Dilatometric model for determining the formation of austenite during continuous heating in medium carbon steel

A model was developed to predict the formation of austenite and dilatometric behavior during continuous heating in AISI 1045 steel, which has an initial microstructure composed of ferrite and pearlite. The model is proposed in two parts based on the heating rate and steel behavior to estimate the volume fraction of austenite during continuous heating. The first part of the transformation model is based on the diffusive model from Johnson–Mehl–Avrami–Kolmogorov at a heating rate interval of 0.083–0.383 °C s⁻¹. The kinetic parameters k and n of the Avrami equation were considered to be dependent on the heating rate. In the second part of the model, the non-isothermal transformation rate model from Johnson–Mehl–Avrami–Kolmogorov was used for a heating rate interval of 0.383–1 °C s⁻¹. The dilatometric behavior before and after austenite formation was associated with the instantaneous coefficient of thermal expansion for each phase or microconstituent present in the steel and during transformation, which was estimated using the volume fraction of austenite and a phase mixing rule. The dilatometric model considers a stage for homogenization of carbon into austenite based on an exponential diffusive model. A dilatometric curve analysis technique was used to determine the kinetic parameters, the instantaneous coefficient of thermal expansion, and the critical temperatures of the AISI 1045 steel. Finally, the model was validated by comparing its predictions with the dilatation deformation obtained in an experiment.