perature), the carbides are partially dis-
                                                                                                solved, and the matrix is also altered. It
What is tool steel?                            In soft annealed tool steel, most of the
                                                                                                is transformed from ferrite to austenite.
                                               alloying elements are bound up with
Uddeholm has concentrated its tool                                                              This means that the iron atoms change
                                               carbon in carbides. In addition to these
steel range on high alloyed types of                                                            their position in the atomic lattice and
                                               there are the alloying elements cobalt
steel, intended primarily for purposes                                                          make room for atoms of carbon and
                                               and nickel, which do not form carbides
such as plastics moulding, blanking and                                                         alloying elements. The carbon and alloy-
                                               but are instead dissolved in the matrix.
forming, die casting, extrusion, forging                                                        ing elements from the carbides are dis-
                                                  When the steel is heated for harden-
and wood-working.                                                                               solved in the matrix.
                                               ing, the basic idea is to dissolve the car-
   Conventional high speed steels and                                                               If the steel is quenched sufficiently
                                               bides to such a degree that the matrix
powder metallurgy (PM) steels are also                                                          rapid in the hardening process, the car-
                                               acquires an alloying content that gives
included in the range.                                                                          bon atoms do not have time to reposi-
                                               the hardening effect—without becom-
   Tool steel is normally delivered in the                                                      tion themselves to allow the reforming
                                               ing coarse grained and brittle.
soft annealed condition. This is to make                                                        of ferrite from austenite, i.e. as in an-
the material easy to machine with cutt-                                                         nealing. Instead, they are fixed in posi-
ing tools and to give it a microstructure                     = Iron atoms                      tions where they really do not have
suitable for hardening.                                       = Possible positions for carbon   enough room, and the result is high
   The microstructure consists of a soft                         atoms                          microstresses that can be defined as in-
matrix in which carbides are embedded.                                                          creased hardness. This hard structure is
In carbon steel, these carbides consist of                                                      called martensite. Thus, martensite can
iron carbide, while in the alloyed steel                                                        be seen as a forced solution of carbon
they are chromium (Cr), tungsten (W),                                                           in ferrite.
molybdenum (Mo) or vanadium (V)                                                                     When a steel is hardened, the matrix
carbides, depending on the composition                                                          is not completely converted into mar-
of the steel. Carbides are compounds of                                                         tensite. Some austenite is always left
carbon and these alloying elements and                              2,86 A                      and is called “retained austenite”. The
are characterized by very high hardness.                                                        amount increases with increasing alloy-
A higher carbide content means higher          Unit cell in a ferrite crystal                   ing content, higher hardening tempera-
resistance to wear.                            Body centred cubic (BCC)                         ture and longer soaking times.
   In alloy steels, it is important that the                                                        After quenching, the steel has a
carbides are evenly distributed.                                                                microstructure consisting of martensite,
   Other alloying elements are also                                                             retained austenite and carbides. This
used in tool steel, such as cobalt (Co)                                                         structure contains inherent stresses that
and nickel (Ni), but these do not form                                                          can easily cause cracking. But this can
carbides. Cobalt is normally used to im-                                                        be prevented by reheating the steel to a
prove red hardness in high speed steels,                                                        certain temperature, reducing the stres-
nickel to improve through-hardening                                                             ses and transforming the retained aus-
properties.                                                         3,57 A
                                                                                                tenite to an extent that depends upon
                                                                                                the reheating temperature. This reheat-
                                               Unit cell in an austenite crystal                ing after hardening is called tempering.
                                               Face centred cubic (FCC)                         Hardening of a tool steel should always
                                                                                                be followed immediately by tempering.
                                                                                                    It should be noted that tempering at
                                                                                                low temperatures only affects the mar-
                                                                                                tensite, while tempering at high tem-
                                                                                     2.98 A     perature also affects the retained auste-
                                                                                                nite.
                                                                                                    After one tempering at high tem-
                                                                                                perature, the microstructure consists of
                                                                                                tempered martensite, newlyformed
                                                                  2,85 A
                                                                                                martensite, some retained austenite and
                                               Unit cell in a martensite crystal                carbides.

