Enhanced microstructures and properties of spray-formed M3:2 high-speed steels by niobium addition and thermal-mechanica
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Enhanced microstructures and properties of spray-formed M3:2 high-speed steels by niobium addition and thermalmechanical treatment Hebin Wang1, Longgang Hou2,a), Ping Ou1, Xiaofeng Wang3 Hongjin Zhao1,b), Hua Cui2, Jishan Zhang2
, Yabin Li1, Li Shen1,
1
School of Materials Science and Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China The Faculty of Mechanical Engineering and Mechanics, Ningbo University, Ningbo 315211, China a) Address all correspondence to these authors. e-mail: [email protected] b) e-mail: [email protected] 2 3
Received: 5 July 2018; accepted: 15 November 2018
M3:2 high-speed steel (HSS) billets with or without Nb addition were prepared by spray deposition. The effects of Nb and post-thermal-mechanical processing (decomposition treatment and hot forging), as well as heat treatment, on the microstructure and properties of M3:2 HSS were investigated. The microstructure of the asdeposited M3:2 HSS consisted of equiaxed grains with a mean size of approximately 25 lm and discontinuous plate-like M2C and irregular MC carbides distributed along grain boundaries. 0.5% Nb addition can refine the M2C plates and spheroidize MC carbides. With 2% Nb addition, the refined grains with a mean size of approximately 12 lm and continuous net of M6C and a uniform distribution of NbC carbides were obtained. The decomposition of metastable M2C carbides can be accelerated with 0.5% Nb addition due to the refined size and lower thermodynamic stability of M2C plates. With the increased degree of decomposition of M2C carbide, the M6C and MC carbides became refined and more uniformly distributed after optimal thermalmechanical processing and heat treatment, which leads to a significant increase in bend strength and toughness.
Introduction Although production and demand for the tool materials such as sintered carbides, ceramic materials, and extremely hard materials increased dramatically in recent years, high-speed steels (HSSs) are still one of the main materials used for the production of cutting and plastic working tools, because of their excellent integrated performance, such as high hardness, satisfactory ductility, and good machinability [1, 2, 3]. Such properties are derived from the microstructure comprising heavily twinned martensite matrix strengthened by evenly distributed nanosized precipitates, and a population of primary carbides [4, 5, 6]. However, due to the slow cooling rate in conventional ingot metallurgy (IM), many coarse primary carbides such as dentrite and conglomeration fishbone eutectic carbide are typically precipitated from melt, leading to damage toughness and ductility. Even though these carbides can be
ª Materials Research Society 2019
broken up during hot working process, the formed angular carbide with zonal distribution will reduce isotropy and decrease toughness in the transverse direction [7, 8]. To solve this problem, powder metall
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