Effects of Annealing on Carbides Size and Distribution and Cold Formability of 1.0C-1.5Cr Bearing Steel
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IN order to improve workability, hypereutectoid steels, such as high-speed steel and bearing steel, are usually subjected to spheroidization annealing.[1,2] The cementite lamellae gradually change their shapes into spherical particles during spheroidization, which leads to the decrease of tensile strength and increase of fracture toughness.[3–5] Hence, the spheroidization of cementite lamellae is beneficial to the cold formability of steels. In addition, the size and distribution of carbides particles would have a dramatic influence on the final properties due to the microstructure heredity.[6] Therefore, it is of great significance to investigate the spheroidizing annealing of steels. The spheroidization of pearlite steels is closely related with the initial microstructure characteristics including the density of holes presented in cementite lamellae, interlamellar spacing of cementite, and pearlite colony size. Tian and Kraft[7,8] studied the mechanisms and kinetics of pearlite spheroidization, they concluded that the recession of cementite plates terminations and expansion of holes led to the breakup of large cementite plates into several smaller segments. Wang et al.[9] investigated the pearlite spheroidization using threedimensional tomography, they found that the holes presented in cementite lamellae could cause the shape instability and induce the shape evolution. The work by ZHEN-XING LI and JIAN ZHANG, Ph.D. Students, and CHANG-SHENG LI, Professor, are with the State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, P.R. China. Contact e-mail: [email protected] BING QIAO and ZHONG-ZHI LI, Senior Engineers, are with the Dongbei Special Steel Group Co., Ltd, Dalian 116105, P.R. China. Manuscript submitted November 20, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
Ko et al.[10] and Arruabarrena et al.[11] indicated that the deformation led to the formation of defects on both cementite and ferrite matrix, which could accelerate the breakup of cementite lamellae during spheroidization. Gang et al.[12] reported that due to the increase of interface area between cementite and ferrite matrix, the refining of interlamellar spacing could also facilitate the breakup of lamellar cementite. There are mainly two types of spheroidization annealing for pearlite steels in industry production.[13,14] The one is subcritical spheroidization, in which steels are held below A1 temperature for several hours and then cooled to room temperature. The other one is the intercritical spheroidization, in which steels are held within A1-A3 or A1-Acm for a few hours at first and then cooled to a certain temperature below A1. Subsequently, they were held for a period of time at this temperature or slowly cooled to a lower temperature. For dissolving grain boundary cementite, hypereutectoid steels are usually subjected to intercritical spheroidization.[15,16] The work by Verhoeven and Gibson[17] showed that when austenite is cooled through A1 temperature, two modes of transformation (i.e., pearlite reac
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