HEAT TREATMENT OF

HEAT TREATMENT OF TOOL STEEL

1 Cover photos from left to right: Böhler Uddeholm Czech Republic, Uddeholms AB/HÄRDtekno, Sweden and Ionbond Sweden AB.

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, 03.2012 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 OF TOOL STEEL

CONTENTS

What is tool steel? 4 Hardening and tempering 4 Dimensional and shape stability 11 Surface treatment 12 Testing of mechanical properties 14 Some words of advice to tool designers 15 Hardness after hardening and tempering 17 Hardness conversion table 18

3 HEAT TREATMENT OF TOOL STEEL

The purpose of this brochure is to lighter parts, greater precision and Hardening provide a general idea of how tool increased reliability. steel is heat treated and how it Uddeholm has concentrated its and tempering behaves during this process. Special tool steel range on high alloyed types When a tool is hardened, many attention is paid to hardness, tough- of steel, intended primarily for pur- factors influence the result. ness and dimensional stability. poses such as plastic moulding, blan- king and forming, die casting, extru- sion, forging, wood-working industry, Some theoretical aspects What is tool steel? recycling industry and component In soft annealed condition, most of business. Powder metallurgy (PM) the carbide-forming alloying elements To ol steels are high-quality steels steels are also included in the range. are bound up with carbon in car- made to controlled chemical compo- To ol steel is normally delivered in bides. sition and processed to develop the soft annealed condition; this When the steel is heated up to properties useful for working and makes the material easy to machine hardening temperature, the matrix is shaping of other materials. The car- with cutting tools and it provides a transformed from ferrite to . bon content in tool steels may range microstructure suitable for harden- This means that the Iron atoms from as low as 0.1% to as high as ing. change their position in the atomic more than 1.6% C and many are The soft annealed microstructure lattice and generate a new lattice alloyed with alloying elements such as consists of a soft matrix in which with different crystallinity. , and vana- carbides are embedded. See picture dium. below. = Iron atoms To ol steels are used for applica- In , these carbides are = Possible positions for tions such as blanking and forming, Iron carbides, while in alloyed steel carbon atoms plastic moulding, die casting, extru- they are chromium (Cr), tungsten sion and forging. (W), molybdenum (Mo) or vanadium Alloy design, the manufacturing (V) carbides, depending on the route of the steel and quality heat composition of the steel. Carbides treatment are key factors in order to are compounds of carbon and develop tools or parts with the en- alloying elements and are character- hanced properties that only tool ized by very high hardness. Higher steel can offer. carbide content means a higher 2.86 A Benefits like durability, strength, resistance to wear. Unit cell in a ferrite crystal. corrosion resistance and high-tem- Also non-carbide forming alloying Body centred cubic (BCC). perature stability are also attractive elements are used in tool steel, such for other purposes than pure tool as cobalt (Co) and nickel (Ni) which applications. For this reason, tool are dissolved in the matrix. Cobalt is steel is a better choice than con- normally used to improve red hard- struction or engineering steel for ness in high speed steels, while nickel strategic components in the different is used to improve through-hardening industries. properties and also increase the More advanced materials easily toughness in the hardened condi- result in lower maintenance costs, tions. 3.57 A Unit cell in an austenite crystal. Face centred cubic (FCC).

2.98 A

Uddeholm Dievar, soft 20µm 2.85 A annealed structure. Unit cell in a martensite crystal. Tetragonal. 4 HEAT TREATMENT OF TOOL STEEL

