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

作为硬质相形成的合金元素，硼是提高工具钢摩擦学性能的一种经济有效的方法。定向稳定渗硼相用于使马氏体硬化(由碳促进)从硬质相形成中分离出来。特别是M2B型(Fe2B、Cr2B、(Fe，Cr)2B)相较M7C3型碳化物具有更高的硬度和断裂韧性。在传统的工具钢概念中，这些富铬M7C3碳化物被用作硬质相。在第一个资助期收集的结果表明，高的B/(B+C)-比促进了M2B型硼化物的形成。此外，合金元素铬进一步稳定了M2B相，改善了其硬度和断裂韧性等微观力学性能。就Fe-C-B和Fe-Cr-C-B系统的凝固顺序而言，除了M2B型硼化物外，还形成了M3(C，B)或M23(C，B)6型碳化物。这些较软碳化物的形成与C和Cr元素的消耗有关。这抵消了金属基质的马氏体淬透性，并由于M2B相中溶质铬含量的减少而降低了M2B相的硬度。初步试验和文献数据表明，元素Mn和Mo进一步稳定了M2B型相，同时避免了在Fe-C-B和Fe-Cr-C-B系统中形成不需要的碳化物。但是，表征和了解合金元素Mo和Mn在Fe-C-B和Fe-Cr-C-B系统中的作用仍然是一个研究兴趣的领域。在工具钢的发展方面，重点研究了凝固顺序、所产生的组织和相组成、固态相变和形成的硬质相的微观力学性能。在更新建议的范围内，同时使用实验室熔体的热力学计算和金相表征来揭示元素铬和锰或钼在潜在机制中的相互依存关系。此外，元素B对二次硬化析出的影响也引起了人们的高度关注。通常，二次硬化碳化物是由合金元素Mo、Cr、V和C在二次硬化区回火工具钢时形成的。我们将研究，硼的低溶解度和其在金属基质中的高扩散系数是否能够形成二次硬化的硼化物或碳化物层。结果将被用来推断B合金化工具钢的合金化概念。利用这一概念，应该开发一种合金，其特征最好是球晶M2B硼化物，它嵌入在马氏体金属基质中。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基参比合金具有可比性。

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.