The dynamic transformation of deformed austenite at temperatures above the Ae3

Recent observations regarding the dynamic transformation of deformed austenite at temperatures above the Ae(3) are reviewed. Experimental results obtained on four different steels over the temperature range from 743 to 917 C and at strains up to epsilon = 5 are described. It is shown that there is a critical strain for the formation of superequilibrium ferrite and that the volume fraction of transformed ferrite increases with the strain. The structures observed are Widmanstatten in form and appear to have nucleated displacively. The effect of deformation on the Gibbs energy of austenite 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 making suitable assumptions about two-phase flow. By further taking into account the inhomogeneity of the dislocation density, Gibbs energy contributions (driving forces) are derived that are sufficient to promote transformation as much as 100 degrees C above the Ae(3). The C diffusion times required for the dynamic formation of the cementite particles observed are estimated. These range from similar to 25 to 100 mu s and are therefore consistent with the times available during rolling. The Gibbs energy calculations suggest that growth of the Widmanstatten ferrite is followed by C diffusion at the lower carbon contents, while it is accompanied by C diffusion at the higher carbon levels. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.