A model for predicting the austenite grain growth during a common heating process including continuous and isothermal heating processes in medium carbon alloy steel 42CrMo was developed. The isothermal austenite grain growth kinetics were studied with conditions involving soaking time and soaking temperature. The time exponent n in the model was obtained considering the influence of the initial grain size rather than simply utilizing Beck’s equation. The results showed that the value of the time exponent n is 3.55 ± 0.30 when the temperature is above 1000 °C, while the value is 8.33 when the temperature is below 1000 °C. When the temperature is below 1000 °C, the pinning effect of carbides contributes to the higher value of the time exponent. Based on the isothermal model and the rule of additivity, a model for predicting the grain growth occurring during continuous heating was proposed. A reasonable agreement between the calculations and experimental measurements of grain size was obtained. According to the model, the effect of the initial austenite grain size on the final austenite grain size during induction quenching was analyzed. The initial austenite grain size has a significant effect on the final austenite grain size. In order to obtain a refined quenched microstructure, it is necessary to refine the microstructure at room temperature.