Austenite has a higher solubility limit Precipitated secondary (newly It is possible to make use of different for carbon and alloying elements, and formed) carbides and newly formed combinations of these factors that the carbides will dissolve into the martensite can increase hardness will result in the same hardness level. matrix to some extent. In this way during high temperature tempering. Each of these combinations corre- the matrix acquires an alloying con- Typical of this is the so called sec- sponds to a different heat treatment tent of carbide-forming elements that ondary hardening of e.g. high speed cycle, but certain hardness does not gives the hardening effect, without steels and high alloyed tool steels. guarantee any specific set of proper- becoming coarse grained. Usually a certain hardness level is ties of the material. The material If the steel is quenched sufficiently required for each individual applica- properties are determined by its rapidly in the hardening process, the tion of the steel, and therefore heat microstructure and this depends on carbon atoms do not have the time treatment parameters are chosen to the heat treatment cycle, and not on to reposition themselves to allow the some extent in order to achieve the the obtained hardness. reforming of ferrite from austenite, as desired hardness. It is very important Quality heat treatment delivers not in for instance annealing. Instead, they to have in mind that hardness is the only desired hardness but also opti- are fixed in positions where they mized properties of the material for really do not have enough room, and Hardness the chosen application. the result is high micro-stresses that To ol steels should always be at contribute to increased hardness. C least double tempered. The second This hard structure is called marten- B tempering takes care of the newly site. Thus, martensite can be seen as a D formed martensite during cooling forced solution of carbon in ferrite. after the first tempering. When the steel is hardened, the Three temperings are recom- matrix is not completely converted mended in the following cases: A into martensite. There is always some • high speed steel with high carbon Tempering temperature austenite that remains in the struc- content ture and it is called retained austenite. A = martensite tempering B = carbide precipitation • complex hot work tools, especially The amount increases with increasing C = transformation of retained austenite to in the case of die casting dies alloying content, higher hardening martensite • big moulds for plastic applications temperature, longer soaking times D = tempering diagram for high speed steel and high alloy tool steel • when high dimension stability is and slower quenching. A+B+C = D a demand (such as in the case of After quenching, the steel has a gauges or tools for integrated microstructure consisting of marten- The diagram shows the influence of different factors on the secondary circuits) site, retained austenite and carbides. hardening. This structure contains inherent stresses that can easily cause crack- ing. But this can be prevented by result of several different factors, reheating the steel to a certain tem- such as the amount of carbon in the perature, reducing the stresses and martensitic matrix, the micro- transforming the retained austenite stresses contained in the material, the to an extent that depends upon the amount of retained austenite and the reheating temperature. This reheating precipitated carbides during temper- after hardening is called tempering. ing. Hardening of tool steel should always be followed immediately by temper- ing. It should be noted that tempering at low temperatures only affects the martensite, while tempering at high temperature also affects the retained austenite. After one tempering at a high tem- perature the microstructure consists of tempered martensite, newly formed martensite, some retained Uddeholm Dievar, austenite and carbides. 20µm hardened structure.

5 HEAT TREATMENT OF TOOL STEEL

Stress relieving exceptions cheaper than making di- In the case of big tools with complex Distortion due to hardening must be mensional adjustments during finish geometry a third preheating step taken into account when a tool is machining of a hardened tool. close to the fully austenitic region is recommended. rough machined. Rough machining The correct work sequence before hard- causes thermal and mechanical ening operaiton is: stresses that will remain embedded rough machining, stress relieving and Holding time at in the material. This might not be semi-finish machining. hardening temperature significant on a symmetrical part of It is not possible to briefly state exact simple design, but can be of great recommendations to cover all heat- importance in an asymmetrical and Heating to ing situations. complex machining, for example of hardening temperature Factors such as furnace type, hard- one half of a die casting die. Here, As has already been explained, ening temperature, the weight of the stress-relieving heat treatment is stresses contained in the material charge in relation to the size of the always recommended. will produce distortion during heat furnace, the geometry of the different This treatment is done after rough treatment. For this reason, thermal parts in the charge, etc., must be machining and before hardening and stresses during heating should be taken into consideration in each case. entails heating to 550–700°C (1020– avoided. The use of thermocouples permits 1300°F). The material should be The fundamental rule for heating an overview of the temperature in heated until it has achieved a uniform to hardening temperature is there- the different areas of the various temperature all the way through, fore, that it should take place slowly, tools in the charge. where it remains 2–3 hours and then increasing just a few degrees per The ramping step finishes when the cooled slowly, for example in a fur- minute. In every heat treatment, the core of the parts in the furnace reach nace. The reason for a necessary heating process is named ramping. the chosen temperature. Then the slow cooling is to avoid new stresses The ramping for hardening should be temperature is maintained constant of thermal origin in the stress-free made in different steps, stopping the for a certain amount of time. This is material. process at intermediate tempera- called holding time. The idea behind stress relieving is tures, commonly named preheating The generally recommended hold- that the yield strength of the material steps. The reason for this is to equal- ing time is 30 minutes. In the case of at elevated temperatures is so low ise the temperatures between the high speed steel, the holding time will that the material cannot resist the surface and the centre of the part. be shorter when the hardening tem- stresses contained in it. The yield Typically choosen preheating tem- perature is over 1100°C (2000°F). If strength is exceeded and these peratures are 600–650°C (1100– the holding time is prolonged, micro- stresses are released, resulting in a 1200°F) and 800–850°C (1450– structural problems like grain growth greater or lesser degree of plastic 1560°F). can arise. deformation. The excuse that stress relieving takes too much time is hardly valid when the potential consequences are considered. Rectifying a part during semi-finish machining is with few

MPa

Yield strength

Residual stresses contained in the material

Plastic deformation

Temperature

. The use of thermocouples gives an overview of the temperature in different areas during heat treatment. Photo: Böhler Uddeholm Czech Republic

