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

硼作为一种硬相形成合金元素，是提高工具钢摩擦学性能的一种经济可行的方法。硼化物相的定向稳定用于从硬相形成中分离马氏体硬化（由碳促进）。特别是M2B型硼化物（Fe2B, Cr2B, (Fe,Cr)2B）比M7C3型碳化物具有更高的硬度和断裂韧性。这些富含cr的M7C3碳化物在传统的工具钢概念中用作硬相。在第一个资助期收集的结果表明，高B/(B+C)-比率促进M2B型硼化物的形成。此外，合金元素Cr进一步稳定了M2B相，提高了其硬度和断裂韧性等微观力学性能。从Fe-C-B和Fe-Cr-C-B体系的凝固顺序来看，除M2B型碳化物外，还形成M3（C，B）型或M23(C，B)6型碳化物。这些较软的碳化物的形成与C和Cr元素的消耗有关。这抵消了金属基体因C耗尽而产生的马氏体淬透性，并且由于M2B相中溶质Cr含量的降低而降低了M2B相的硬度。初步试验和文献数据表明，Mn和Mo元素进一步稳定了M2B型相，同时避免了Fe-C-B和Fe-Cr-C-B体系中不良碳化物的形成。然而，表征和理解合金元素Mo和Mn在Fe-C-B和Fe-Cr-C-B体系中的作用仍然是一个研究兴趣领域。对于工具钢的发展，重点研究的是凝固顺序、组织和相组成、固态相变和形成的硬相的微观力学性能。在更新建议的范围内，热力学计算和实验室熔体的金相表征同时用于揭示Cr和Mn或Mo元素在潜在机制中的相互依赖性。此外，B元素对二次硬化析出的影响也备受关注。通常，二次硬化碳化物是由Mo、Cr、V和C等合金元素在淬火工具钢回火过程中形成的。硼的低溶解度及其同时在金属基体中的高扩散性是否会导致二次硬化硼化物或碳硼化物的形成，这将被研究。结果将用于推导b合金工具钢的合金化概念。利用这一概念，应该开发一种具有优选球晶M2B硼化物的合金，这些硼化物嵌入马氏体金属基体中。最后，对该合金的摩擦学性能进行了表征，使其与传统的Fe-Cr-C基基准合金具有可比性。

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.