Heat Treatment of Uddeholm Tool Steels

HEAT TREATMENT OF TOOL STEEL

HEAT TREATMENT OF UDDEHOLM TOOL STEELS

1 ww

HEAT TREATMENT OF TOOL STEEL

Cover photos from left to right: Böhler Uddeholm Czech Republic, Uddeholms AB/HÄRDtekno, Eifeler Werkzeuge, Germany.

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 9, 10.2017 2 ww

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 rovide a general idea of how tool steel increased reliability. is heat treated and how it behaves Uddeholm has concentrated its tool AND TEMPERING during this process. Special attention steel range on high alloyed types of When a tool is hardened, many is paid to hardness, toughness and steel, intended primarily for purposes factors inﬂuence the result. dimensional stability. such as plastic moulding, blanking and forming, die casting, extrusion, SOME THEORETICAL forging, wood-working industry, recy- ASPECTS WHAT IS cling industry and component In soft annealed condition, most of business. Powder metallurgy (PM) TOOL STEEL? the carbide-forming alloying elements steels are also included in the range. are bound up with carbon in carbides. Tool steels are high-quality steels Tool steel is normally delivered in the When the steel is heated up to made to controlled chemical com- soft annealed condition; this makes the hardening temperature, the matrix position and processed to develop material easy to machine with cutting is transformed from ferrite to auste- properties useful for working and tools and it provides a microstructure nite. This means that the Iron atoms shaping of other materials. The car- suitable for hardening. change their position in the atomic bon content in tool steels may range The soft annealed microstructure lattice and generate a new lattice with from as low as 0.1% to as high as consists of a soft matrix in which different crystallinity. more than 1.6% C and many are carbides are embedded. See picture alloyed with alloying elements such below. as chromium, molybdenum and In carbon steel, these carbides are = Iron atoms vanadium. Iron carbides, while in alloyed steel = Possible positions for Tool steels are used for applications they are chromium (Cr), tungsten (W), carbon atoms such as blanking and forming, plastic molybdenum (Mo) or vanadium (V) moulding, die casting, extrusion and carbides, depending on the com- forging. position of the steel. Carbides are Alloy design, the manufacturing route compounds of carbon and alloying of the steel and quality heat treatment elements and are characterized by are key factors in order to develop tools very high hardness. Higher carbide or parts with the enhanced properties content means a higher resistance to that only tool steel can offer. wear. 2.86 A Beneﬁts like durability, strength, Also non-carbide forming alloying Unit cell in a ferrite crystal. corrosion resistance and high-tem- elements are used in tool steel, such Body centred cubic (BCC). perature stability are also attractive as cobalt (Co) and nickel (Ni) which for other purposes than pure tool are dissolved in the matrix. Cobalt is applications. For this reason, tool normally used to improve red hard- steel is a better choice than construc- ness in high speed steels, while nickel tion or engineering steel for strategic is used to improve through-hardening components in the different indu- properties and also increase the stries. toughness in the hardened condi- More advanced materials easily tions. result in lower maintenance costs, 3.57 A

Unit cell in an austenite crystal. Face centred cubic (FCC).

2.98 A

Uddeholm Dievar, 20µm 2.85 A soft annealed 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, formed) carbides and newly formed combinations of these factors that and the carbides will dissolve into the martensite can increase hardness will result in the same hardness level. matrix to some extent. In this way the during high temperature tempering. Each of these combinations cor- matrix acquires an alloying content of Typical of this is the so called secon- responds to a different heat treatment carbide-forming elements that gives dary hardening of e.g. high speed cycle, but certain hardness does not the hardening effect, without beco- steels and high alloyed tool steels. guarantee any specific set of proper- ming 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, desired hardness. It is very important Quality heat treatment delivers not as in for instance annealing. Instead, to have in mind that hardness is the only desired hardness but also they are fixed in positions where they optimized properties of the material

really do not have enough room, and Hardness for the chosen application. the result is high micro-stresses that Tool 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 D formed martensite during cooling a forced solution of carbon in ferrite. after the ﬁrst tempering. When the steel is hardened, the Three temperings are recommended matrix is not completely converted in the following cases: A into martensite. There is always some • high speed steel with high carbon Tempering temperature austenite that remains in the structure content 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 martensite alloying content, higher hardening • 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 cracking. But this can be prevented by re- result of several different factors, heating the steel to a certain tem- such as the amount of carbon in the perature, reducing the stresses and martensitic matrix, the micro-stresses transforming the retained austenite contained in the material, the amount to an extent that depends upon the of retained austenite and the precipi- reheating temperature. This reheating tated carbides during tempering. after hardening is called tempering. Hardening of tool steel should always be followed immediately by tempering. 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 temperature the microstructure consists of tempered martensite, newly formed martensite, some retained Uddeholm Dievar, austenite and carbides. 20µm hardened structure. Unit cell in a martensite crystal. Tetragonal. 5 HEAT TREATMENT OF TOOL STEEL