                                                                                                                                        3
HEAT TREATMENT


Precipitated secondary (newly formed)            HOW HARDENING AND TEMPERING                  combined. Heating and cooling rates
carbides and newly formed martensite             IS DONE IN PRACTICE                          can be compared with salt bath. The
can increase hardness during high-               Distortion due to hardening must be          Al-oxides and gas used as protective
temperature tempering. Typical of this is        taken into consideration when a tool is      atmosphere are less detrimental to the
the so called secondary hardening of             rough-machined. Rough machining              environment than salt bath.
e.g. high speed steel and high alloyed           causes local heating and mechanical              It is important that the tools are
tool steels.                                     working of the steel, which gives rise to    protected against oxidation and decar-
                                                 inherent stresses. This is not serious on    burization. The best protection is pro-
Hardness                                                                                      vided by a vacuum furnace, where the
                                                 a symmetrical part of simple design, but
                                                 can be significant in asymmetrical           surface of the steel remains unaffected.
                                  C
                                      B          machining, for example of one half of a          Furnaces with a controlled protective
                                                 die casting die. Here, stress relieving is   gas atmosphere or salt baths also pro-
                                  D
                                                 always recommended.                          vide good protection.
                                                                                                  If an electric muffle furnace is used,
                                                 Stress relieving                             the tool can be protected by packing it
                                   A             This treatment is done after rough           in spent charcoal or cast iron chips.
                         Tempering temperature   machining and entails heating to 550–            It should be observed that these
A = martensite tempering                         650°C (1020–1200°F). The material            packing materials can have a carburiz-
B = carbide precipitation
                                                 should be heated until it has achieved       ing effect if the steels have a low car-
C = transformation of retained austenite to                                                   bon content, such as conventional hot
    martensite                                   uniform temperature all the way
D = tempering diagram for high speed steel       through and then cooled slowly, for          work steels.
    and high alloy tool steel
                                                 example in a furnace.
A+B+C = D
                                                    The idea behind stress relieving is
The diagram shows the influence of different     that the yield strength of the material at
parameters on the secondary hardening.           the elevated temperature is so low that
                                                 the material cannot resist the inherent
                                                 stresses. The yield strength is exceeded
Tool steel should always be double-              and these stresses are released, result-
tempered. The second tempering takes             ing in a greater or lesser degree of dis-
care of the newly formed martensite              tortion.
formed after the first tempering. Three
tempers are recommended for high                 The correct work sequence is:                                           Vacuum furnace
speed steel with a high carbon content.          rough machining, stress relieving and
                                                 semifinish machining.
                                                 The excuse that stress relieving takes
                                                 too much time is hardly valid. Rectifying
                                                 a part during semifinish machining of
                                                 an annealed material is with few excep-
                                                 tions cheaper than making dimensional
                                                 adjustments during finish machining of
                                                 a hardened tool.

                                                 Heating to hardening temperature
           Tempered once.                                                                                               Salt bath furnace
                                                 The fundamental rule for heating to
                                                 hardening temperature is that it should
                                                 take place slowly. This minimizes distor-
                                                 tion.
                                                    In vacuum furnaces and furnaces
                                                 with controlled protective gas atmos-
                                                 phere, the heat is increased gradually.
                                                 When molten salt baths are used, pre-
                                                 heating is employed, whereas heating is
                                                 automatically slow in a muffle furnace
           Tempered twice.     1000x
                                                 when steel is packed in castiron chips.
                                                    In a fluidized bed the advantages of                               Batch type furnace
         Uddeholm Rigor, hardened                                                                            with a controlled atmosphere
                                                 salt bath and protective atmosphere are
        and tempered.
4
                                                                                                                     HEAT TREATMENT


Wrapping in stainless steel foil also          in the microstructure can take place,        Temperature
provides good protection when heating          risking a poor tool performance.                             AC3
in a muffle furnace.                               Water is used as a quenching me-                         AC1
   Decarburization results in low sur-         dium for unalloyed steels. 8–10% so-
face hardness and a risk of cracking.          dium chloride (salt) or soda should be
   Carburization results in a harder           added to the water in order to achieve
surface layer, which can have negative         optimum cooling efficiency. Water hard-
effects.                                       ening can often cause problems in the
                                               form of distortion and quench cracks.
Holding time at                                Oil hardening is safer, but hardening in                      Core
                                               air or martempering is best of all.
hardening temperature                                                                        Surface
                                                   Oil should be used for low alloyed
It is not possible to state exact recom-       steels. The oil should be of good quality,
mendations briefly to cover all heating        and preferably of the rapid quenching
situations.                                    type. It should be kept clean and must         MS
     Factors such as furnace type, furnace     be changed after a certain period of use.
rating, temperature level, the weight of       Hardening oils should have a tempera-
the charge in relation to the size of the      ture of 50–70°C (140–160°F) to give
                                               the best cooling efficiency. Lower tem-        Martensite
furnace etc., must be taken into consid-
eration in each case.                          peratures mean higher viscosity, i.e. the
     We can, however, give one recom-          oil is thicker.                              The quenching process as expressed in a
mendation that is valid in virtually all           Hardening in oil is not the safest way   TTT graph
situations:                                    to quench steel, in view of the risks of
• when the steel has reached hardening         distortion and hardening cracks. These       ture of the salt bath is normally kept at
   temperature through its entire thick-       risks can be reduced by means of mar-        about 500°C (930°F). This temperature
   ness, hold at this temperature for          tempering. In this process, the material     ensures a relatively mild thermal shock,
   30 minutes. An exception to this rule       is quenched in two steps. First it is        but a sufficient cooling rate to avoid
   is for thin parts heated in salt baths at   cooled from hardening temperature in a       phase transformations.
   high temperature, or high speed steel.      salt bath whose temperature is just             Full martensite transformation has,
   Here the entire period of immersion is      above the MS temperature. It is kept         in many cases, time to occur when the
   often only a few minutes.                   there until the temperature has equal-       steel is cooled in air from the martem-
                                               ized between the surface and the core,       pering bath temperature. However, if
Quenching                                      after which the tool can be allowed to       the dimensions are big, it is often nec-
The choice between a fast and slow             cool freely in air down through the          essary to use a forced quenching rate
quenching rate is usually a compromise;        martensite transformation range.             depending of the hardenability of the
to get the best microstructure and tool            When martempering oil hardening          steel.
performance, the quenching rate should         steels, it should also be kept in mind
be rapid; to minimize distortion, a slow       that the material transforms relatively      Temperature