6 HEAT TREATMENT OF TOOL STEEL

Quenching Temperature in two steps. First it is cooled from the hardening temperature until the The choice between a fast and a slow A C3 temperature at the surface is just quenching rate is usually a compro- AC1 above the M temperature. Then it mise. To get the best microstructure s and tool performance the quenching must be held there until the tem- rate should be rapid. To minimize perature has been equalised between distortion, a slow quenching rate is the surface and the core. After this, recommended. the cooling process continues. This Slow quenching results in less method permits the core and the temperature difference between the Core surface to transform into martensite surface and the core of a part, and Surface at more or less the same time and sections of different thickness will diminishes thermal stresses. Step have a more uniform cooling rate. quenching is also a possibility when This is of great importance when quenching in vacuum furnaces. M quenching through the martensite S The maximum cooling rate that can be obtained in a part depends on the range, below the Ms temperature. Martensite formation leads to an heat conductivity of the steel, the increase in volume and stresses in Martensite cooling capacity of the quenching the material. This is also the reason media and the cross-section of the why quenching should be interrupted part. The quenching process as expressed in a before room temperature has been CCT graph. reached, normally at 50–70°C (120– 160°F). Temperature However, if the quenching rate is Air hardening is reserved for steel Hardening temperature too slow, especially with heavier with high hardenability, which in most cross-sections, undesirable transfor- of the cases is due to the combined Oil mations in the microstructure can presence of , chrome and Air take place, risking a poor tool per- molybdenum. Polymer Vacuum formance. Risk of distortion and hardening Salt bath MS Quenching media used for alloyed cracks can be reduced by means of Water steel nowadays are: hardening oil, step quenching or martempering. In Room temperature polymer solutions, air and inert gas. this process the material is quenched Time Cooling rates for various media.

Temperature A poor quenching rate will lead to carbide precipitation at the grain A C3 boundaries in the core of the part, A C1 and this is very detrimental to the mechanical properties of the steel. Also the obtained hardness at the surface of larger parts could be lower Batch prepared for heat treatment. Photo: Böhler Uddeholm Czech Republic. for tools with bigger cross-sections Core than that for smaller parts, as the high amount of heat that has to be It is still possible to find some heat transported from the core through treatment shops that use salt baths, Surface the surface produces a self-tempering but this technique is disappearing due MS effect. to environmental aspects. Oil and polymer solutions are usually utilised for low alloyed steel Martensite and for tool steel with low carbon Time contents. Martempering or step-quenching.

7 HEAT TREATMENT OF TOOL STEEL

SOME PRACTICAL ISSUES VACUUM TECHNOLOGY •When the furnace reaches a tem- At high temperature, steel is very Vacuum technology is the most used perature of approx. 850°C (1560°F), likely to suffer oxidation and varia- technology nowadays for hardening the effect of radiation heating tions in the carbon content (carburi- of high alloyed steel. mechanisms will overshadow that of zation or decarburization). Protected Vacuum heat treatment is a clean the convection ones in the heat atmospheres and vacuum technology process, so the parts do not need to transfer process. Therefore the are the answer to these problems. be cleaned afterwards. It also offers a Nitrogen pressure is lowered, in Decarburization results in low sur- reliable process control with high order to optimize the effects of face hardness and a risk of cracking. automation, low maintenance and radiation and convection heating Carburization, on the other hand, environmental friendliness. All these mechanisms are negligible under can result in two different problems: factors make vacuum technology these new physical conditions. The • the first and easiest to identify is especially attractive for high-quality new value of the nitrogen pressure the formation of a harder surface parts. is around 0.7 mbar. The reason for layer, which can have negative having this remaining pressure is to effects avoid sublimation of the alloying • the second possible problem is retained austenite at the surface Top gas flap Heating elements Retained austenite can in many cases be confused with ferrite when ob- Furnace Heat exchanger serving it through the optical micro- vessel scope. These two phases also have similar hardness, and therefore, what at first sight can be identified as a decarburization can in some cases be

Bottom gas flap Cooling fan

Hot zone Convection fan

Cooling phase, top cooling. Illustration from Schmetz GmbH Vacuum Furnaces, Germany.

The different steps in the functioning elements, i.e. to avoid the loss of of a vacuum furnace can schematically alloying elements to the vacuum. be listed as follows: This low pressure condition will be •When the furnace is closed after maintained invariant during the last charging operation, air is pumped part of the heating process, as well Batch type furnace with controlled out from the heating chamber in as during the holding time at the atmosphere. Photo: Bodycote Stockholm, order to avoid oxidation. chosen hardening temperature. Sweden. • An inert gas (most commonly • The cooling down will be carried Nitrogen) is injected into the heat- out by a massive injection of inert gas (most commonly nitrogen) the completely opposite problem. ing chamber until a pressure of into the heating chamber in alter- For these reasons it is very impor- around 1–1.5 bar is reached. tant that the atmosphere in which • The heating system is started. the heat treatment takes place does The presence of the inert gas will not affect the carbon content of the make possible the heat transfer part. process through convection mecha- Wrapping in a hermetically closed nisms. This is the most efficient way stainless-steel foil also provides some to heat up the furnace to a tem- protection when heating in a muffle perature of approx. 850°C(1560°F). furnace. The steel foil should be re- moved before quenching. Hot zone with graphite insulation. Photo: Schmetz GmbH Vacuum Furnaces, Germany.