STRESS RELIEVING few exceptions cheaper than making In the case of big tools with complex Distortion due to hardening must be dimensional 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 causes thermal and mechanical stres- hardening operation is: ses that will remain embedded in the rough machining, stress relieving and HOLDING TIME AT material. This might not be significant semi-finish machining. HARDENING TEMPERATURE on a symmetrical part of simple de- It is not possible to briefly state exact sign, but can be of great importance recommendations to cover all heating in an asymmetrical and complex HEATING TO situations. machining, for example of one half of HARDENING TEMPERATURE Factors such as furnace type, a die casting die. Here, stress As has already been explained, hardening temperature, the weight of relieving heat treatment is always stresses contained in the material the charge in relation to the size of recommended. will produce distortion during heat the furnace, the geometry of the diffe- This treatment is done after rough treatment. For this reason, thermal rent parts in the charge, etc., must be machining and before hardening stresses during heating should be taken into consideration in each case. and entails heating to 550–700°C avoided. The use of thermocouples permits (1020–1300°F). The material should The fundamental rule for heating an overview of the temperature in the be heated until it has achieved a to hardening temperature is there- different areas of the various tools in the uniform temperature all the way fore, that it should take place slowly, charge. through, where it remains 2–3 hours increasing just a few degrees per The ramping step finishes when the and then cooled slowly, for example minute. In every heat treatment, the core of the parts in the furnace reach the in a furnace. The reason for a neces- heating process is named ramping. chosen temperature. Then the tempera- sary slow cooling is to avoid new The ramping for hardening should be ture is maintained constant for a certain stresses of thermal origin in the made in different steps, stopping the amount of time. This is called holding stress-free material. process at intermediate temperatures, time. The idea behind stress relieving is commonly named preheating steps. The generally recommended hol- that the yield strength of the material The reason for this is to equalise the ding time is 30 minutes. In the case at elevated temperatures is so low temperatures between the surface of high speed steel, the holding time that the material cannot resist the and the centre of the part. Typically will be shorter when the hardening stresses contained in it. The yield choosen preheating temperatures temperature is over 1100°C (2000°F). strength is exceeded and these stres- are 600–650°C (1100–1200°F) and If the holding time is prolonged, ses are released, resulting in a greater 800–850°C (1450–1560°F). microstructural problems like grain or lesser degree of plastic deforma- growth can arise. tion. 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

MPa

Yield strength

Residual stresses contained in the material

Plastic deforma-

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 tempera- rate should be rapid. To minimize ture has been equalised between the distortion, a slow quenching rate is 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 at more or less the same time and surface and the core of a part, and Surface 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 heat range, below the Ms temperature. Martensite formation leads to an conductivity of the steel, the cooling increase in volume and stresses in Martensite capacity of the quenching media and the material. This is also the reason the cross-section of the part. why quenching should be interrup- The quenching process as expressed in a ted before room temperature has CCT graph. been reached, nor mally at 50–70°C (120–160°F). Temperature

However, if the quenching rate is too Air hardening is reserved for steel Hardening temperature slow, especially with heavier cross-sec- with high hardenability, which in most tions, undesirable transformations in the of the cases is due to the combined Oil microstructure can take place, risking a presence of manganese, chrome and Air poor tool performance. molybdenum. Polymer Vacuum Quenching media used for alloyed Risk of distortion and hardening Salt bath MS steel nowadays are: hardening oil, cracks can be reduced by means of Water polymer solutions, air and inert gas. step quenching or martempering. In Room this process the material is quenched temperature 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 than that for smaller parts, as the Core high amount of heat that has to be It is still possible to ﬁnd some heat transported from the core through the treatment shops that use salt baths, Surface surface produces a self-tempering effect. but this technique is disappearing MS due to environmental aspects. Oil and polymer solutions are usually utilised for low alloyed steel and for tool Martensite steel with low carbon contents. Time Martempering or step-quenching.

