NEW BÖHLER POWDER METALLURGY HIGH SPEED STEEL WITH EXCELLENT HOT HARDNESS

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1 NEW BÖHLER POWDER METALLURGY HIGH SPEED STEEL WITH EXCELLENT HOT HARDNESS I. Maili, R. Rabitsch, W. Liebfahrt, H. Makovec and E. Putzgruber BÖHLER EDELSTAHL GMBH & Co KG Mariazellerstr. 25 A 8605 Kapfenberg AUSTRIA Abstract Keywords: High Performance Cutting that means reducing production costs by using higher cutting speeds and by reducing the amount of coolant and lubricant (for example using dry cutting). For the cutting tools this means a much higher requirement for strength and hot hardness than under normal cutting conditions. Böhler has therefore developed a new super-hard powder metallurgy high speed steel which is more highly alloyed than state-of-the-art high speed steels. This new Böhler grade has a well-balanced chemical analysis and is produced via the new Böhler-Uddeholm-Powder-Technology route, which includes the most modern production facility available for powder metallurgy steels. This brings the advantages of high hardness up to 70 HRc, high wear resistance, superior hot hardness in combination with very good machinability and a better ductility than comparable Powder Metallurgy High Speed Steels. With all these advantages and especially with the advantage in its hot hardness, the new Böhler steel is excellent for cutting tools such as end mills, milling cutters, hobs, etc. Due to its high wear resistance and very high compression yield strength this steel grade can also be used for special cold work applications. Powder metallurgy, High speed steel, Hot hardness 377

2 378 6TH INTERNATIONAL TOOLING CONFERENCE INTRODUCTION The development of the first high-speed steels started over 100 years ago. High speed steels were developed to machine metals at high temperatures and with minimum tool wear. New high speed steels with different chemical compositions were continuously developed to meet the ever increasing demands placed on cutting tools. Today, there are definite trends in the machining industry towards cost reduction, which means increasing the cutting speed and decreasing the amount of lubricant as far as dry cutting. These extreme requirements for cutting tools have led to the fact that state-of-the-art high-speed steels are often not sufficient, so that cemented carbide tools are used although there are numerous disadvantages. A cemented carbide tool has a very high price; it is much more difficult to machine and because of its modest toughness a tool made of cemented carbides is susceptible to impact which leads to untimely breakage. This is the reason that higher alloyed high speed steels are still in demand [1, 2]. These highly alloyed steels can only be produced by powder metallurgy methods, which is why Böhler Edelstahl reacted by installing the most modern PM-plant in the world [3]. Accordingly for Böhler Edelstahl this resulted in the development of a new highly alloyed steel grade, which is distinguished in an optimum combination of relevant properties. NEW STEEL GRADE: BÖHLER S290PM MICROCLEAN BACKGROUND TO THE ALLOY DESIGN AND CHEMICAL COMPOSITION At the moment, Böhler s top benchmark in the field of PM-HSS is Böhler S390PM Microclean. With its advantages of a hardness up to 68 HRc, a very good wear resistance and hot hardness in combination with good toughness and best machinability, Böhler S390PM Microclean has fixed its position in the market. New trends in the cutting industry, such as high-speed-cutting or dry cutting, increase the demands placed on cutting-tools regarding wear resistance and hot hardness. For this reason, cemented carbides are used even though the machinability and the toughness of the tools are inadequate.