8 HEAT TREATMENT OF TOOL STEEL

Vertical cooling

From top to bottom From bottom to top

Horizontal cooling

From right to left From left to right

Cooling phase. Nitrogen gas stream passes through the heating chamber in different directions. Illustration from Schmetz GmbH Vacuum Furnaces, Germany.

nating directions and reaching the overpressure that was previously chosen when programming the furnace. The maximum over- pressure is a nominal characteristic of each furnace and it gives an idea of its cooling capacity.

Charging operation. Photo: Böhler Uddeholm Czech Republic.

Vacuum furnace. Photo: Schmetz GmbH Vacuum Furnaces, Germany.

9 HEAT TREATMENT OF TOOL STEEL

Tempering Precipitation of secondary carbides The basic rule of quenching is to The material should be tempered will occur when tempering highly interrupt at 50–70°C (120–160°F). immediately after quenching. alloyed steel at a high temperature. Therefore a certain amount of auste- Quenching should be stopped at a This will be detrimental to its corro- nite remains untransformed when the sion resistance but will give to it material is ready to be tempered. temperature of 50–70°C (120–160°F) and tempering should be done at somewhat higher wear resistance. If When the material cools after tem- once. If this is not possible, the the tool is to be electrical discharge pering, most of the austenite is trans- material must be kept warm, e.g. in a machined (EDM) or coated, high- formed to newly formed martensite special “hot cabinet” awaiting tem- temperature tempering is necessary. (untempered). A second tempering pering. gives the material optimum tough- HOW MANY TEMPERS ness at the chosen hardness level. Please, notice that the stresses ARE REQUIRED? contained in the as-quenched Two tempers are generally recom- HOLDING TIMES material can result in breakage mended for tool steel, except in the IN CONNECTION WITH of the crystalline structure and cases of large cross-sections, parts TEMPERING the formation of cracks if the with complex geometries or very Here there is also a general rule, tempering is not done immedi- high demands on dimensional sta- applicable in most of the cases: once ately after the quenching pro- bility. In these cases, a third tempering the tool has been heated through, cess. This breakage of the crys- is usually needed. hold it for at least two hours at full talline structure can take place temperature each time. in a violent way. Therefore the importance of tempering as soon as possible is not only to safeguard the part from cracks, Austenite but it is also a matter of per- Untempered martensite sonal safety. Tempered martensite Uddeholm has made a wide range of Retained experiments and measurements and austenite collected the resulting data regard- } ing hardness, toughness, dimensional changes and retained austenite in graphs. These graphs are available for the different steel grades and are of great help in order to choose the Evolution of the phase correct tempering temperature. After After After first After second After third content along the heating quenching tempering tempering tempering* The first priority when choosing different steps of the *HSS steel and big high-pressure die casting dies the tempering temperature should be heat treatment. the mechanical properties, as some small dimensional adjustments can be made in a last fine machining step. The mechanical and physical proper- ties obtained after tempering will depend greatly on the chosen tem- pering temperature. High-tempera- ture tempering will result in a lower content of retained austenite than low-temperature tempering. The material will therefore have higher compressive strength and improved dimensional stability (in service and A lower die for at surface coating). aluminium rim just When tempering at high tempera- before heat treatment ture, other differences in properties on charging grid. are also noticeable, like higher heat Photo: ASSAB Çelik conductivity. (Turkey)

