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.

Microstructures and austenite grain growth behavior of the alumina-forming austenitic (AFA) steel subjected to normalizing and annealing at various temperatures were investigated. A modified kinetic model of austenite grain growth was constructed based on consideration of the heating history. Abnormal growth of austenite grain occurs when the temperature is increased to 1473 K, and some special large particles of the precipitates located at grain boundaries form when the sample is normalized at the temperature of 1523 K. Both NbC and NiAl precipitates are identified using routine x-ray diffraction. The fitted data based on the kinetic model used and the consideration of the heating history is in agreement with the changes in the austenite grain growth in the AFA steel even when there is abnormal grain growth. The grain growth exponents are shown to be 2.85 and 2.42 for normalizing and annealing, respectively.