Austenite grain growth behavior of a GCr15 bearing steel cast billet in the homogenization heat treatment process
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Ruijie Zhang Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China
Zhi Li CISDI Thermal & Environmental Engineering Co., Ltd., CISDI Group Co., Ltd., Chongqing 400013, China (Received 5 March 2016; accepted 9 June 2016)
Isothermal homogenization heat treatments for a GCr15 bearing steel cast billet were performed at temperatures of 1000–1250 °C and holding times of 30–180 min. The grain size of austenite was measured with a metallographic microscope through the linear intercept method. Experimental results show that the grain size of austenite increases with the increase in heating temperature and holding time. The relationship between grain size and homogenization cycles was established. The homogeneity of the cast billet has an obvious effect on the austenite grain size distributions. Small and large grains were observed in the high- and low-concentration regions, respectively. The log-normal function can describe the grain size distributions more accurately than other functions after heating at low temperatures for short times. However, the Weibull function fits the grain size data well when the heating temperatures and holding times are improved.
I. INTRODUCTION
The most common material utilized to manufacture ball and roller bearing rings is alloy steel GCr15 (the alternative corresponding designations are AISI/SAE 52100 and DIN 100Cr6). GCr15 comprises 1.0% carbon steel and 1.3–1.6% chromium.1 The segregation that occurs during solidification leads to coarse carbide formation along the center of the casting2–4 and uneven component distribution.5–7 The large carbides and homogenization of the components seriously influence hot workability8,9 and machinability.10 Homogenization heat treatment before hot rolling is usually applied to reduce microsegregation and dissolve large core carbides. However, long-time homogenization at high temperatures leads to coarse austenite grain sizes. Coarse prior austenite grains significantly influence the structure and mechanical properties of steels after hot rolling.11–15 The austenite grain size distribution is inhomogeneous because the distribution of the components in the cast billet is also inhomogeneous. Research on grain growth and homogenization is necessary. The austenite grain growth kinetics of hot-rolled GCr15 bearing bars have been investigated by annealing the samples for short times at low temperatures.16 Recently, Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.248 J. Mater. Res., Vol. 31, No. 14, Jul 28, 2016
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some research works have been published on the normal/ abnormal grain growth.17,18 A study on the grain size, morphology, and grain size distribution of austenite also has been reported.19 Homogenization heat treatment has also been implemented in different situations (i.e., inhomogeneous distribution, long times, and high temperatures). This study aims to i
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