10 HEAT TREATMENT OF TOOL STEEL

Dimensional and In order to reduce distortion while thermal stresses. But as earlier men- heating during the hardening process, tioned, a faster quenching will result shape stability a stress relieving operation should be in better mechanical properties. carried out prior to the hardening It is important that the quenching Distortion during operation. It is recommended to medium is applied as uniformly as hardening and tempering of stress relieve the material after rough possible. This is especially valid when tool steel machining. Any distortion can then be forced air or protective gas atmos- adjusted during semi-finish machining phere (as in vacuum furnaces) is used. When a piece of tool steel is hard- prior to hardening operation. Otherwise temperature differences ened and tempered, some warpage in the tool can lead to significant or distortion normally occurs. This is distortion. well known and it is normal practice THERMAL STRESSES to leave some machining allowance Thermal stresses arise every time on the tool prior to hardening, mak- there is a temperature gradient in the TRANSFORMATION STRESSES ing it possible to adjust the tool to material, i.e. when the temperature is Transformation stresses arise when the correct dimensions after harden- not even all over the part. the microstructure of the steel is ing and tempering by grinding, for Thermal stresses grow with in- transformed. This is because the example . creasing heating rate. Uneven heating three phases in question—ferrite, austenite and martensite—have How does distortion can result in local variations in vol- ume due to uneven dilatation rates different densities, i.e. volumes. take place? and this will also contribute to the Out of all the microstructural The cause is stresses in the material. arising of stresses and distortion. changes that take place during heat These stresses can be divided into: In order to tackle this problem, it treatment, the biggest contribution •machining stresses is common practice to heat up the to transformation stresses is caused • thermal stresses material in steps, in order to equalise by the transformation of austenite • transformation stresses the temperature between the surface into martensite. This causes a volume and the centre. increase. Excessively rapid and uneven MACHINING STRESSES quenching can also cause local mar- Linear expansion mm/100 mm Machining stresses are generated tensite formation and thereby vol- during machining operations such as ume increases locally in a piece and turning, milling and grinding or any 0.8 gives rise to stresses in this section. type of cold working. These stresses can lead to distortion 0.6 If stresses have built up in a part, and, in some cases, hardening cracks. they will be released during heating. 0.4 Heating reduces strength, releasing 0.2 Volume stresses through local distortion. This can lead to overall distortion. 100 200 300 400 500 600 °C 210 390 570 750 930 1110 °F Temperature Yield strength Rp0.2 MPa Effect of temperature on the linear expan- Transformation sion of Uddeholm ORVAR Supreme, soft Trans- to austenite 400 annealed. formation 350 to martensite M AC1 A 300 s C3 250 Temperature 200 An attempt should always be made to 150 heat slowly enough so that the tem- Volume changes due to structural 100 perature remains virtually equal transformation. 50 throughout the piece. 100 200 300 400 500 600 °C What has been said regarding 210 390 570 750 930 1110 °F heating, applies also to cooling. Temperature Very powerful stresses arise during Effect of temperature on the yield quenching. As a general rule, the strength of Uddeholm Orvar Supreme, slower quenching can be done, the soft annealed. less distortion will occur due to

11 HEAT TREATMENT OF TOOL STEEL

How can distortion SUB-ZERO TREATMENT each tempering, Always finish with a be reduced? Retained austenite in a tool can tempering as last operation, in order to avoid the existence of untempered Distortion can be minimized by: transform into martensite during martensite in the part. •keeping the design simple and service. This will lead to local distor- symmetrical tion and embrittlement of the tool • eliminating machining stresses due to the presence of untempered by stress relieving after rough martensite. Therefore the require- Surface treatment machining ment of maximum dimensional stabil- • heating up slowly to hardening ity in service has an implied demand Nitriding temperature for very low or no retained austenite Nitriding is performed by exposing • using a suitable grade of steel content. This can be achieved by the parts to some media rich in • quenching the piece as slowly as using sub-zero treatment after nitrogen under certain physical con- possible, but quick enough to quenching or by high temperature ditions that will result in the diffusion obtain a correct microstructure tempering. of nitrogen atoms into the steel and in the steel The sub-zero treatment leads to the formation of nitrides. The part •by usage of martempering or step a reduction of retained austenite surface will then be harder and have quenching content by exposing the tool or part a higher wear resistance in its outer • tempering at a suitable temperature to very low temperatures. The most layer. commonly used are about -80°C In the case of corrosion resistant The following values for machining (-110°F) and -196°C (-320°F). This, in steel with high-chromium content, it allowances can be used as guidelines. turn, will result in a hardness in- is very important to take into con- crease of up to 1–2 HRC, in com- sideration the fact that nitriding has a Machining allowance parison to non sub-zero treated detrimental effect on the corrosion Uddeholm on length and diameter tools, if low temperature tempering Steel grade as % of dimension resistance of the material. In other is used. For high temperature tem- cases nitriding can have a positive ARNE 0,25 % pered tools there will be little or no effect on the corrosion resistance. 0,25 % hardness increase. CALMAX/CARMO 0,20 % Appropriate steel to be nitrided CHIPPER/VIKING 0,20 % Tools that are high temperature are usually medium-carbon steel RIGOR 0,20 % tempered, even without a sub-zero with nitride-forming elements such SLEIPNER 0,25 % treatment, will normally have a low as chromium, aluminium, molyb- SVERKER 3 0,20 % retained austenite content and in denum and vanadium. 0,20 % most cases, a sufficient dimensional The core should act as a stable VANADIS 4 EXTRA 0,15 % stability. However, for high demands VANADIS 6 0,15 % substrate regarding mechanical prop- VANADIS 10 0,15 % on dimensional stability in service it erties and microstructure. This VANADIS 23 0,15 % is also recommended to use a sub- means that for hardened material it VANCRON 40 0,20 % zero treatment in combination with is necessary to temper above the CORRAX 0,05–0,15 %* high temperature tempering. nitriding temperature in order to ELMAX 0,15 % For the highest requirements on MIRRAX ESR 0,20 % avoid softening of the core during STAVAX ESR 0,15 % dimensional stability, sub-zero treat- the nitriding process. UNIMAX 0,30 % ment in liquid nitrogen is recom- It should be noted that a nitrided ALVAR 0.20 % mended after quenching and after surface cannot be machined with ALVAR 14 0.20 % DIEVAR 0,30 % HOTVAR 0,30 % No treatment ORVAR 2 MICRODIZED 0,20 % Sub-zero treatment ORVAR SUPREME 0,20 % Hardness HRC Retained austenite % QRO 90 SUPREME 0,30 % 75 24 VIDAR SUPERIOR 0,25 % 70 Hardness 21 THG 2000 0.20 % 65 18 * Depending on ageing temperature 60 15 Uddeholm Sleipner. 55 12 Hardness and retained 50 9 austenite as function of 45 Retained austenite 6 tempering temperature 40 3 with and without sub-zero 35 treatment. 150 250 350 450 550 650°C 300 480 660 840 1020 1200°F Tempering temperature °C

