Effect of Pre-deformation on Decomposition and Spheroidization of M 2 C Carbide in High-Speed Steel
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HIGH-SPEED steel (HSS) is a high-carbon multi-component alloy and exhibits superior mechanical properties both at ambient and elevated temperatures such as high hardness, good wear resistance, and reasonable toughness. Its outstanding properties originate from the representative microstructure, i.e., substantial primary carbides embedded in tempered martensite decorated with appreciable nano-sized precipitates. Primary carbides can act as hard particles and determine wear resistance and toughness of HSS. Nevertheless, primary carbides that are products of eutectic solidification are very coarse and heterogeneously distributed along grain boundaries. Large-sized carbides fracture more readily under applied stress and
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 e-mail: [email protected] WEICHAO ZHANG, ZHIXIA ZHENG, DI LIU, 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 September 30, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
cause a deteriorated fracture toughness. Hence, obtaining small carbides with a homogeneous distribution has been a main challenge for microstructure design of HSS. Different types of primary carbides may form in HSS, such as M2C, M6C, and MC, depending upon chemical compositions and cooling rates.[1] Among them, M2C is the most predominant type and exists in almost all HSS. Metastable M2C carbides decompose into a mixture of M6C and MC carbides at elevated temperatures, facilitating carbide spheroidization and refinement.[2,3] M2C decomposition and spheroidization occur during the diffusion annealing step in industrial productions of HSS. In most cases, however, diffusion annealing requires a high temperature (1100 C to 1200 C) and a prolonged time (1 to 4 hours) for complete carbide decomposition.[3,4] This may induce pronounced coarsening of carbides and consequently a deteriorated toughness of HSS. In order to overcome shortcomings of diffusion annealing, further heavy plastic deformation such as forging and rolling is usually employed in the industrial production. Post-deformation following diffusion annealing can disperse decomposed carbide mixtures and refine carbide dimensions to some extent. Effective carbide refinement occurs only at very large strains. Nevertheless, large strains cannot be guaranteed in
many cases. For instance, large-sized carbides remain a big issue in large-section wrought HSS products with limited forging ratios. Deformation can exert a significant influence on thermodynamics and kinetics of phase transformation. Deformation-induced phase transformation, such as deformation-induced martensite transformation (DIMT)[5] and ferrite transformation (
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