quenching rate is recommended.                 rapid and should not be kept too long at
    Slow quenching results in less tem-        the martempering bath temperature.
                                                                                                            AC3
perature difference between the surface        This can lead to excessive bainite trans-
                                                                                                            AC1
and core of a part, and sections of dif-       formation and the risk of low hardness.
ferent thickness will have a more uni-             High alloy steels can be hardened in
form cooling rate.                             oil, a martempering bath or air. The
    This is of great importance when           advantages and disadvantages of the
quenching through the martensite               different methods can be discussed.
range, below the Ms temperature. Mar-              Oil gives a good finish and high                        Core
tensite formation leads to an increase in      hardness, but it also maximizes the risk
volume and stresses in the material. This      of excessive distortion or cracking. In
is also the reason why quenching               the case of thick parts, quenching in oil     Surface
should be interrupted before room tem-         is often the only way to achieve maxi-          MS
perature has been reached, normally at         mum hardness.
50–70°C (120–160°F).                               Martempering in salt bath produces
    However, if the quenching rate is too                                                          Martensite
                                               a good finish, high hardness and less
slow, especially with heavier cross-           risk of excessive distortion or cracking.                                              Time
sections, undersirable transformations         For certain types of steel, the tempera-     Martempering

                                                                                                                                             5
HEAT TREATMENT


Air quenching entails the least risk of            expected. Hardness in the centre of         important consideration, the choice of
excessive distortion. A tendency to-               heavy sections is even lower.               tempering temperature must often be a
wards lower hardness is noticeable at                 This effect can be critical with high    compromise. If possible, however, prior-
greater thicknesses. One disadvantage              speed steel and hot work steel, where a     ity should be given to toughness.
is a poorer finish.                                centre section can be cooled so slowly
    Some oxidation takes place when the            that carbide precipitation takes place on   How many tempers are required?
material comes into contact with air               the way down. Here, the matrix be-          Two tempers are recommended for tool
and cools slowly from the high harden-             comes depleted of carbon and carbide-       steel and three are considered neces-
ing temperatures.                                  forming alloying elements. The result is    sary for high speed steel with a high
    The choice of quenching medium                 reduced hardness and strength of the        carbon content, e.g. over 1%.
must be made from job to job, but a                core.                                           Two tempers are always recom-
general recommendation could perhaps                                                           mended. If the basic rule in quenching is
be made as follows:                                Tempering                                   followed—to interrupt at 50–70°C
                                                   The material should be tempered imme-       (120–160°F)—then a certain amount of
Temperature                                        diately after quenching. Quenching          austenite remains untransformed when
Hardening temperature                              should be stopped at a temperature of       the material is to be tempered. When
                                                   50–70°C (120–160°F) and tempering           the material cools after tempering, most
                                                   should be done at once. If this is not      of the austenite is transformed to mar-
       Oil                                         possible, the material must be kept         tensite. It is untempered. A second tem-
                       Air
      Surface
                                Salt bath          warm, e.g. in a special “hot cabinet”,      pering gives the material optimum
    MS                                             awaiting tempering.                         toughness at the hardness in question.
                         Core
                                                       The choice of tempering temperature         The same line of reasoning can be
    Room temperature                               is often determined by experience. How-     applied with regard to retained auste-
                                                   ever, certain guidelines can be drawn       nite in high speed steel. In this case,
                                            Time   and the following factors can be taken      however, the retained austenite is highly
Cooling rates for various media
                                                   into consideration:                         alloyed and slow transforming. During
                                                   • hardness                                  tempering, some diffusion takes place in
A martempering bath is the safest in               • toughness                                 the austenite, secondary carbides are
most cases.                                        • dimension change.                         precipitated, the austenite becomes
    Air is used when dimensional stabil-              If maximum hardness is desired, tem-     lower alloyed and is more easily trans-
ity is crucial.                                    per at about 200°C (390°F), but never       formed to martensite when it cools after
    Oil should be avoided and used only            lower than 180°C (360°F). High speed        tempering. Here, several temperings can
when it is necessary to achieve satisfac-          steel is normally tempered at about         be beneficial in driving the transforma-
tory hardness in heavy sections.                   20°C (36°F) above the peak of the sec-      tion of the retained austenite further to
    Three well known quenching meth-               ondary hardening temperature.               martensite.
ods have been mentioned here. Some                    If a lower hardness is desired, this
new concepts have been introduced                  means a higher tempering temperature.       Holding times in connection
with modern types of furnaces, and the             Reduced hardness does not always            with tempering
technique of quenching at a controlled             mean increased toughness, as is evident     Here also, one should avoid all compli-
rate in a protective gas atmosphere or             from the toughness values in our pro-       cated formulae and rules of thumb, and