12 HEAT TREATMENT OF TOOL STEEL cutting tools and can only be ground for this purpose. The temperature Surface coating with difficulty. A nitrided surface will range for this process is 550°C to Surface coating of tool steel has cause problems in weld repairing as 580°C (1020°F to 1075°F) and the become a common practise. The well. time of exposure is between 30 min- general aim for these kinds of pro- There are several technologies utes and 5 hours. cesses is to generate an outer layer available in the field of nitriding; the After the exposure the part should with a very high hardness and low main ones are gas nitriding, high pres- be cooled down rapidly. friction that results in good wear sure nitriding (carried out in vacuum resistance, minimising the risk for furnaces) and plasma nitriding. Case hardening adhesion and sticking. To be able to Two common problems of con- use these properties in an optimal Case hardening is a process in which ventional nitriding technologies are way a tool steel of high quality should a finished part is exposed to a possible over-tempering of the sub- be chosen. carburizing atmosphere and high strate material and thickening of the The most commonly used coating temperature simultaneously. The nitrided layer in the sharp corners. methods are: Pulsed plasma nitriding technology temperature range is 850°C–950°C • physical vapour deposition diminishes the possibility of over- (1560°F–1740°F). This exposure coating (PVD coating) tempering by applying the plasma generates a layer with higher carbon intermittently on the part. This pro- content, normally 0.1–1.5 mm thick. • chemical vapour deposition vides a better control over the local After the layer has been formed, the coating (CVD coating) temperatures during the process. part is to be quenched in order for Chemical vapour deposition coating Active screen plasma nitriding is also a the layer to transform into marten- can also be carried out with a development of plasma nitriding tech- site with higher carbon content, and plasma assisted technology nology. This technology promises a it will therefore have a higher hard- (PACVD) uniform thickness of the nitride layer ness. Tempering of the part should independently of its geometry. follow.

Nitrocarburizing Thermal diffusion Nitrocarburizing is a process in Thermal diffusion is a process in which the parts are to be enriched in which vanadium diffuses into the nitrogen and also in carbon, the en- material and reacts with existing richment is carried out by exposure carbon, to form a vanadium carbide to atmosphere rich in these two layer. The steel must have a minimum elements. A mixture of ammonia gas of 0.3% carbon. This surface treat- and carbon monoxide or dioxide is ment provides a very high level of an example of a suitable atmosphere abrasive wear resistance.

Surface coated mould for production of golf balls. Photo Ionbond Sweden AB.

Platings Chromium and nickel metallic platings are commonly used for a variety of tooling applications, like plastic injection moulds. Platings may be deposited over most steel grades and they will prevent seizing and galling, reduce friction, increase surface hardness and prevent or reduce corrosion of the substrate’s surface.

Plasma nitriding. Photo Böhler Uddeholm Czech Republic.