7 HEAT TREATMENT OF TOOL STEEL

SOME PRACTICAL ISSUES VACUUM TECHNOLOGY • When the furnace reaches a At high temperature, steel is very Vacuum technology is the most used temperature of approx. 850°C likely to suffer oxidation and varia- technology nowadays for hardening (1560°F), the effect of radiation tions in the carbon content (carburi- of high alloyed steel. heating mechanisms will over- zation or decarburization). Protected Vacuum heat treatment is a clean shadow that of the convection atmospheres and vacuum technology process, so the parts do not need to ones in the heat transfer process. are the answer to these problems. be cleaned afterwards. It also offers Therefore the Nitrogen pressure is Decarburization results in low sur- a reliable process control with high lowered, in order to optimize the face hardness and a risk of cracking. automation, low maintenance and effects of radiation and convection Carburization, on the other hand, environmental friendliness. All these heating mechanisms are negligible can result in two different problems: factors make vacuum technology under these new physical condi- • the first and easiest to identify is especially attractive for high-quality tions. The new value of the nitro- the formation of a harder surface parts. gen pressure is around 1–7 mbar. layer, which can have negative The reason for having this remain- effects ing pressure is to avoid sublimation • 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 observ- Heat exchanger ing it through the optical micro- Furnace 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 the completely opposite problem. 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 of the alloying elements, i.e. to of a vacuum furnace can schemati- avoid the loss of alloying elements cally be listed as follows: to the vacuum. This low pressure • When the furnace is closed after condition will be maintained invari- charging operation, air is pumped ant during the last part of the heat Batch type furnace with controlled out from the heating chamber in ing process, as well as during the atmosphere. Photo: Bodycote Stockholm, order to avoid oxidation. holding time at the chosen harden Sweden. • An inert gas (most commonly ing temperature. Nitrogen) is injected into the heat- • The cooling down will be carried out by a massive injection of inert For these reasons it is very important ing chamber until a pressure of gas (most commonly nitrogen) that the atmosphere in which the heat around 1–1.5 bar is reached. treatment takes place does not affect • The heating system is started. the carbon content of the part. The presence of the inert gas will Wrapping in a hermetically closed make possible the heat transfer stainless-steel foil also provides some process through convection protection when heating in a mufﬂe mechanisms. This is the most efﬁ- furnace. The steel foil should be re- cient way to heat up the furnace to moved before quenching. a temperature of approx. 850°C (1560°F).

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

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

into the heating chamber in alter- nating directions and reaching the overpressure that was previously chosen when programming the furnace. The maximum overpressure 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 will occur when tempering highly interrupt at 50–70°C (120–160°F). The material should be tempered alloyed steel at a high temperature. Therefore a certain amount of aus- immediately after quenching. This will be detrimental to its corro- tenite remains untransformed when Quenching should be stopped at a sion resistance but will give to it the material is ready to be tempered. temperature of 50–70°C (120–160°F) somewhat higher wear resistance. When the material cools after tem- and tempering should be done at If the tool is to be electrical discharge pering, most of the austenite is trans- once. If this is not possible, the mate- machined (EDM) or coated, high- formed to newly formed martensite rial must be kept warm, e.g. in a temperature tempering is necessary. (untempered). A second tempering special “hot cabinet” awaiting tem- gives the material optimum tough- pering. 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 cases of large cross-sections, parts TEMPERING and the formation of cracks with complex geometries or very high if the tempering is not done Here there is also a general rule, demands on dimensional stability. immediately after the quenching applicable in most of the cases: once In these cases, a third tempering is process. This breakage of the the tool has been heated through, usually needed. crystalline structure can take hold it for at least two hours at full place in a violent way. Therefore temperature each time. 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 personal Untempered martensite safety. Tempered martensite Uddeholm has made a wide range of experiments and measurements and Retained collected the resulting data regarding }austenite 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. content along the After After After first After second After third The ﬁrst priority when choosing the heating quenching tempering tempering tempering* different steps of the heat treatment. tempering temperature should be the *HSS steel and big high-pressure die casting dies mechanical properties, as some small dimensional adjustments can be made in a last ﬁne machining step. The mechanical and physical properties obtained after tempering will depend greatly on the chosen tempering temperature. High-temperature 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 at A lower die for surface coating). aluminium rim just When tempering at high temperature, before heat treatment other differences in properties are also on charging grid. noticeable, like higher heat conductivity. Photo: ASSAB Çelik (Turkey)