in a vacuum furnace with gas is becom-             duct brochures. Avoid tempering within      adopt the following recommendation:
ing increasingly widespread. The cooling           temperature ranges that reduce tough-       After the tool is heated through, hold
rate is roughly the same as in air for             ness. If dimensional stability is also an   the material for at least 2 hours at full
protective gas atmosphere, but the                                                             temperature each time.
problem of oxidized surfaces is elimi-
nated. Modern vacuum furnaces have
the possibility to use overpressure
during quenching which increases the
quenching speed. The surfaces are com-
pletely clean after a vacuum hardening,
    With these techniques, as with
quenching in air, the risks of excessively
slow cooling must be borne in mind,
even for vacuum furnaces if no over-
pressure is used. The effect is that sur-            Convection type
face hardness is normally lower than               tempering furnace
6
                                                                                                                         HEAT TREATMENT



Dimensional and                                  material be stress relieved after rough         quenching can be done, the less distor-
                                                 machining. Any distorsion can then be           tion will occur due to thermal stresses.
shape stability                                  adjusted during semifinish machining               It is important that the quenching
DISTORTION DURING THE                            prior to the hardening operation.               medium is applied as uniformly as pos-
HARDENING AND TEMPERING OF                                                                       sible. This is especially valid when
TOOL STEEL                                       Thermal stresses                                forced air or protective gas atmosphere
                                                                                                 (as in vacuum furnaces) is used. Other-
When a piece of tool steel is hardened           These stresses are created when a piece
                                                                                                 wise temperature differences in the tool
and tempered, some warpage or distor-            is heated. They increase if heating takes
                                                                                                 can lead to significant distortion.
tion normally occurs. This distortion is         place rapidly or unevenly. The volume
usually greater at high temperature.             of the steel is increased by heating.
   This is well known, and it is normal          Uneven heating can result in local varia-       Transformation stresses
practice to leave some machining allow-          tions in volume growth, leading to stres-
                                                                                                 This type of stress arises when the
ance on the tool prior to hardening. This        ses and distortion.
                                                                                                 microstructure of the steel is trans-
makes it possible to adjust the tool to              As an alternative with large or com-
                                                                                                 formed. This is because the three micro-
the correct dimensions after hardening           plex parts, heating can be done in pre-
                                                                                                 structures in question—ferrite, auste-
and tempering by grinding, for example.          heating stages in order to equalize the
                                                                                                 nite and martensite—have different
                                                 temperature in the component.
                                                                                                 densities, i.e. volumes.
How does distortion take place?                                                                      The greatest effect is caused by
                                                 Linear expansion mm/100 mm
The cause is stresses in the material.                                                           transformation from austenite to mar-
These stresses can be divided into:                                                              tensite. This causes a volume increase.
• machining stresses                             0,8                                                 Excessively rapid and uneven
• thermal stresses                                                                               quenching can also cause local marten-
                                                 0,6
• transformation stresses.                                                                       site formation and thereby volume
                                                 0,4                                             increases locally in a piece and give rise
Machining stresses                                                                               to stresses in this section. These stresses
                                                 0,2
This type of stress is generated during                                                          can lead to distortion and, in some
machining operations such as turning,                                                            cases, quenching cracks.
                                                        100   200     300 400     500   600°C
milling and grinding. (For example, such
                                                                    Temperature
stresses are formed to a greater extent                                                          Volume
during cold forming operations such as           Effect of temperature on the linear expansion
blanking, bending and drawing.)                  of Uddeholm ORVAR 2 Microdized, soft
   If stresses have built up in a part,          annealed
they will be released during heating.                An attempt should always be made
Heating reduces strength, releasing              to heat slowly enough so that the
stresses through local distortion. This           temperature remains virtually equal             Trans-                      Transformation
can lead to overall distortion.                                                                   formation                   to austenite
                                                 throughout the piece.
                                                                                                  to martensite            AC1           AC3
   In order to reduce this distortion               What has been said regarding heat-                            Ms
while heating during the hardening               ing also applies to quenching. Very                                                Temperature
process, a stress relieving operation can        powerful stresses arise during quench-          Volume changes due to structural
be carried out prior to the hardening            ing. As a general rule, the slower that         transformation
operation. It is recommended that the

