HEAT TREATMENT OF TOOL STEEL
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
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-
After one tempering at high tem-
perature, the microstructure consists of
tempered martensite, newlyformed
martensite, some retained austenite and
Unit cell in a martensite crystal carbides.
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.
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-
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
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-
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
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.
perature difference between the surface This can lead to excessive bainite trans-
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
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-
Salt bath warm, e.g. in a special “hot cabinet”, pering gives the material optimum
MS awaiting tempering. toughness at the hardness in question.
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
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-
• 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
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
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
100 200 300 400 500 600°C
Effect of temperature on the yield strength of
Uddeholm Orvar 2 Microdized, soft annealed
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-
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
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
Ion nitriding plant • PVD coating: performed at 200–
500°C (390–930°F) (PVD = Physical
• CVD coating: performed at about
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
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
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
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
Surface of specimen
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.
TENSILE STRENGTH For the most part, tool steel has a
rather low toughness by reason of its
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
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
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-
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.
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.
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