HEAT TREATMENT OF TOOL STEEL

DIMENSIONAL AND In order to reduce distortion while thermal stresses. But as earlier heating during the hardening process, SHAPE STABILITY mentioned, a faster quenching will a stress relieving operation should result in better mechanical properties. DISTORTION DURING be carried out prior to the harde- It is important that the quenching HARDENING AND TEMPER- ning operation. It is recommended to medium is applied as uniformly as stress relieve the material after rough ING OF TOOL STEEL possible. This is especially valid machining. Any distortion can then be when forced air or protective gas When a piece of tool steel is harde- adjusted during semi-ﬁnish machining atmosphere (as in vacuum furnaces) ned and tempered, some warpage prior to hardening operation. is used. Otherwise temperature diffe- or distortion normally occurs. This is rences in the tool can lead to signi- well known and it is normal practice ﬁcant distortion. to leave some machining allowance THERMAL STRESSES on the tool prior to hardening, making Thermal stresses arise every time it possible to adjust the tool to the there is a temperature gradient in the TRANSFORMATION STRESSES correct dimensions after harde- material, i.e. when the temperature is Transformation stresses arise when ning and tempering by grinding, for not even all over the part. the microstructure of the steel is example. Thermal stresses grow with in- transformed. This is because the creasing heating rate. Uneven heating three phases in question—ferrite, How does distortion can result in local variations in volume austenite and martensite—have take place? due to uneven dilatation rates and this different densities, i.e. volumes. The cause is stresses in the material. will also contribute to the arising of Out of all the microstructural These stresses can be divided into: stresses and distortion. changes that take place during heat • machining stresses In order to tackle this problem, it treatment, the biggest contribution • thermal stresses is common practice to heat up the to transformation stresses is caused by the transformation of austenite • transformation stresses material in steps, in order to equalise the temperature between the surface and into martensite. This causes a volume MACHINING STRESSES the centre. increase. Excessively rapid and uneven Machining stresses are generated quenching can also cause local during machining operations such as Linear expansion mm/100 mm martensite formation and thereby turning, milling and grinding or any volume increases locally in a piece type of cold working. 0.8 and gives rise to stresses in this If stresses have built up in a part, section. These stresses can lead to they will be released during heating. 0.6 distortion and, in some cases, harde- Heating reduces strength, releasing 0.4 ning cracks. stresses through local distortion. This can lead to overall distortion. 0.2