Yield strength Rp0,2
MPa

400

300
250
200

100


         100   200     300 400 500     600°C
                       Temperature
Effect of temperature on the yield strength of
Uddeholm Orvar 2 Microdized, soft annealed
                                                                                                                                               7
HEAT TREATMENT


HOW CAN DISTORTION                         Immediately after quenching, the tool         Nitriding is done in gas at about 510°C
BE REDUCED?                                should be sub-zero treated to –70 to –        (950°F) and in salt or gas at about
Distortion can be minimized by:            80°C (–95 to –110°F), soaking time 1–         570°C (1060°F) or as ion nitriding,
• keeping the design simple and            3 hours, followed by tempering.               normally at around 500°C (930°F). The
  symmetrical                                 The sub-zero treatment leads to a          process therefore requires steels that
                                           reduction of retained austenite content.      are resistant to tempering in order not
• eliminating machining stresses by
                                           This, in turn, will result in a hardness      to reduce the core strength.
  stress relieving after rough machining
                                           increase of 1–2 HRC in comparison to
• heating slowly during hardening
                                           not sub-zero treated tools if low tem-        Examples of applications
• using a suitable grade of steel          perature tempering is used. For high          • Nitriding is used in some cases on
• quenching the piece as slowly as         temperature tempered tools there will           prehardened plastic moulds in order
  possible, but quick enough to obtain     be little or no hardness increase and           to prevent indentation and defects on
  a correct microstructure in the steel    when referencing the normal tempering           the parting faces. It should be noted,
• tempering at a suitable temperature.     curves, a 25 to 50°C (45 to 90°F) lower         however, that a nitrided surface can-
                                           tempering temperature should be cho-            not be machined with cutting tools
The following values for machining
                                           sen to achieve the required hardness.           and can only be ground with diffi-
allowances can be used as guidelines.
                                              Tools that are high temperature              culty. A nitrided surface will cause
                   Machining allowance     tempered, even without a sub-zero               problems in weld repairing as well.
Grade of steel   on length and diameter    treatment, will normally have a low
                      as % of dimension                                                    Nitriding can also have a stress reliev-
                                           retained austenite content and in most          ing effect. Heavily machined parts
UDDEHOLM ARNE                  0,25 %
UDDEHOLM RIGOR                 0,20 %
                                           cases, a sufficient dimensional stability.      may, therefore, undergo some distor-
UDDEHOLM SVERKER 21            0,20 %      However, for high demands on dimen-             tion during nitriding due to the re-
UDDEHOLM SVERKER 3             0,20 %      sional stability in service it is also rec-     lease of residual stresses from machi-
UDDEHOLM CARMO                 0,20 %      ommended to use a sub-zero treatment            ning and in such a case, a stress re-
UDDEHOLM SLEIPNER              0,25 %      in combination with high temperature
UDDEHOLM CALDIE                0,25 %                                                      lieving between rough and finish
                                           tempering.                                      machining is recommended.
UDDEHOLM VANADIS 4 EXTRA       0,15 %
UDDEHOLM VANADIS 6             0,15 %         For the highest requirements on
                                                                                         • The life of forging dies can be in-
UDDEHOLM VANADIS 10            0,15 %      dimensional stability, sub-zero treat-
                                                                                           creased by nitriding. It must be noted,
UDDEHOLM VANADIS 23            0,15 %      ment in liquid nitrogen is recommended
UDDEHOLM VANCRON 40            0,20 %
                                                                                           though, that the treatment can give
                                           after quenching and after each temper-
UDDEHOLM CALMAX                0,20 %                                                      rise to higher susceptibility to crack-
                                           ing.
UDDEHOLM GRANE                 0,15 %                                                      ing in sharp corners. Furthermore, the
UDDEHOLM STAVAX ESR            0,15 %                                                      edge of the flash land must be given
UDDEHOLM MIRRAX ESR            0,20 %
UDDEHOLM ELMAX                 0,15 %      Surface treatment                               a rounded profile.
UDDEHOLM UNIMAX                0,30 %                                                    • Extrusion dies of Uddeholm Orvar 2
UDDEHOLM CORRAX           0,05–0,15 %
                                           NITRIDING                                       Microdized can be nitrided to advan-
UDDEHOLM ORVAR 2 MICRODIZED    0,20 %      The purpose of nitriding is to increase         tage—especially in the case of alumi-
UDDEHOLM ORVAR SUPREME         0,20 %      the surface hardness of the steel and           nium alloys. Exceptions can be pro-
UDDEHOLM VIDAR SUPERIOR        0,20 %
                                           improve its wear properties. This treat-        files with sharp corners and thin sec-
UDDEHOLM QRO 90 SUPREME        0,30 %
UDDEHOLM HOTVAR                0,40 %
                                           ment takes place in a medium (gas or            tions of the dies.
UDDEHOLM DIEVAR                0,30 %      salt) which gives off nitrogen. During
                                           nitriding, nitrogen diffuses into the steel   NITROCARBURIZING
                                           and forms hard, wear resistant nitrides.      A widely known method is nitriding in a
Note: Uddeholm Corrax is a precipita-      This results in an intermetallic surface      salt bath.
tion hardening steel. Machining allow-     layer with good wearing and frictional           The temperature is normally 570°C
ance is needed to compensate for           properties.                                   (1060°F). Due to aeration the cyanate
shrinkage during ageing. The shrinkage                                                   content of the bath can be better con-
depends on ageing temperature (see                                                       trolled and the nitriding effect is very
product information brochure). No dis-                                                   good.
tortion occurs.                                                                             A nitrocarburizing effect can also be
                                                                                         achieved in gas atmosphere at 570°C
SUB-ZERO TREATMENT                                                                       (1060°F). The results after these
Tools requiring maximum dimensional                                                      methods are comparable.
stability in service can be sub-zero                                                        The total nitriding time must be var-
treated as follows:                        Nitrided case shown at a magnification of     ied for different tool types and sizes. In
                                           100X Uddeholm Orvar 2 Microdized              the case of large sizes, the heating time

