OPTIMIZING MICROSTRUCTURE FOR HIGH TOUGHNESS COLD-WORK TOOL STEELS

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1 OPTIMIZING MICROSTRUCTURE FOR HIGH TOUGHNESS COLD-WORK TOOL STEELS D. Viale, J. Béguinot, F. Chenou and G. Baron USINOR INDUSTEEL Abstract Increasing toughness and machinability at given high level of wear resistance are consistently growing requirements for cold-work tool steels. Improving microstructure characteristics, especially coarse carbides distribution and their chemical composition, reveals an appropriate way to meet such requirements. Referring to the archetypal cold work tool steel AISI D2, an improvement of coarse carbides hardness by higher alloying with strong carbides formers allows a moderate reduction of their volume fraction, resulting in increased toughness and machinability performances. Also, the increase of ultimate resistance of surrounding matrix by improved secondary hardening preventing premature pulling off of carbides in service contributes to longer service life, while reasonably increased silicon content leads to still better machinability. A further step towards increased toughness and machinability may result from slightly refining the coarse carbides sizes through moderate addition of fine titanium nitrides acting as precipitation promoters for M 7C 3 type carbides. INTRODUCTION Cold-work tool steels have been developed and used for more than a century and have been designed, mainly on an empirical basis [1] in order to cope with a large variety of often contradictory properties, among which : high strength level to resist against permanent deformations resulting from high levels of applied stress wear resistance during use, including abrasive wear, adhesive wear, surface fatigue 299

2 300 6TH INTERNATIONAL TOOLING CONFERENCE toughness (fracture resistance and also fatigue resistance) dimensional stability during application (thermal treatment and subsequent use) uniformity and isotropy of microstructure acceptable machinability, at least in the annealed state acceptable corrosion resistance, especially against pitting corrosion in some demanding applications acceptably limited susceptibility to excessive hardening and associated crack sensitivity in thermal affected zones, related to EDM and weld deposits. Conventional cold-work tool steel such as 12 % chromium, 1,5 % carbon (AISI D2 type) have long proved to be a satisfying solution especially regarding an equilibrated answer between deformation, wear and corrosion resistances and dimensional sensibility. On the other hand, the high volume fraction ( 10 to 15 %) of coarse ( 20 µm) eutectic M 7 C 3 carbides in these steels is largely responsible for low levels of toughness since these carbides are intrinsically prone to easy cracking and contribute thus to excessive sensitivity of steel to fracture initiation and propagation. Times going on, with steadily increasing deformation stresses applied to working material, it appeared that insufficient toughness becomes far more a cause of failure of tools with AISI D2 type steels than was an insufficient wear resistance. Accordingly, increasing demands from tool manufacturers and end users became in favour of improved fracture resistant cold work tool steels. In this respect, and considering the major detrimental role of coarse eutectic M 7 C 3 chromium- molybdenum carbides on the generation cracks during use, it seems quite logical to try to modify the steel composition of D2 type steel in order to decrease the volume fraction of these eutectic carbides, and in this scope, to reduce significantly the carbon and chromium contents which govern, in major part, this volume fraction. On the other hand, as these carbides contribute strongly to the wear resistance, it seems necessary to compensate their lower volume fraction by a still higher intrinsic hardness. This is likely to be obtained by a significant enrichment of strong carbides-forming elements like molybdenum and even, if necessary,