Volume 100 200 300 400 500 600 °C Yield strength Rp0.2 210 390 570 750 930 1110 °F MPa Temperature 400 350 Effect of temperature on the linear 300 expansion of Uddeholm ORVAR Supreme, 250 soft annealed. 200 Transformation 150 Trans- to austenite formation 100 to martensite M A A An attempt should always be made s C1 C3 50 to heat slowly enough so that the Temperature 100 200 300 400 500 600 °C temperature remains virtually equal 210 390 570 750 930 1110 °F Volume changes due to structural Temperature throughout the piece. transformation. What has been said regarding Effect of temperature on the yield heating, applies also to cooling. strength of Uddeholm Orvar Supreme, Very powerful stresses arise soft annealed. during quenching. As a general rule, the slower quenching can be done, the 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 Distortion can be minimized by: transform into martensite during to avoid the existence of untempered martensite in the part. • keeping the design simple and service. This will lead to local distor- symmetrical tion and embrittlement of the tool due to the presence of untempered • eliminating machining stresses martensite. Therefore the requirement by stress relieving after rough SURFACE of maximum dimensional stability in machining service has an implied demand for TREATMENT • heating up slowly to hardening very low or no retained austenite temperature NITRIDING content. This can be achieved by • using a suitable grade of steel Nitriding is performed by exposing using sub-zero treatment after the parts to some media rich in • quenching the piece as slowly as quenching or by high temperature nitrogen under certain physical con- possible, but quick enough to tempering. ditions that will result in the diffusion obtain a correct microstructure The sub-zero treatment leads of nitrogen atoms into the steel and in the steel to a reduction of retained austenite the formation of nitrides. The part • by usage of martempering or step content by exposing the tool or part surface will then be harder and have quenching to very low temperatures. The most a higher wear resistance in its outer • tempering at a suitable temperature commonly used are about -80°C layer. (-110°F) and -196°C (-320°F). This, in The following values for machining In the case of corrosion resistant turn, will result in a hardness increase allowances can be used as guide- steel with high-chromium content, it of up to 1–2 HRC, in comparison to lines. is very important to take into con- non sub-zero treated tools, if low sideration the fact that nitriding has temperature tempering is used. For a detrimental effect on the corrosion Machining allowance high temperature tempered tools Uddeholm on length and diameter resistance of the material. In other steel grade as % of dimension there will be little or no hardness cases nitriding can have a positive increase. ARNE 0.25 % effect on the corrosion resistance. Tools that are high temperature CALDIE 0.25 % Appropriate steel to be nitrided are tempered, even without a sub-zero CALMAX/CARMO 0.20 % usually medium-carbon steel with CHIPPER/VIKING 0.20 % treatment, will normally have a low nitride-forming elements such as RIGOR 0.20 % retained austenite content and in chromium, aluminium, molybdenum SLEIPNER 0.25 % most cases, a sufﬁcient dimensional and vanadium. SVERKER 3 0.20 % stability. However, for high demands The core should act as a stable SVERKER 21 0.20 % on dimensional stability in service it is substrate regarding mechanical VANADIS 4 EXTRA also recommended to use a sub-zero SUPERCLEAN 0.15 % properties and microstructure. This treatment in combination with high VANADIS 8 means that for hardened material it SUPERCLEAN 0.15 % temperature tempering. is necessary to temper above the VANADIS 23 For the highest requirements SUPERCLEAN 0.15 % nitriding temperature in order to avoid on dimensional stability, sub-zero VANCRON 40 softening of the core during the SUPERCLEAN 0.20 % treatment in liquid nitrogen is recom- nitriding process. CORRAX 0.05–0.15 %* mended after quenching and after ELMAX SUPERCLEAN 0.15 % MIRRAX ESR 0.20 % No treatment STAVAX ESR 0.15 % Sub-zero treatment UNIMAX 0.30 % Hardness HRC Retained austenite % ALVAR 14 0.20 % 75 24 DIEVAR 0.30 % 70 Hardness 21 FORMVAR 0.30 % 65 18 ORVAR 2 MICRODIZED 0.20 % 60 15 Uddeholm Sleipner. ORVAR SUPREME 0.20 % Hardness and retained 55 12 austenite as function of QRO 90 SUPREME 0.30 % 50 9 tempering temperature VIDAR SUPERIOR 0.25 % 45 Retained austenite 6 with and without sub-zero BURE 0.20 % 40 3 treatment. 35 * Depending on ageing temperature 150 250 350 450 550 650°C 300 480 660 840 1020 1200°F Tempering temperature °C

12 HEAT TREATMENT OF TOOL STEEL

It should be noted that a nitrided elements. A mixture of ammonia gas SURFACE COATING surface cannot be machined with and carbon monoxide or dioxide is an Surface coating of tool steel has cutting tools and can only be ground example of a suitable atmosphere for become a common practise. The with difficulty. A nitrided surface will this purpose. The temperature range general aim for these kinds of pro- cause problems in weld repairing as for this process is 550°C to 580°C cesses is to generate an outer layer well. (1020°F to 1075°F) and the time of with a very high hardness and low There are several technologies exposure is between 30 minutes and friction that results in good wear available in the field of nitriding; the 5 hours. After the exposure the part resistance, minimising the risk for main ones are gas nitriding, high should be cooled down rapidly. adhesion and sticking. To be able to pressure nitriding (carried out in use these properties in an optimal vacuum furnaces) and plasma nitri- CASE HARDENING way a tool steel of high quality should ding. Case hardening is a process in which be chosen. Two common problems of conven- a finished part is exposed to a The most commonly used coating tional nitriding technologies are carburizing atmosphere and high tem- methods are: possible over-tempering of the sub- perature simultaneously. The tem- • physical vapour deposition strate material and thickening of the perature range is 850°C–950°C coating (PVD coating) nitrided layer in the sharp corners. (1560°F–1740°F). This exposure • chemical vapour deposition Pulsed plasma nitriding technology generates a layer with higher carbon coating (CVD coating) diminishes the possibility of over- content, normally 0.1–1.5 mm thick. tempering by applying the plasma Chemical vapour deposition coating After the layer has been formed, the intermittently on the part. This pro- can also be carried out with a plasma part is to be quenched in order for vides a better control over the local assisted technology (PACVD) the layer to transform into martensite temperatures during the process. with higher carbon content, and it Active screen plasma nitriding is also will therefore have a higher hardness. a development of plasma nitriding Tempering of the part should follow. technology. This technology promises a uniform thickness of the nitride layer independently of its geometry. THERMAL DIFFUSION Thermal diffusion is a process in which vanadium diffuses into the NITROCARBURIZING material and reacts with existing Nitrocarburizing is a process in carbon, to form a vanadium carbide which the parts are to be enriched layer. The steel must have a minimum in nitrogen and also in carbon, the of 0.3% carbon. This surface treat- enrichment is carried out by expo- ment provides a very high level of CVD TiC/TiN. Photo Eifeler Werkzeuge, sure to atmosphere rich in these two abrasive wear resistance. Germany.