8
                                                                                                                    HEAT TREATMENT


to the specified nitriding temperature        ceive a thicker deposit than large flat        Certain demands are put on the tool
can be considerably longer than in the        surfaces or the holes. If the chromium         steel depending on: coating method, the
case of small tools.                          layer is damaged, the exposed steel may        design of the tool and the tolerances
                                              corrode rapidly.                               needed. PVD coating is used for the
ION NITRIDING                                    Another advantage of the chromium           highest demands on tolerances. When
This is a new nitriding technology. The       layer is that it greatly reduces the coeffi-   using this method a tool steel with high
method can be summarized as follows:          cient of friction on the surface.              tempering resistance must be used and
    The part to be nitrided is placed in a       During the chromium plating process,        the surface coating has to be performed
process chamber filled with gas, mainly       hydrogen absorption can cause a brittle        as the last operation, after the heat
nitrogen. The part forms the cathode          surface layer. This nuisance can be elimi-     treatment. At CVD coating, hardening
and the shell of the chamber the anode        nated by tempering immediately after           and tempering are done after the coat-
in an electric circuit. When the circuit is   plating at 180°C (360°F) for 4 hours.          ing. When using the CVD method there
closed, the gas is ionized and the part is                                                   is a risk for dimensional changes. The
subjected to ion bombardment. The gas         SURFACE COATING                                method is therefore not recommended
serves both as heating and nitriding                                                         for tools with requirements of very
                                              Surface coating of tool steel is becom-
medium.                                                                                      narrow tolerances.
                                              ing more common. Not only for cold
    The advantages of ion nitriding in-                                                          The most suitable steels for the men-
                                              work applications, but also for plastic
clude a low process temperature and a                                                        tioned methods are Uddeholm Vanadis
                                              moulds and hot work dies.
hard, tough surface layer. The depth of                                                      4 Extra, Uddeholm Vanadis 6, Uddeholm
                                                  The hard coating normally consists
diffusion is of the same order as with                                                       Vanadis 10, Uddeholm Vanadis 23 and
                                              of titanium nitride and/or titanium
gas nitriding.                                                                               Uddeholm Caldie.
                                              carbide. The very high hardness and low
                                                                                                 Surface coating of tools and moulds
                                              friction gives a very wear resistant
                                                                                             should be discussed from case to case
                                              surface, minimizing the risk for adhe-
                                                                                             considering the application, coating
                                              sion and sticking.
                                                                                             method and tolerance requirements .
                                                  To be able to use these properties in
                                              an optimal way one has to choose a
                                              tool steel of high quality or a powder
                                              metallurgy manufactured steel as sub-
                                              strate. The two most common coating
                                              methods are:
Ion nitriding plant                           • PVD coating: performed at 200–
                                                  500°C (390–930°F) (PVD = Physical
                                                  Vapour Deposition).
                                              • CVD coating: performed at about
CASE HARDENING
                                                  1000°C (1830°F) (CVD = Chemical
In this method, the steel is heated in a          Vapour Deposition).
medium that gives off carbon (gas, salt
or dry carburizing compound). The car-
bon diffuses into the surface of the
material and after hardening this gives
a surface layer with enhanced hardness
and wear resistance. This method is
used for structural steel, but is not gen-
erally recommended for alloy tool steels.