13 HEAT TREATMENT OF TOOL STEEL

Testing is converted into a hardness number (HRC) which is read directly from a of mechanical scale on the tester dial or read-out. properties When the steel is hardened and tem- pered, its strength is affected, so let VICKERS (HV) F us take a closer look at how these Vickers is the most properties are measured. universal of the three testing methods. In 136° Hardness testing Vickers hardness d1 d2 testing a pyramid- Hardness testing is the most popular shaped diamond with way to check the results of hardening. a square base and a Hardness is usually the property that peak angle of 136 is is specified when a tool is hardened. ° pressed under a load It is easy to test hardness. The Principle of Vickers hardness testing. F against the material whose hard- material is not destroyed and the ness is to be determined. After apparatus is relatively inexpensive. unloading, the diagonals d and d of The most common methods are 1 2 the impression are measured and the BRINELL (HBW) Rockwell C (HRC), Vickers (HV) and hardness number (HV) is read off a Brinell (HBW). This method is suitable for soft an- table. The old expression “file-hard” nealed condition and prehardened When the test results are re- should not be entirely forgotten. steel with relatively low hardness. ported, Vickers hardness is indicated In order to check whether hardness In Brinell hardness testing, a tungsten with the letters HV and a suffix is satisfactory, for example above (W) ball is pressed against the mate- indicating the mass that exerted the 60 HRC, a file of good quality can rial whose hardness is to be deter- load and (when required) the loading provide a good indication. mined. After unloading, two measure- period, as illustrated by the following ments of the diameter of the impres- example: sion are taken at 90° to each other ROCKWELL (HRC) HV 30/20 = Vickers hardness deter- (d1 and d2) and the HBW value is This method is suitable for hardened mined with a load of 30 kgf exerted read off a table, from the average of material and never for material in for 20 seconds. d1 and d2. soft annealed condition. In Rockwell When the test results are re- hardness testing, a conical diamond is ported, Brinell hardness is indicated first pressed with a force F0, and then with the letters HBW and a suffix with a force F0+F1 against a specimen indicating ball diameter, the mass of the material which hardness is to with which the load was exerted and be determined. After unloading to F0, (when required) the loading period, the increase (e) of the depth of the as illustrated by the following exam- impression caused by F1 is deter- ple: HBW 5/750/15 = Brinell hard- mined. The depth of penetration (e) ness determined with 5 mm tungsten (W) ball and under load of 750 kgf exerted for 15 seconds.

F0 F0+F1=F F0

HRC D h0 h e

F Surface of specimen 100 h 0 d e h 0,2 mm h HRC 0

Hardness scale Hardness

Principle of Rockwell hardness testing. Principle of Brinell hardness testing.

14 HEAT TREATMENT OF TOOL STEEL

Tensile strength Impact testing Some words Tensile strength is determined on a A certain quantity of energy is re- of advice to tool test piece which is gripped in a ten- quired to produce a fracture in a sile testing machine and subjected to material. This quantity of energy can designers a successively increasing tensile load be used as a measure of the tough- until fracture occurs. The properties ness of the material, a higher absorp- Design that are normally recorded are yield tion of energy indicating better Avoid: strength Rp0.2 and ultimate tensile toughness. The most common and • sharp corners strength Rm, while elongation A5 and simplest method of determining • notch effects reduction of area Z are measured on toughness is impact testing. A rigid • large differences in section the test piece. In general, it can be pendulum is allowed to fall from a thicknesses said that hardness is dependent upon known height and to strike a test yield strength and ultimate tensile specimen at the lowest point of its These are often causes of hardening strength, while elongation and reduc- swing. The angle through which the cracks, especially if the material is tion of area are an indication of pendulum travels after breaking the cooled down too far or allowed to toughness. High values for yield and specimen is measured, and the stand untempered. ultimate tensile strength generally amount of energy that was absorbed mean low values for elongation and in breaking the specimen can be Unsuitable Preferred reduction of area. calculated. design alternative Several variants of impact testing are in use. The various methods differ in the shape of the specimens. Fillet These are usually provided with a V- or U-shaped notch, the test methods being then known as Charpy V and ✗ Charpy U respectively. For the most part, tool steel has a rather low toughness by reason of its high strength. Materials of low tough- ness are notch sensitive, for which reason smooth, unnotched speci- ✗ mens are often used in the impact testing of tool steel. The results of the tests are commonly stated in joules, or alternatively in kgm (strictly speaking kgfm), although Heat treatment J/cm2 or kgm/cm2 is sometimes used Choose suitable hardnesses for the instead, specially in Charpy U testing. application concerned. Be particularly careful to avoid temperature ranges Tensile test. that can reduce toughness after tempering. Tensile tests are used mostly on Keep the risk of distortion in mind structural steel, seldom on tool steel. and follow recommendations con- It is difficult to perform tensile tests cerning machining allowances. at hardnesses above 55 HRC. Tensile It is a good idea to specify stress tests may be of interest for tougher relieving on the drawings. types of tool steel, especially when they are used as high strength struc- tural materials. These include e.g. Uddeholm Impax Supreme and Uddeholm Orvar Supreme.

Impact testing machine.

15 HEAT TREATMENT OF TOOL STEEL

Vacuum furnace.