3 New Böhler Powder Metallurgy High Speed Steel with Excellent Hot Hardness379 Böhler saw the necessity to develop a new high-speed-steel which can fill the gap between cemented carbides and Böhler S390PM, see Fig. 1. This Figure 1. Background to the development of the new PM-HSS grade S290PM Microclean. is the reason that the target was set to produce a new PM-HSS with the following properties: a hardness capability of about 70 HRc a much higher thermal stability than that of S390PM adequate toughness properties similar machinability to S390PM and better machinability than other highly alloyed PM-HSS with more than 9 wt% cobalt The new alloy was designed with an increased volume fraction of primary carbides compared to Böhler S390PM. But the high hardness capability was not only reached because of a very high carbide content. By a specific intervention in the chemical composition of the matrix, the microstructural coarsening and the growth of the secondary carbides were positively influenced regarding hot hardness. A good toughness was achieved by balancing the chemical composition, which influences the matrix and leads to a limited number of primary carbides. The chemical composition of the new PM-HSS grade S290PM Microclean in comparison to Böhler S390PM is shown in Table 1.

4 380 6TH INTERNATIONAL TOOLING CONFERENCE Table 1. Chemical composition of the new PM-HSS grade S290PM Microclean in comparison to Böhler S390PM Microclean Steel grade % C % Cr % W % Mo % V % Co S390PM S290PM PROPERTIES OF THE NEW STEEL GRADE BÖHLER S290PM MICROCLEAN Hardening and Tempering Behaviour. Figure 2 shows the hardening and tempering behaviour of Böhler S290PM in comparison to Böhler S390PM for austenitising temperatures ranging from 1150 Cup to 1210 C. Figure 2. Hardening and tempering behaviour of the new PM-HSS grade S290PM Microclean in comparison to Böhler S390PM Microclean. All of the samples were austenitised in a vacuum furnace, quenched by N 2 gas down to room temperature, and tempered three times for 2 hours each. As can be seen in Fig. 2, within the temperature range relevant for highly alloyed high-speed-steels, hardness values up to 70 HRc are achievable.

5 New Böhler Powder Metallurgy High Speed Steel with Excellent Hot Hardness381 All of the samples austenitised at 1190 Cand 1210 Cachieved about 70 HRc after tempering 3 times at 560 C, which is the minimum tempering temperature usually recommended for the elimination of retained austenite and optimum toughness. At this stage the hardness of the S290PM is clearly higher than that of the S390PM. Hardening from 1150 Cand tempering 3 times at 540 Cor 560 Cresulted in a hardness of 69+ HRc which is an excellent value when considering that this heat treatment is a typical commerical heat treatment for non-cutting tool applications. Toughness. The toughness of the new PM-HSS grade S290PM Microclean is positively affected by: a very well balanced chemical analysis of the matrix a very homogeneous microstructure very fine and homogeneously distributed carbides very low non-metallic inclusion content (K0 = 0 1 ) [4] Figure 3 compares the toughness, in the form of impact energy, of the new steel-grade to Böhler S390PM and its competitors. Figure 3. Toughness of the new PM-HSS grade S290PM Microclean in comparison to Böhler S390PM Microclean and its competitors. All of the samples were hardened from 1150 Cin vacuum, cooled by N 2 -gas quenching to room temperature and were tempered three times to a hardness value of 67 HRc.

6 382 6TH INTERNATIONAL TOOLING CONFERENCE Böhler S390PM has a well-known reputation for a very good toughness even at a high hardness value. As is shown in Fig. 3, it reaches an impact energy value of more than 30 J/cm 2 at the chosen heat treatment parameters. Böhler S290PM Microclean is of course not as tough as the Böhler S390PM Microclean because of its higher alloying element content but it reaches an impact energy value of more than 20 J/cm 2 which is an amazing result especially when compared to its highly alloyed competitors. Hot Hardness. Figure 4 was constructed by testing the materials using the laboratory facility at the Materials Center Leoben [5]. Figure 4. Hot hardness of the new PM-HSS grade S290PM Microclean at a temperature of 650 Cin comparison to its competitors. The effect of isothermal tempering on the hardness changes of the new PM-HSS grade S290PM Microclean in comparison to its competitors is shown in Fig. 4. All the samples were hardened at 1190 Cin a vacuum furnace and tempered to a hardness value of 68 HRc. The hot hardness shown in Fig 4 was measured at a temperature of 650 Cover time. The temperature of 650 Cwas chosen because it is a typical temperature at the tip of a tool for example when cutting non-ferrous-metals (e.g. aluminium). The hot hardness is expressed by dynamic hardness values which cannot easily be converted to the Vickers scale. But by testing fully heat-treated steel grades this method has been proven as a new and powerful tool for obtaining quantitative data for softening at high temperatures.