3 Optimizing Microstructure for High Toughness Cold-Work Tool Steels 301 by a complementary contribution of very strong MC carbides forming elements such as vanadium or niobium, which are in moderate content in D2 type steels. In addition, the higher bulk content of strong carbides-forming elements may also contribute to improve wear resistance of the steel through their residual contents in the matrix. The stronger secondary precipitation of Mo, V, Nb-enriched fine carbides should make the matrix itself more resistant to wear solicitations and thus reinforce the ability of the matrix to resist to the pulling off of the coarse carbides during service, in severe conditions. Also, to limit in an other way the detrimental effect of coarse carbides on steel toughness, it seems as possibly useful to retain a minimum quantity of austenite (even after moderate tempering) embedding the coarse carbides. Ductile austenite acts as a mean to reduce stresses concentrations around these carbides during use and, by the way, the solicitations for premature cracking. At this point of view, sufficient silicon addition, which increases carbon solubility in austenite and thus acts indirectly as an austenite stabilisating factor, may be considered (silicon is also interesting for its generally recognised beneficial role on machinability answer). Eventually, the alloys equilibrium for such a newly designed tool steel, especially regarding chromium and carbon contents, should be preferably optimised regarding dimensional stability of the steel and also referring to corrosion resistance. Here, it is not the bulk alloys contents but the contents in the matrix, as depleted in carbon and carbides formers by the previous precipitation of eutectic carbides, which has to be considered. As D2 type steels may be considered as fairly well optimised regarding dimensional stability, the trend should be that the matrix composition of the new steels lay close to that of D2 matrix. As concern corrosion resistance, the parallel decrease of the bulk contents of chromium and carbon may lead to a substantially unchanged level of passivating soluble chromium in the matrix. In addition, the significant increase of both bulk and solute molybdenum content is intended to promote a higher pitting corrosion resistance [2]. As regard other properties, lowering carbon content looks positive as a way to reduce excessive hardening and crack sensitivity of thermal affected zones, along EDM cuts or welding deposits. Considering machinability, things are a lot complex since reduced volume fraction of carbides looks favourable and, on the other hand, higher enrichment of carbides with molybdenum and vanadium/niobium looks unfavourable. At least for the specific steel conception presented hereafter, the balance reveals positive with significant improve-

4 302 6TH INTERNATIONAL TOOLING CONFERENCE ment of machinability observed in practice, both in annealed and end-treated conditions. Incidentally this positive balance between contradictory effects of reducing volume fraction and increasing intrinsic hardness of carbides is in agreement with similar observations on quite different types of steels [3]. These metallurgical trends had substantiated considerable work, especially among Japanese researchers, for example MATSUDA and SUDOH, and were at the origin of the so called "8 % Cr. 1 % C" new generation of cold work tool steels. The "core composition" of this new concept is: 1 % carbon, 8 % chromium, 2.5 % molybdenum, 0.2% to 0.6% V + Nb/2, 1 % silicon, to be compared to the typical composition of AISI D2 type steel: 1.5 % carbon, 12 % chromium, 0.8 % molybdenum, 0.25 % vanadium, 0.3 % silicon. This evolution of alloy composition puts in a concrete form the application of the metallurgical trends described above. Several tool steels manufacturers subsequently derived a lot of variants, around this central concept. TOWARDS A STILL FINER AND REGULAR DISTRIBUTION OF COARSE EUTECTIC CARBIDES MICROSTRUCTURE Undoubtedly, the reduction of coarse eutectic carbides volume fraction acts decisively for improvement of the cold work tool steels toughness, according to the new concept [4]. But the accompanying decrease of coarse carbides average sizes (reduced to 5 10 µm as compared to µm for D2 type steels) is likely to play also a significant role in this respect [5]. Indeed, refining the microstructure and especially coarse carbides sizes proves very efficient, for example when comparing results obtained by powder metallurgy and by conventional processes, applied to the same steel composition with similar hot rolling ratios. Unfortunately, specific steel making routes, such as powder metallurgy, remain quite expensive. However, from laboratory and industrial experiments, it was recently observed than it is possible to improve the microstructure and toughness through specific micro-additions. Namely, the addition, in small contents, of elements of the titanium family, in the melt prior to casting, under severely controlled conditions, proves to be efficient in this respect. This refining effect is tentatively attributed to an indirect consequence of the fine precipitation of small titanium nitrides directly in the melt, according to the very high thermal stability of these compounds. These fine nitrides particles in spite of