16 HEAT TREATMENT OF TOOL STEEL

Approx. hardness after hardening and tempering

Austenitizing HRC at tempering temperature °C, 2 x 2 h Uddeholm temperature grade °C 200 250 500 525 550 600

ALVAR 14 8501) 54 53 45 – 42 38 ALVAR 900 54 53 45 – 43 41 ARNE 8301) 62 60 45 43 41 38 CALDIE 1020 – – – 61*** 59 50 CALMAX 960 59 58 53 53 50 43 CARMO 960 59 58 53 53 50 43 CHIPPER 1010 59 57 59* 58 56 48 CORRAX 8502) – – – – –– DIEVAR 1025 53 52 52* – 52 47 ELMAX3) 1080 59 58 60** 59** 58** – FERMO – Delivered in prehardened condition FORMAX – Delivered in prehardened condition HOLDAX – Delivered in prehardened condition HOTVAR 1050 – 56 – – 57 53 IMPAX SUPREME – Delivered in prehardened condition MIRRAX ESR 1020 – 50 52** – 42** 36 MIRRAX 40 – Delivered in prehardened condition NIMAX4) – Delivered in prehardened condition ORVAR SUPREME 1020 52 52 54* – 52 46 ORVAR SUPERIOR 1020 52 52 54* – 52 46 ORVAR 2 MICRODIZED 1020 52 52 54* – 52 46 POLMAX 1030 53 52 54** – 53** 37 QRO 90 SUPREME 1020 49 49 51* – 51* 505) RAMAX HH – Delivered in prehardened condition ROYALLOY – Delivered in prehardened condition RIGOR 950 61 59 56* 55* 53 46 SLEIPNER 1030 60 59 – 62*** 60 48 SR 1855 850 63 62 50 48 46 42 STAVAX ESR 1030 53 52 54*** – 43*** 37 SVERKER 3 960 60 59 56 53 – – SVERKER 21 1020 63 59 60 57 54 48 THG 2000 1020 52 52 53* – 52 46 UHB 11 – As-delivered condition (~200 HB) UNIMAX 1020 – – – – 55 49 VANADIS 4 EXTRA3) 1020 – 59 – 61*** 60 52 VANADIS 63) 1050 63 62 – 62*** 59 52 VANADIS 103) 1060 63 62 – 62*** 60 52 3 x 560°C VANCRON 403) 950–1100 57–65 VIDAR SUPERIOR 1000 52 51 51* – 50 45 VIDAR 1 1000 54 53 55* – 52 46 VIDAR 1 ESR 1000 54 53 55* – 52 46 High speed steel 3 x 560°C VANADIS 233) 1050–1180 60–66 VANADIS 303) 1000–1180 60–67 VANADIS 603) 1000–1180 64–69

* This tempering temp. should be avoided due to the risk of temper brittleness. ** For Uddeholm Stavax ESR, Uddeholm Mirrax SER, Uddeholm Polmax and Uddeholm Elmax corrosion resistance is reduced. *** The lowest tempering temperature when high temperature tempering is 525°C. 1) Quench in oil 2) Solution treatment. Ageing: ~50 HRC after 525°C/2 h, ~46 HRC after 575°C/2h, ~40 HRC after 600°C/4h. 3) Powder Metallurgy tool steel 4) The delivery hardness of Uddeholm Nimax can not be increased. Tempering shall be avoided as toughness will be reduced. 5) At 650°C 2 x 2h: 42 HRC

17 HEAT TREATMENT OF TOOL STEEL

Hardness conversion table Approx. comparison between hardness and ultimate tensile strength.

Rockwell Approx. UTS Brinell* Vickers 30 kg HRC HRB N/mm2 kp/mm2

78 133 140 446 46 85 152 160 510 52 91 171 180 570 58 95 190 200 637 65 98 209 220 696 71 228 240 756 77 247 260 824 84 265 280 883 90 30 284 300 951 97 33 303 320 1020 104 35 322 340 1080 110 37 341 360 1150 117 39 360 380 1210 123 41 379 400 1280 130 42 397 420 1340 137 44 415 440 1410 144 46 433 460 1470 150 47 452 480 1530 156 48 471 500 1610 164 50 488 520 1690 172 51 507 540 1770 180 52 525 560 1850 188 53 545 580 1940 198 54 564 600 55 584 620

56 601 640 57 620 660 59 638 680 59 700 60 720 61 740 62 760 63 780 64 800 64 820 65 840 66 860 66 880

* 10 mm ball, 3 000 kg load.

18 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 the Asia Pacific area. Together we secure our position as the world’s leading supplier of tooling materials.

www.assab.com www.uddeholm.com UDDEHOLM 120312.1000 / TRYCKERI KNAPPEN, KARLSTAD 201203199

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 the Asia Pacific area. 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