Correlation Between Secondary Precipitation and Tensile Ductility of High-Speed Steels
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INTRODUCTION
HIGH-SPEED steels are complex ferrous-based Fe-C-X multicomponent alloys, where X represents a series of alloying elements (e.g., W, Mo, Cr, V, Co, Si, or Mn), and are widely used in high-temperature applications owing to their high hardness as well as their superior softening and wear resistance at elevated temperatures. Their excellent mechanical properties are closely related to a high number density of primary and secondary carbides, including M6C, MC, M23C6, and M2C embedded in tempered martensite.[1] Micron-sized primary carbides are formed in the eutectic reaction and develop in distributions and dimensions during the subsequent thermomechanical processing, whereas submicron or nanoscale secondary carbides primarily originate from a supersaturated matrix during the annealing or tempering procedure. The entire amount of primary
XUEFENG ZHOU, WENTAO LI, HONGBING JIANG, FENG FANG, and YIYOU TU are 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] JIANQING JIANG is with Nanjing Forestry University, Nanjing 210037, P.R. China. Manuscript submitted November 2, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
and secondary carbides can reach as high as 20 wt pct and play a significant role in the mechanical properties of high-speed steels.[2] Progress has been made over the past several decades in a performance enhancement of high-speed steels by employing new processing methods and a cost reduction by developing low-alloying steels. Nevertheless, there still exist big challenges such as a very low ductility and a poor formability. Primary carbides exhibit a limited deformation compatibility with neighboring ferritic grains, which leads to a stress concentration or crack nucleation and propagation at the carbide/ferrite interface.[3] With the aim of enhancing mechanical properties, a homogenous distribution and fine dimension of primary carbides has been the core of microstructure modification in high-speed steels. The majority of these efforts involve hot deformation,[4] rapid solidification,[5,6] multiple modifiers,[7,8] and phase transformation refinement,[9] which have been demonstrated as effective methods of enhancing microstructure homogeneity. Nevertheless, these approaches appear to reach a limit in the enhancement of ductility and toughness when primary carbides are reduced to a few microns. In recent years, research work on ultra-fine-grained (UFG) materials has shed more light into the challenge of ductility enhancement. UFG materials show excellent mechanical properties, such as extremely high strength and good toughness, but exhibit a very low ductility in most cases, which is caused by plastic instability due to
higher stress and the lower strain hardening rate of ultrafine grains.[10] Intensive efforts have been devoted to enhance the ductility of UFG materials via an enhancement of
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