5 Optimizing Microstructure for High Toughness Cold-Work Tool Steels 303 Table 1. Chemical analysis C Mn Cr Mo V Others X160Cr Mo V12 / D TENASTEEL Ti their buoyancy in the melt, may act as promoters for carbides precipitation, accordingly making carbides more numerous and consequently finer. In fact this hypothetical mechanism would, logically, more readily address the precipitation of primary carbides stricto-sensu rather than the more delayed precipitation of eutectic carbides considered here. However, whatever be the actual mechanism involved, the average size observed for eutectic carbides was 3.3 µm to be compared to 6 7 µm without titanium addition. Toughness was correspondingly improved from 20 % to near 40 % depending on thermal treatments applied. METALLURGICAL CONCEPT FOR THE DEVELOPMENT OF THE NEW COLD WORK TOOL STEEL OF USINOR INDUSTEEL : TENASTEEL According to the previous considerations, the TENASTEEL differs from standard grade X160 Cr Mo V12 / D2, Table 1, by 3 main points : decreasing in the carbon and chromium contents increasing of the toughness increasing of the molybdenum content to keep quenchability, hardness and wear resistance addition of titanium to refine the structure through fine precipitation of titanium nitrides. High chromium and carbon contents always induce formation of coarse eutectic chromium carbides (Fig. 1) effective in term of wear resistance, but principal causes of brittleness of steel X160 Cr Mo V12 / D2 type. The concentration of these large carbides will be even more intense at mid thickness of the products (segregated lines). Conversely, a low carbon and chromium content guarantees:

6 304 6TH INTERNATIONAL TOOLING CONFERENCE a much more finer carbide distribution (Fig. 1) a better homogeneity in the thickness. These are very good things to improve the toughness, the machinability or the polishability of steel. Figure 1. Structures of D2 and TENASTEEL Moreover, a characterisation of the precipitates in the D2 and TENAS- TEEL grades was carried out by electron microprobe analysis. In the D2 grade steel, there is only one carbide type with the M 7 C 3 stoechiometrie. Of course, the Table 2 shows that it is chromium carbide with a little bit of vanadium, molybdenum and iron. In the TENASTEEL, we can find this chromium carbide so, but moreover there are molybdenum carbides (M 23 C 6 type) and titanium carbo-nitrides (M 4 (C,N 3 )) type). The substitution of chromium carbides by molybdenum

7 Optimizing Microstructure for High Toughness Cold-Work Tool Steels 305 Table 2. Chemical composition of the carbides precipitates in D2 and TENASTEEL grade Atomic % C Si Mn Cr V Mo Ti Fe D2 M 7C 3 31,33 0,01 0,32 34,64 5,26 0,85 / 27,55 TENASTEEL M 7C 3 31,91 0,03 0,36 31,01 3,14 3,82 0,09 29,64 TENASTEEL M 23C 6 20,73 4,55 0,17 7,22 1,45 30,21 0,25 35,39 TENASTEEL M 4(C, N 3) 0,33 0,03 0,55 4,09 1,35 39,5 1,98 carbides gives a good wear resistance to the TENASTEEL because of their hardness (Mo carbides: 1800 HV, Cr carbides: 1500 HV). HEAT TREATMENTS AND MECHANICAL PROPERTIES OF TENASTEEL To obtain the mechanical characteristics on a given steel, with a good compromise between strength and toughness, it is essential to optimize its metallurgical structure as well as the size, the distribution, the density, the homogeneity of the carbides which it contains. This double aim can be achieved by an adapted composition, as it is the case for TENASTEEL, but that is not enough. The mechanical characteristics of a steel with a given composition can be widely improved by the heat treatments which it will undergo. While influencing its structure, they will be able to increase or decrease the strength of metal and decrease its brittleness. A heat treatment does not change the chemical composition of metal, but it modify (Fig. 2) : its structure by controlling of carbide precipitation (size, distribution ) as well as the control of the nature and the proportion of the components (ferrite, austenite, martensite ) ; the mechanical equilibrium in the metal (internal stress, expansion ). Technically, a heat treatment is defined by a variation in temperature according to time. A thermal cycle practised on steel can be divided into three distinct stages : a reheating to the desired temperature ; a stage at the temperature defined according to the practiced heat treatment, and depending on its final purpose (homogenization, hardening, softening, increase in ductility, internal stress relaxation )

