B-alloyed tool steels - thermodynamic calculation and metallographic characterization of the effect of the alloying elements Mn and Mo in the quaternary system Fe-C-B-Cr

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As a hard-phase forming alloying element, boron is a cost-effective possibility to improve the tribological performance of tool steels. Targeted stabilization of boride phases is used to uncouple martensitic hardening (promoted by carbon) from hard phase formation. In particular, borides of M2B type (Fe2B, Cr2B, (Fe,Cr)2B) feature higher hardness and fracture toughness than the carbide of type M7C3. These Cr-rich M7C3 carbides are used as hard phase in conventional tool steel concepts. Results gathered in the first funding period show that high B/(B+C)-ratios promote the formation of M2B type borides. In addition, the alloying element Cr further stabilizes the M2B phase and improves its micro-mechanical properties like hardness and fracture toughness. With respect to the solidification sequence in the system Fe-C-B and Fe-Cr-C-B, carboborides of M3(C,B) or M23(C,B)6 type are formed besides the M2B type boride. The formation of these softer carboborides is associated by a consumption of the elements C and Cr. This counteracts a martentisic hardenability of the metal matrix by C depletion and decreases the hardness of M2B phase due to decreased solute Cr-content inside the M2B phase. Preliminary test as well as literature data indicate that the elements Mn and Mo further stabilize the M2B type phase, while avoiding the formation of undesirable carboborides in the systems Fe-C-B and Fe-Cr-C-B.However, characterization and understanding of the effects of the alloying elements Mo and Mn in the systems Fe-C-B and Fe-Cr-C-B remains an area of research interest. Regarding the development of tool steels, focus of the investigations is placed on solidification sequence, resulting microstructure and phase composition, phase transformation in solid-state and micro-mechanical properties of the formed hard phases. Within the scope of the renewal proposal, thermodynamic calculation and metallographic characterization of laboratory melts are simultaneously used to uncover the interdependencies of the elements Cr and Mn or Mo in the underlying mechanisms. Furthermore, the effect of the element B on secondary-hardening precipitations is of high interest. Commonly, secondary-hardening carbides are formed by the alloying elements Mo, Cr, V and C during tempering a quenched tool steel in the secondary hardening regime. It will be investigated, whether the low solubility of boron and its simultaneously high diffusivity in the metal matrix enable the formation of secondary-hardening borides or carboborides. Results will be used to deduce an alloying concept for a B-alloyed tool steel. Using this concept, an alloy featuring preferably spherolitic M2B borides, which are embedded inside a martensitic metal matrix should be developed. Finally, tribological properties of this alloy are characterized to enable comparability with a conventional Fe-Cr-C based reference alloy.