Thermodynamics of dynamic transformation of hot deformed austenite in four steels of increasing carbon contents

Recent observations regarding the dynamic transformation of deformed austenite at temperatures above the Ae3 are reviewed for four different steels of increasing carbon contents. The structures observed are Widmanstätten in nature and appear to have formed displacively. The effect of deformation on the Gibbs energy of austenite in these steels is estimated by assuming that the austenite continues to work harden after initiation of the transformation and that its flow stress and dislocation density can be derived from the experimental flow curve. By further taking into account the inhomogeneity of the dislocation density, Gibbs energy contributions are derived that are sufficient to promote transformation as much as 100 °C above the Ae3. The present calculations indicate that the dislocation densities in the regions that experience transformation are 2-15 times higher than the average values. It is also suggested that, at the lower carbon levels, plate growth is followed by C diffusion, while it is accompanied by C diffusion at the higher carbon contents.