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 OF MECHA NICAL (HRC) which is read directly from a scale on the tester dial or read-out. PROPER TIES When the steel is hardened and tempered, its strength is affected, VICKERS (HV) so let us take a closer look at how Vickers is the most these properties are measured. universal of the three testing methods. HARDNESS TESTING In Vickers hardness Hardness testing is the most popular testing a pyramid- way to check the results of hardening. shaped dia mond with Hardness is usually the pro perty that a square base and a is specified when a tool is hardened. peak angle of 136° It is easy to test hardness. The is pressed under a Principle of Vickers hardness testing. material is not destroyed and the load F against the material whose apparatus is relatively inexpen sive. hardness is to be determined. After

The most common methods are unload ing, the diagonals d1 and d2 Rockwell C (HRC), Vickers (HV) and of the impression are measured and BRINELL (HBW) Brinell (HBW). the hardness number (HV) is read off This method is suitable for soft The old expression “file-hard” a table. annealed condition and prehardened should not be entirely forgotten. When the test results are reported, steel with relatively low hardness. In order to check whether hardness is Vickers hardness is indicated with the In Brinell hardness testing, a tung- satisfactory, for example above letters HV and a sufﬁx indicating the sten (W) ball is pressed against the 60 HRC, a file of good quality can mass that exerted the load and (when material whose hardness is to be provide a good indication. required) the loading period, as illustra- determined. After unloading, two ted by the following example: measurements of the diameter of the ROCKWELL (HRC) HV 30/20 = Vickers hardness deter- impression are taken at 90° to each This method is suitable for hardened mined with a load of 30 kgf exerted other (d1 and d2) and the HBW value material and never for material in soft for 20 seconds. is read off a table, from the average annealed condition. In Rockwell hard- of d1 and d2. ness testing, a conical diamond is When the test results are reported, first pressed with a force F0, and then Brinell hard ness is indicated with the with a force F0+F1 against a speci- letters HBW and a sufﬁx indicating men of the material which hardness is ball diameter, the mass with which to be determined. After unloading to the load was exerted and (when re- F0, the increase (e) of the depth of the quired) the loading period, as illustra- impression caused by F1 is deter- ted by the following example: HBW mined. The depth of penetration (e) 5/750/15 = Brinell hardness 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 OF ADVICE TO test piece which is gripped in a required to produce a fracture in tensile testing machine and sub- a material. This quantity of energy TOOL DESIGNERS jected to a successively increasing can be used as a measure of the tensile load until fracture occurs. The toughness of the material, a higher DESIGN properties that are nor mally recorded absorption of energy indicating bet- Avoid: are yield strength Rp0.2 and ulti mate ter toughness. The most common • sharp corners tensile strength Rm, while elongation and simplest method of determi- • notch effects A5 and reduction of area Z are meas- ning toughness is impact testing. • large differences in section ured on the test piece. In general, it A rigid pendulum is allowed to fall thicknesses can be said that hardness is depen- from a known height and to strike a These are often causes of hardening dent upon yield strength and ultimate test specimen at the lowest point of cracks, espe cially if the material is tensile strength, while elongation and its swing. The angle through which cooled down too far or allow ed to reduction of area are an indication of the pendulum travels after breaking stand untempered. toughness. High values for yield and the specimen is measured, and the ultimate tensile strength generally amount of energy that was absorbed mean low values for elongation and in breaking the specimen can be calculated. Unsuitable Preferred reduction of area. design alternative Several variants of impact testing are in use. The various methods differ in the shape of the specimens. These Fillet are usually provided with a V- or U- shaped notch, the test methods being then known as Charpy V and Charpy U respectively. x For the most part, tool steel has a rather low toughness by reason of its high strength. Materials of low toughness are notch sensitive, for which reason smooth, unnotched speci- x mens are often used in the impact testing of tool steel. The re sults of the tests are commonly stated in joules, or alternatively in kgm (strictly speaking kgfm), al though J/cm2 or HEAT TREATMENT kgm/cm2 is sometimes used in stead, Choose suitable hardnesses for the specially in Charpy U testing. application con cerned. Be particularly careful to avoid tempera ture ranges Tensile test. that can reduce toughness after tempering. Keep the risk of distortion in mind and follow recommendations concerning Tensile tests are used mostly on machining allow an ces. structural steel, seldom on tool steel. It is a good idea to specify stress It is difficult to per form tensile tests relieving on the drawings. at hardnesses above 55 HRC. Tensile tests may be of interest for tougher types of tool steel, especially when they are used as high strength structural 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 AM CORRAX 8502) –––––– ARNE 8301) 62 60 45 43 41 38 BURE 1020 52 52 53* – 52 46 BALDER6) – –––––– CALDIE 1020 3 x 525°C*** 3 x 540°C 3 x 560°C 60 59 56 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 FORMVAR 1025 53 52 52* – 52 47 HOLDAX – Delivered in prehardened condition IDUN – Delivered in prehardened condition IMPAX SUPREME – Delivered in prehardened condition MIRRAX ESR 1020 – 50 52** – 42** 36 MIRRAX 40 – Delivered in prehardened condition NIMAX ESR4) – 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 M 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 3 x 525°C*** 3 x 540°C 3 x 560°C 62 60 58 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 UHB 11 – As-delivered condition (~200HB) UNIMAX 1020 – – – 57*** 55 49 VANADIS 4 3 x 525°C*** 3 x 540°C 3 x 560°C EXTRA3) 10207) 61 60 59 11508) 64 64 63 VANADIS 83) 10207) 61 60 59 11808) 64 64 63 VANAX3) 1080°C 60 – – – – – VANCRON 403) 950–1100 3 x 560°C 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 ESR, 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: ~51 HRC after 525°C/4h, ~44 HRC after 575°C/4h, ~41 HRC after 600°C/4h. 3) Powder Metallurgy SuperClean tool steel 4) The delivery hardness of Uddeholm Nimax ESR and Nimax can not be increased. Tempering shall be avoided as toughness will be reduced. 5) At 650°C 2 x 2h: 42 HRC 6) Uddeholm Balder is delivered pre-hardened, tempered at 590°C (1090°F) /2 x 2h 7) For better toughness 8) For better wear resistance 17 HEAT TREATMENT OF TOOL STEEL