8 306 6TH INTERNATIONAL TOOLING CONFERENCE Figure 2. Incidence of the structure on the steel properties. a cooling speed will fix the structure of metal in terms of components and precipitation where the several speeds of cooling can follow one another before reaching the temperature of end of processing. The implementation of the heat treatments thus requires the comprehension of the principal phenomena involved i.e. especially for tool steels, precipitation and dissolution of carbides, as well as the evolution of structures, their transformations and conditions under which they occur. In order to facilitate its machining, TENASTEEL is delivered in annealed condition to give a low hardness structure. Softened steel can be formed, but a heat treatment will be then necessary to give the final mechanical characteristics to the pieces. It is a quenching to harden the metal followed by temperings to eliminate its brittleness and to increase its toughness. The processing of hardening consists in:

9 Optimizing Microstructure for High Toughness Cold-Work Tool Steels 307 slow heating, to limit deformation and to avoid cracking (due to stresses) up to a temperature just below AC1, then holding (time depending on thickness) to homogenize the temperature through the whole thickness. Then, re-heating up to austenitization temperature (>AC3) ; holding at austenitization temperature to get an homogeneous temperature in the whole piece, to transform the steel into austenite and to dissolve a maximum of carbides previously formed ; cooling in an adapted cooling medium to get a martensitic structure. In order to get a martensitic transformation, it is necessary to have a cooling speed higher than critical quenching rate of the steel (minimum speed allowing cooling without transformation into ferrite-pearlite). The lower the critical quenching rate is, the more steel will be able to harden deeply. The hardenability of a steel depends primarily on its chemical composition. All the alloy elements, except cobalt, tend to increase hardenability. The TENASTEEL exhibits a critical quenching rate relatively low. Its hardenability is comparable with that of steel D2. After hardening, the structure of steel is not completely martensitic, there remains a part of austenite called retained austenite, and carbides. The more the steel is alloyed and the larger the temperature and time of reaustenitization are, the more there is retained austenite. This complex structure shelters internal stresses which increase the brittleness of the steel. To decrease the harmful effects of hardening, a new heat treatment will be perform on the pieces : the tempering which consists in carrying them at a temperature lower than AC1, to avoid modifying the crystalline iron (α) structure, then to cool them quickly. The reheating of martensite tends to bring back it in a state of balance because the carbon is rejected out of the structure and precipitates to give iron ε carbide (Fe 2 C) and cementite (Fe 3 C). This precipitation is accompanied by a contraction of metal and by a reduction in hardness (internal stress relaxation). This softening due to the transformation of martensite, is attenuated by a hardening caused by the transformation of retained austenite in secondary martensite or bainite during cooling ; this reaction is accompanied by an expansion of metal. A second tempering is generally practiced to transform this new martensite. Lastly, during temperings carried out at high temperature (starting from 500 C), a secondary hardening is also produced by the special carbides precipitation : vanadium and molybdenum carbides in the