HARD CHROMIUM PLATING
Hard chromium plating can improve the
wear resistance and corrosion resistance
of a tool. Hard chromium plating is done
electrolytically. The thickness of the
plating is normally between 0,001 and
0,1 mm (0,00004–0,004 inch). It can be
difficult to obtain a uniform surface
layer, especially on complex tools, since
projecting corners and edges may re-          Coated tools


                                                                                                                                    9
HEAT TREATMENT



Testing                                                            Vickers (HV)
                                                                   In Vickers hardness testing a pyramid-
of mechanical                                                      shaped diamond with a square base
properties                                                         and a peak angle of 136° is pressed
                                                                   under a load F against the material
When the steel is hardened and tem-                                whose hardness is to be determined.
pered, its strength is affected, so let us                         After unloading, the diagonals d1 and d2
take a closer look at how these proper-                            of the impression are measured and the
ties are measured.                                                 hardness number (HV) is read off a
                                                                   table.
HARDNESS TESTING
                                                                       When the test re-
Hardness testing is the most popular                                                                    F
                                                                   sults are reported,
way to check the results of hardening.
                                                                   Vickers hardness is
Hardness is usually the property that is
                                                                   indicated with the
specified when a tool is hardened.                                                                            136°
                                                                   letters HV and a
    It is easy to test hardness. The mate-                                                                                      d1                  d2
                                                                   suffix indicating the
rial is not destroyed and the apparatus
                                                                   mass that exerted the
is relatively inexpensive. The most com-
                                                                   load and (when re-
mon methods are Rockwell C (HRC),
                                                                   quired) the loading
Vickers (HV) and Brinell (HBW).
                                                                   period, as illustrated by
    We shouldn’t entirely forget the old                                                                               Principle of Vickers hardness testing
                                                                   the following example:
expression “file-hard”. In order to check
                                                                   HV 30/20 = Vickers
whether hardness is satisfactory, for
                                                                   hardness determined
example above 60 HRC, a file of good                                                                          Brinell (HBW)
                                                                   with a load of 30 kgf
quality can provide a good indication.                                                                        In Brinell hardness testing, a Tungsten
                                                                   exerted for 20 seconds.
                                                                                                              (W) ball is pressed against the material
Rockwell (HRC)                                                                                                whose hardness is to be determined.
In Rockwell hardness testing, a conical                                                                       After unloading, two measurements of
diamond is first pressed with a force F0,                                                                     the diameter of the impression are
and then with a force F0+F1 against a                                                                         taken at 90° to each other (d1 and d2)
specimen of the material whose hard-                                                                          and the HBW value is read off a table,
ness is to be determined. After unload-                                                                       from the average of d1 and d2.
ing to F0, the increase (e) of the depth                                                                          When the test results are reported,
of the impression caused by F1 is deter-                                                                      Brinell hardness is indicated with the
mined. The depth of penetration (e) is                                                                        letters HBW and a suffix indicating ball
converted into a hardness number                                                                              diameter, the mass with which the load


                            F0                     F0+F1=F           F0                                                              D


                  h0                          h               e            HRC
                                                                                                        F


                                                                                                                                     d
               Surface of specimen
                                                                                                                                                     h
                      100                         h0
                             Hardness scale




                                                        e
               0,2 mm
                                                  h
                                                             HRC
                        0
                                                                                                                        Principle of Brinell hardness testing

Principle of Rockwell hardness testing                                                                        was exerted and (when required) the
                                                                                                              loading period, as illustrated by the
(HRC) which is read directly from a scale                                                                     following example: HBW 5/750/15 =
on the tester dial or read-out.                                                                               Brinell hardness determined with 5 mm
                                                                                                              Tungsten (W) ball and under load of
                                                                                                              750 kgf exerted for 15 seconds.