7 New Böhler Powder Metallurgy High Speed Steel with Excellent Hot Hardness383 The new PM-HSS grade S290PM Microclean exhibits a higher hot hardness level than Böhler S390PM and other highly alloyed PM-HSS with more than 9 wt% cobalt. For a cutting tool during operation this means that a Böhler S290PM-tool does not soften as fast, and this indicates that the life time of the tool would probably be substantially higher. This was confirmed in practical cutting tests carried out by tool-makers. Wear resistance / compressive strength. Wear behaviour and compressive strength are very important criteria for the efficiency of alloyed tool steels. They are characterised by parameters such as carbide hardness, carbide percentage, carbide size and distribution, and matrix hardness. Figure 5 compares the optical micrographs of Böhler S290PM and Böhler S390PM. The samples were austenitised at 1210 Cand multiply tempered at 560 C. Figure 5. Polished and etched microstructure of Böhler S390PM Microclean (a) and the new PM-HSS grade S290PM Microclean (b). Samples austenitised at 1210 C The total carbide volume in Böhler S290PM is significantly greater than that of Böhler S390PM. Because of the higher amount of carbides in Böhler S290PM and because of the higher alloyed matrix, Böhler S290PM is of course better in its wear resistance and its compressive strength. Despite this, tool makers have reported that Böhler S290PM is not more difficult to machine than Böhler S390PM.

8 384 6TH INTERNATIONAL TOOLING CONFERENCE SUMMARY The most modern Powder Metallurgy plant in the world has been installed at Böhler Edelstahl GmbH & Co KG in Kapfenberg, Austria. This plant makes it possible to produce highly alloyed steel grades. A new steel grade, BÖHLER S290PM Microclean, has been developed for use in cutting applications such as end mills, milling cutters, hobs, etc. and it can also be successfully used for particular cold work applications such as punching metal flats or pressing metal powder. A very well balanced chemical analysis resulted in a high hardness of about 70 HRc, however the high hardness is not only achieved by an extremly high carbide content, but also by a specific influence on the matrix. This - in combination with production via the brand new Powder Metallurgy method at Böhler with its advantages of best cleanliness, finest powder-size, finest carbide-size and highly homogeneous carbide-distribution ensures that BÖHLER S290PM Microclean shows best toughness and best hot hardness and is easy to machine. All of these extraordinary advantages make the new steel grade BÖHLER S290PM Microclean a cost effective alternative to carbide cutting-tools and to cemented carbides in cold-work applications, which may be too brittle or to difficult to machine. ACKNOWLEDGMENTS The authors are grateful to the Institut für Metallkunde und Werkstoffprüfung at the Montanuniversität Leoben and to the Materials Center Leoben for carrying out the hot hardness investigations. REFERENCES [1] P. HELLMAN, "High speed steels by powder metallurgy", Veitsch-Radex Rundschau 1/1999, p.16 [2] P. HELLMAN, D. FRANCHI and G. PANIALE, International Conference on Tooling Materials, Interlaken, Sept. 1992, p [3] C. TORNBERG, A. FÖLZER, "Fully Dense PM Tool Steels and High Speed Steels with 3rd Gereration Processing Technology", PM2001, p.121. [4] DIN [5] H. LEITNER, R. EBNER, B. MAJOR and G. PÖCKL, "Investigations on the thermodynamics and kinetics of secondary hardening carbides in high speed steels", 5th

9 New Böhler Powder Metallurgy High Speed Steel with Excellent Hot Hardness385 International Conference on Tooling, Leoben 1999, p.369.