The Effects of Austenitizing Conditions on the Microstructure and Wear Resistance of a Centrifugally Cast High-Speed Ste
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INTRODUCTION
HIGH-SPEED steel (HSS) rolls have been widely used in hot rolling mills due to their high wear resistance and hardness.[1–5] The wear characteristics of HSS rolls are determined by their microstructures, particularly hard carbides.[6–10] Several different types of carbides, such as MC, M2C, and M6C (M is a metallic element), form by complex eutectic reactions during solidification.[11–15] A typical solidification sequence of HSS rolls is as follows: formation of pro-eutectic austenite (c), a primary eutectic reaction (liquid fi c + MC), and a secondary eutectic reaction (liquid fi c + M2C).[13–16] Much research has been performed on the effects of both the type and volume fraction of eutectic carbides on the wear resistance of HSS rolls. Park et al. [1] and Kang et al. [8] reported that the wear resistance depends mainly on the amount of primary eutectic MC particles, not of the secondary eutectic M2C particles. For centrifugally cast HSS rolls, coarse secondary eutectic M2C particles form along the boundaries of cellular primary eutectic cells with MC, resulting in a network of M2C particles.[17] During hot rolling, fatigue cracks usually start at the interfaces between carbides and the matrix on the roll surface, due to the repetitive high mechanical and thermal stresses. The network of MINWOO KANG, Ph.D., formerly Graduate Student with Phase Transformation Laboratory, Department of Materials Science and Engineering, Yonsei University, Seoul, 03722, Korea, is now Researcher with Metallic Materials Research Laboratory, Materials Development Center, R&D division, Hyundai Motor Company, Hwaseong, 18280, Korea. YOUNG-KOOK LEE, Professor, is with the Phase Transformation Laboratory, Department of Materials Science and Engineering, Yonsei University. Contact e-mail: yklee@ yonsei.ac.kr Manuscript submitted October 10, 2015. Article published online April 29, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
eutectic M2C particles provides the routes of crack propagation, and makes cracks deeper. This causes some problems, such as roll spalling.[18,19] Therefore, to improve resistance to crack propagation without degradation of wear resistance, several attempts were made to break the network of eutectic M2C particles into isolated small particles by controlling the austenitizing conditions. For example, some researchers observed that M2C particles were decomposed into a mixture of M6C and MC particles at high temperatures of over 1273 K (1000 C) because M2C particles are a metastable phase.[11,17,20] However, the network morphology of carbides did not disappear. Hashimoto et al. [20] investigated the effect of austenitizing time at 1273 K (1000 C) on the morphology of eutectic M2C particles. They realized that the austenitizing time did not significantly change the morphology, size, and distribution of eutectic M2C particles. Hwang et al. [11] also reported that the distribution of eutectic carbides was not altered by austenitizing temperature up to 1373 K (1100 C); this indicates that both thermal fatigue and f
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