10 308 6TH INTERNATIONAL TOOLING CONFERENCE case of TENASTEEL. This new hardening is also accompanied by an expansion of metal. In summary, softening with tempering results from several simultaneous phenomena (Fig. 3): the softening of martensite (primary then secondary) the transformation of residual austenite the special carbide precipitation if the tempering is carried out at high temperature. The thermal cycle of hardening - tempering to TENASTEEL takes into account of these metallurgical considerations as schematised Fig. 4. The reheating of austenitization will be practiced under vacuum, or at least in a controlled atmosphere to prevent the risks of decarburization of steel. The temperature of reaustenitization can be selected between 1000 and 1100 C. The hardness evolution of TENASTEEL after complete heat treatment is shown on Fig. 5 according to austenitizing and tempering temperatures. It should be noted that this very great interval of austenitizing temperatures makes it possible to be compatible with the temperatures usually used for many other steels (D2 in particular). This allows an optimization of furnace productivity, and thus a reduction of the costs of heat treatment, as well as a reduction of the risks of errors related to the non-observance of austenitizing temperatures. Whatever the austenitizing temperature, a hardness range between 58 and 62 HRC (standard of use for this type of steel) can be obtained if the TENASTEEL undergone two temperings between 500/550 C(930/1020 F) to 575 C(1065 F). While an austenitization between 1000 and 1100 C(1830/2010 F) leads to a good hardness of our steel, the best properties will be obtained after reheating around C( F). Indeed, Fig. 6 shows that the toughness of metal is maximum in this range of temperature. Indeed, if the austenitizing temperature is too low, a large part of fine chromium molybdenum carbides will not be dissolved. They will remain coarse and will not increase hardness and wear resistance of steel. In addition, for the highest reheating temperatures, the hardening obtained by the refinement of subsequent secondary precipitation of carbides is counterbalanced by softening due to the increasing in retained austenite rate after hardening. A third tempering should be then necessary to completely destabilise this retained austenite. Number and temperature of temperings used

11 Optimizing Microstructure for High Toughness Cold-Work Tool Steels 309 Figure 3. Various metallurgical phenomena leading to a softening with the tempering. Figure 4. Thermal cycles of hardening and tempering practices on TENASTEEL.

12 310 6TH INTERNATIONAL TOOLING CONFERENCE Evolution of hardness of TENASTEEL with austenitizing and tempering tem- Figure 5. peratures. to soften martensitic structure after quenching will allow TENASTEEL to obtain final mechanical characteristics and final using properties. The softening curves of the TENASTEEL are compared with those of the D2 Fig. 7 for an austenitizing temperature of 1050 C(1920 F). These softening curves of TENASTEEL make it possible to draw some interesting conclusions : TENASTEEL and D2 grades are treated in the same ranges of temperatures, for an identical temperature of tempering, the TENASTEEL is harder than D2, lastly, TENASTEEL makes it possible to obtain high hardnesses (> 60 HRC) after tempering at high temperature ( C). This last possibility is a very good advantage regarding the aptitude for the surface coating which requires for nitriding (gas, bath of salts, ionic...) or PVD, for example, relatively long holding time at high temperatures. These curves of soften show that for processing performed between 550 and

13 Optimizing Microstructure for High Toughness Cold-Work Tool Steels 311 Figure 6. Effect of austenitizing temperature on the toughness of TENASTEEL Figure 7. Softening curve of TENASTEEL compared with that of the D2.

14 312 6TH INTERNATIONAL TOOLING CONFERENCE 575 C, the TENASTEEL is able to keep a hardness of the matrix higher than 60 HRC, whereas the Z160CDV12 sees its hardness breaking down (50 58 HRC) in this temperature range. The evolution of mechanical characteristics obtained after heat treatment is shown on Fig. 8. Figure 8. Evolution of mechanical characteristics of TENASTEEL following hardness obtained after heat treatment. The toughness of TENASTEEL strongly grows up with the reduction in the hardness of steel, whereas its abrasive wear resistance increases slightly with hardness. The best compromise, for a standard application is obtained after a double tempering between 525 and 575 C. Moreover, the Fig. 9 shows than the toughness of TENASTEEL is twice better than of D2 steel. And this is always true in the range of the hardness used for these cold work tool steels : HRC. In the other hand, the abrasive wear resistance of the two steels is comparable, then, TENASTEEL exhibits the best compromise between wear resistance and toughness.