HARDNESS CONVERSION TABLE

These conversions are based on EN-ISO 18265:2013. Approx. comparison between hardness and ultimate tensile strength.

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

26 259 273 873 89 27 265 279 897 92 28 272 286 919 94 29 279 294 944 96 30 287 302 970 99 31 295 310 995 101 32 303 318 1024 104 33 311 327 1052 107 34 320 336 1082 110 35 328 345 1111 113 36 337 355 1139 116 37 346 364 1168 119 38 354 373 1198 122 39 363 382 1227 125 40 373 392 1262 129 41 382 402 1296 132 42 392 412 1327 135 43 402 423 1362 139 44 413 434 1401 143 45 424 446 1425 145 46 436 459 1478 151 47 448 471 1524 155 48 460 484 1572 160 49 474 499 1625 166 50 488 513 1675 171 51 502 528 1733 177 52 518 545 1793 183 53 532 560 1845 188 54 549 578 1912 195 55 566 596 1979 202 56 585 615 2050 209 57 603 634 2121 216 58 654 59 675 60 698 61 720 62 746 63 773 64 800

* 10 mm ball, 3 000 kg load.

18 HEAT TREATMENT OF TOOL STEEL

NETWORK OF EXCELLENCE Uddeholm is present on every continent. This ensures you high-quality Swedish tool steel and local support wherever you are. We secure our position as the world’s leading supplier of tooling materials.

19 UDDEHOLM 10.2017.200 / STROKIRK-LANDSTRÖMS, Karlstad

HEAT TREATMENT OF TOOL STEEL

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 development 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. We act worldwide. For us it is all a matter of trust – in long-term partnerships as well as in developing new products.

For more information, please visit www.uddeholm.com