Microstructural Evolution, Behavior of Precipitates, and Mechanical Properties of Powder Metallurgical High-Speed Steel
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RODUCTION
RECENTLY, fine-blanking has rapidly developed, and subsequently, the demand for high-strength highspeed steels (HSSs) increased.[1–3] To improve the service lifetime of fine-blanking tools, casting HSSs have been gradually substituted by powder metallurgical (PM) HSSs. In particular, PM HSS S390 has been widely used to manufacture fine-blanking punches. The excellent microstructure of PM HSS resulted in improved wear resistance, compressive strength, and fatigue strength.[4,5] In addition to using high-quality steel, cryogenic treatment,[6,7] quenching,[8] and tempering
HANLIN PENG, XIANGLIN ZHANG, XING WEI, and JIANYU ZHOU are with the State Key Laboratory of Material Processing and Die and Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China. Contact e-mail: [email protected] LING HU is with the School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China and also with Thayer School of Engineering, Dartmouth College, Hanover, NH 03755. Contact e-mail: [email protected] LIEJUN LI is with the School of Mechanical and Automotive Engineering, South China University of Technology. This work was financially supported by the Science and Technology Support Program of Hubei Province (Grant No. 2015BAA015). Manuscript submitted April 10, 2018. Article published online November 29, 2018 874—VOLUME 50A, FEBRUARY 2019
treatments[9,10] have been the main approaches to optimize the mechanical properties of HSSs. To date, extensive studies have been conducted on PM HSS S390. Niederkofler et al.[11] observed needleshaped precipitates and spherical particles with sizes of approximately 40 nm in annealed PM HSS S390 samples. Godec et al.[12] confirmed that the carbides observed in these HSSs were MC and M6C carbides. However, no studies on the crystallographic orientation relationship (OR) between the precipitated particles and matrix have been published. Torres et al.[13] investigated the fatigue behavior of PM HSS S390 and assessed the fatigue crack growth–fatigue life relationship. Additionally, the influence of cryogenic treatment and simultaneous pulse plasma nitriding on the wear resistance of PM HSS S390 has been reported.[14,15] Recently, our previous papers[8,16] reported the significant effect of the austenitizing temperature on the mechanical properties of PM HSS S390, and we determined the appropriate austenitizing temperature to be in the 1120 °C to 1180 °C range. However, no studies on the influence of the tempering temperature and times on PM HSSs have been published. It is well known that quenching and tempering are conventional heat treatment methods for most tool steels.[17,18] When quenching low-carbon steel, precipitates usually dissolve into the matrix and a martensitic lath structure featuring high dislocation density forms
METALLURGICAL AND MATERIALS TRANSACTIONS A
during rapid cooling.[19] The subsequent changes in microstructural and mechanical properties during high-temperature tempering are very complex[20,21]: (
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