15 Optimizing Microstructure for High Toughness Cold-Work Tool Steels 313 Figure 9. The toughness of TENASTEEL grade is almost twice better than of D2. SURFACE TREATMENT In order to increase their resistance to seizing up and to minimize the friction in service, tool steels are more and more frequently surface treated by nitriding or coated by metal deposits. This surface treatment also makes it possible to increase the surface hardness of pieces and to increase the tool life subjected to abrasive and/or adhesive wear. Nitriding is a thermochemical process of hard facing by atomic nitrogen diffusion on the surface of the pieces previously treated by hardening and tempering. The insertion of nitrogen atoms and the nitride formation with steel alloying elements, induce a hardening of surface (750 to 1400 HV) bringing the required properties : improvement of resistance to wear and seizing up of materials ; increasing of the stress limit of material because of the compressive stresses created by the processing ; maintain of metallurgical structures of the material and thus of its internal mechanical characteristics if the tempering has been carried out at a temperature higher than that of the surface treatment.

16 314 6TH INTERNATIONAL TOOLING CONFERENCE Comparative gaseous nitriding tests were performed at 525 Con TENAS- TEEL and X160 Cr Mo V12 / D2, both heat-treated to 60 HRC. Nitrided TENASTEEL layer appears homogeneous in depth and morphology, while that of D2 reveals a lot of carbides and exhibit an irregular depth (Fig. 10). For TENASTEEL, depth of nitrided layers measured by micro-hardness Figure 10. After gaseous nitriding at 525 C, TENASTEEL exhibits layers thicker and more homogeneous in depth and morphology than X160CrMoV12 / D2. readings or shown on micrographs are coherent, and the values obtained are respectively 60, 80 and 120 µm after treatment times of 4, 8 and 16 hours. For X160 Cr Mo V12 / D2, maximum depth reach only 50, 70 and 80 µm for same treatment times. But the Fig. 10 shows that in some places the nitrided layer can drop down to 20 µm after 4h of treatment. Moreover, due to the quantity of coarse carbides in the nitrided layer of X160 Cr Mo V12 / D2 and its interface with the substrate, a poor adhesion can be expected with possible shipping of the nitrided layer. The second very important

17 Optimizing Microstructure for High Toughness Cold-Work Tool Steels 315 Figure 11. Comparative hardness of TENASTEEL matrix and X160 Cr Mo V12/D2 matrix after gaseous nitriding at 525 C. point to note is the influence of the hardness of the core of the piece during nitriding. The Fig. 11 shows hardness records measured on the matrix of TENASTEEL and X160 Cr Mo V12 / D2 after 4, 8 16 hours of nitriding at 525 C(975 F). Both steels were heat-treated to 60 HRC before nitriding. TENASTEEL keeps its initial hardness after gaseous nitriding, a treatment time of 16 H induce only a drop of 1 HRC. Conversely, the hardness of X160 Cr Mo V12 / D2 is strongly affected as it drops down from 60 to respectively 56, 55, and 50 HRC after 4, 8 and 16 hours of nitriding. This softening is not surprising, looking at Fig. 7 showing the evolution of hardness versus the holding time at 525 C. Some other tests were carried out with ionic nitriding process at 500 C(930 F). Like after gaseous nitriding, the layer at TENASTEEL surface seems to be homogeneous as well in thickness as in morphology. Moreover, as already mentioned, layers on X160 Cr Mo V12/ D2 include much carbides and present a very irregular thickness. For TENASTEEL, measurement of nitrided layers thickness obtained on micrographs of the Fig. 12 or by micro-hardness measurements, are coherent and gives values of about 100 and 140 µm for 6 and 24 hours treatment time respectively. For X160 Cr Mo V12 / D2, thickness have a maximum