10
                                                                                                                  HEAT TREATMENT


TENSILE STRENGTH                              For the most part, tool steel has a
                                              rather low toughness by reason of its
                                                                                          Some words
Tensile strength is determined on a test
piece which is gripped in a tensile test-     high strength. Materials of low tough-      of advice to tool
ing machine and subjected to a succes-        ness are notch sensitive, for which
                                              reason smooth, un-notched specimens
                                                                                          designers
sively increasing tensile load until frac-
ture occurs. The properties that are          are often used in the impact testing of     CHOICE OF STEEL
normally recorded are yield strength          tool steels. The results of the tests are   Choose air-hardening steels for complex
Rp0,2 and ultimate tensile strength Rm,       commonly stated in joules, or alterna-      tools.
while elongation A5 and reduction of          tively in kgm (strictly speaking kgfm),
area Z are measured on the test piece.        although J/cm2 or kgm/cm2 is some-          DESIGN
In general, it can be said that hardness      times used instead, specially in Charpy     Avoid:
is dependent upon yield strength and          U testing.
                                                                                          • sharp corners
ultimate tensile strength, while elonga-          There are several other variants of
                                                                                          • notch effects
tion and reduction of area are an indica-     impact testing which are used outside
                                                                                          • large differences in section
tion of toughness. High values for yield      Sweden, e.g. DVM, Mesanger and—
                                              especially in English speaking coun-          thicknesses.
and ultimate tensile strength generally
mean low values for elongation and            tries—Izod.                                    These are often causes of quench
reduction of area.                                                                        cracks, especially if the material is
    Tensile tests are used mostly on                                                      cooled down too far or allowed to stand
structural steels, seldom on tool steels.                                                 untempered.
It is difficult to perform tensile tests at
hardnesses above 55 HRC. Tensile tests
may be of interest for tougher types of
                                                                                             Unsuitable        Preferred
tool steel, especially when they are used                                                    design            alternative
as high strength structural materials.
These include e.g. Uddeholm Impax                                                                                            Fillet
Supreme and Uddeholm Orvar 2 Micro-




                                                                                                  ✗
dized.

IMPACT TESTING
A certain quantity of energy is required
to produce a fracture in a material. This




                                                                                                  ✗
quantity of energy can be used as a
measure of the toughness of the mate-
rial, a higher absorption of energy indi-
cating better toughness. The most com-
mon and simplest method of determin-
ing toughness is impact testing. A rigid
pendulum is allowed to fall from a
known height and to strike a test speci-
men at the lowest point of its swing.
The angle through which the pendulum
travels after breaking the specimen is                                                    HEAT TREATMENT
measured, and the amount of energy                                                        Choose suitable hardnesses for the ap-
that was absorbed in breaking the                                                         plication concerned. Be particularly care-
specimen can be calculated.                                                               ful to avoid temperature ranges that can
    Several variants of impact testing                                                    reduce toughness after tempering.
are in use. The various methods differ in                                                     Keep the risk of distortion in mind
the shape of the specimens. These are                                                     and follow recommendations concerning
usually provided with a V- or U-shaped                                                    machining allowances.
notch, the test methods being then                                                            It is a good idea to specify stress
known as Charpy V and Charpy U re-                                                        relieving on the drawings.
spectively.




                                                                                                                                      11
                Network of excellence
                Uddeholm is present on every continent. This ensures you
                high-quality Swedish tool steel and local support wherever you
                are. Assab is our wholly-owned subsidiary and exclusive sales
                channel, representing Uddeholm in various parts of the world.
                Together we secure our position as the world’s leading supplier
                of tooling materials.




www.assab.com                                                                     www.uddeholm.com
                                                                               HAGFORS KLARTEXT U0805XX
Uddeholm is the world’s leading supplier of tooling materials. This
is a position we have reached by improving our customers’ everyday
business. Long tradition combined with research and product develop-
ment equips Uddeholm to solve any tooling problem that may arise.
It is a challenging process, but the goal is clear – to be your number one
partner and tool steel provider.


Our presence on every continent guarantees you the same high quality
wherever you are. Assab is our wholly-owned subsidiary and exclusive
sales channel, representing Uddeholm in various parts of the world.
Together we secure our position as the world’s leading supplier of
tooling materials. We act worldwide, so there is always an Uddeholm
or Assab representative close at hand to give local advice and support.
For us it is all a matter of trust – in long-term partnerships as well as in
developing new products. Trust is something you earn, every day.


For more information, please visit www.uddeholm.com or www.assab.com

				
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