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HIGH-SPEED steel (HSS), which is widely used in manufacturing cutting tools, is characterized by its excellent hardness and wear resistance.[1–3] Among the types of the HSS, AISI M42 HSS is one of the most popular ones due to its excellent hardness at both room and elevated temperatures. Traditionally, AISI M42 HSS is produced in the sequence as follows: induction furnace smelting fi casting consumable electrode fi electroslag remelting (ESR) fi forging fi spheroidizing annealing fi quenching fi multiple tempering.[4,5] The typical cast structure of AISI M42 HSS consisting of dendrites surrounded by coarse eutectic carbides will

YI-WA LUO, HAN-JIE GUO, and JING GUO are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing, 100083, P.R. China and with the Beijing Key Laboratory of Special Melting and Preparation of High-End Metal Materials, Beijing, 100083, P.R. China. Contact e-mail: [email protected] XIAO-LIN SUN and FEI WANG are with the Tianjin Cisri-Harder Materials & Technology Co. LTD, Central Iron and Steel Research Institute (CISRI), Tianjin, 301721, P.R. China. Manuscript submitted March 5, 2018.

METALLURGICAL AND MATERIALS TRANSACTIONS A

severely deteriorate the properties of high-speed steel.[6,7] Such microstructure can be eliminated by a series of heat-treatment processes, and the final microstructure after heat treatment is composed of tempered martensite with fine and uniform carbides.[8–11] Consequently, the microstructure, which achieves the best mechanical properties of the steel, occurs at the final stage of the heat treatment. Thus, microstructural factors such as the distribution and dimension of final carbides and the characteristics of tempered martensite matrix play important roles in optimizing the hardness, toughness, and wear resistance of high-speed steels. A study on the effects of quenching conditions on the microstructure and precipitating behavior of AISI M42 HSS was previously carried out by the authors.[12] Results showed that the microstructure after quenching consisted of martensite, retained austenite, and carbides. An appropriate increase of the quenching temperature would result in a more homogenous carbide distribution in the matrix and increase the hardness of the steel. Moreover, it was found that a high cooling rate depressed the martensitic transformation and increased the fraction of retained austenite, which negatively affects the hardness of the steel. To further improve the mechanical properties of high-speed steels, it is required to optimize the microstructure and refine the

carbide precipitates during the process of tempering. After quenching, high-speed steels are usually tempered to obtain a perfect combination of hardness and impact toughness, together with the secondary hardening caused by carbide precipitates.[13,14] During tempering, the retained austenite decomposes to form ferrite and cementite, and then precipitates at interfaces as fine particles.[15–17] In addi