18 316 6TH INTERNATIONAL TOOLING CONFERENCE Figure 12. After ionic nitriding at 500 C, TENASTEEL exhibits layers thicker and more homogeneous in depth and morphology than X160CrMoV12 / D2 do. size of 50 and 100 µm for a same treatment duration. TENASTEEL allows to minimise nitriding time (6h to obtain 100 µm on TENASTEEL and 24 h to the same thickness on D2) and to obtain a same thickness layer all over the surface of the sheet. It has to be mentioned that the presence of coarse carbides in the nitrided zone and at the interface with substrate will reduce adhesion and lead to chipping of this layer in the case of X160 Cr Mo V12 / D2. Steels are heated at only 500 C(930 F) for this treatment, then TENASTEEL as well as X160 Cr Mo V12 / D2 save their matrix hardness. The cutting, forming tools, as well as molds elements for aluminum and plastic injection are frequently covered by titanium nitride which reduces to a significant degree the coefficient of friction, and very largely improves the abrasive and adhesive wear resistance. These coatings, obtained by vapor condensation on the surface of the substrate make it possible to form

19 Optimizing Microstructure for High Toughness Cold-Work Tool Steels 317 a metal deposit, which will grapple to the heat-treated surfaces. For this type of coating, the preparation of the surface of the substrate is an essential step. It will make it possible to solve possible problems of adherence by a cleaning and an activation of surface. For these PVD (Physical vapor deposition) and CVD (Chemical Vapor Deposition) coatings the main advantage of the TENASTEEL compared to the X160 Cr Mo V12 / D2 is due to the smoothness and the distribution of carbides. Indeed, the presence of large chromium carbides to the interface between the substrate and the coating decreases the adhesion of this one. CONCLUSION The aim of the new cold work tool steel grade with improved toughness, TENASTEEL, is to take the place of X160CrMoV12 / D2, currently most widespread on the market in spite of big problems of rupture, of damage by chipping or adhesion due to its too low toughness. Its chemical composition was adapted in order to decrease the volume fraction of large primary chromium carbides and improve the toughness of steel. An increase in molybdenum content compensates this decrease of the carbon and chromium content to preserve a good wear resistance because of finer and dispersed secondary carbides contribution. Moreover, an addition of titanium refines the structure. After austenitization at 1030 or 1050 C, temperatures compatible with the current practices for the processing of the other grades, the TENASTEEL will be tempered twice between 525 and 575 Cto obtain a standard hardness ranging between 58 and 62 HRC. For particular applications requiring a still increased toughness, tempering at higher temperature could be practised in order to decrease the hardness of steel. At equal level of hardness, the toughness of TENASTEEL is twice better than of D2 for a wear resistance comparable. Moreover, the higher tempering temperatures and the thinner carbides and structure confer to TENASTEEL a very good aptitude for the surface coating : to obtain homogeneous layers in thickness and in morphology ; to reduce heating time ; to improve coating adherence ; to save the matrix hardness.

20 318 6TH INTERNATIONAL TOOLING CONFERENCE Lastly, weldability, polishability and machinability of the TENASTEEL are also higher than that of the D2. For example, compared with grades X160 Cr Mo V12 / D2, TENASTEEL allows an increasing of the tool life during machining : 30% in softened condition 75% in hardened condition The size and the dispersion of the carbides can explain these good properties. REFERENCES [1] R. EBNER, H. LEITNER, F. JEGLITSCH, D. CALISKANOGLU "Tool Steels in the next century" 5th International Conference on Tooling Loeben [2] H. BERNS "New Materials Processes Experiences for Tooling" International European Conference on Tooling Materials Interlaken [3] S. CORRE, C. LE CALVEZ, P. MABELLY, F. CHENOU, J. BEGUINOT "Tool Steels in the next century" 5th Interntional Conference on Tooling Loeben [4] H. JESPERSON "Tool Steels in the next century" 5th Interntional Conference on Tooling Loeben [5] D. YOKOI, N. TSUJII European Patent Application EP A1.