Deformation-Induced Carbide Transformation in M2 High-Speed Steel
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-speed steel (HSS) is a complex ferrous multicomponent alloy and widely used in high-temperature applications, owing to its superior hardness, wear resistance and red-hardness at elevated temperatures. Its excellent mechanical properties are attributed to the representative microstructure, i.e., tempered martensite decorated with a large amount of primary carbides that can reach almost 20 wt pct or even more. Various types of primary carbides may exist in HSS, including M2C, M6C, MC, etc., depending on the chemical composition and processing route.[1] Primary carbides can act as hard particles and significantly enhance the wear resistance of HSS. Nevertheless, high-hardness carbides are
XUEFENG ZHOU is with the School of Materials Science and Engineering, Southeast University, Nanjing 211189, P.R. China, and also with the Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing 211189, P.R. China. Contact email: [email protected] ZHIXIA ZHENG, WEICHAO ZHANG, FENG FANG, and YIYOU TU are with the School of Materials Science and Engineering, Southeast University. JIANQING JIANG is with the College of Mechanical and Electronic Engineering, Nanjing Forestry University, Nanjing 210037, P.R. China. Manuscript submitted August 12, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
susceptible to brittle fracture under applied stress in most cases and hence induce a lower fracture toughness. The trade-off between wear resistance and fracture toughness, which is closely correlated to the mechanical properties of primary carbides, is a main challenge to production of HSS. Primary carbides with excellent intrinsic properties, i.e., a good combination of high hardness and fracture toughness, are a prerequisite for optimizing the mechanical properties of HSS. The mechanical properties of carbides are inherently determined by their crystal structure and chemical composition. M2C carbide enriched in Mo and W presents a hexagonal closepacked (HCP) structure and is the predominant carbide type in M2 HSS. Nonetheless, it only exhibits a hardness of 1500 to 2000 HV (14.7 to 19.6 Gpa) and a fracture toughness of 0.9 to 1.1 MPa m1/2, respectively,[2] so has low resistance to wear and fracture. Fortunately, M2C carbide is thermodynamically metastable at elevated temperatures and transforms into a mixture of M6C and MC carbides. M6C carbide enriched with Fe, W or Mo has a complex face-centered cubic (FCC) structure, whereas V-rich MC carbide exhibits a NaCl (B1) crystal structure.[3] MC and M6C carbides display significantly enhanced hardness and fracture toughness, which are 20.3 to 26.1 GPa, 2.2 to 3.7 MPa m1/2 and ~ 18.1 GPa, ~ 3.3 MPa m1/2, respectively.[4] The transformation of M2C into MC and M6C favors a simultaneous enhancement of wear and fracture resistance of M2 HSS.[5,6] However, M2C usually starts to decompose at high temperatures (> 850 C) since M2C decomposition is controlled by the diffusion of substitutional elements in carbides and a high activation energy is necessary.[7] Such a high transfo
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