Criteria for the dilatometric analysis to determine the transformation kinetics during continuous heating

Abstract

A method was established to determine the amount of austenite formed, by stages, in a low-carbon steel through dilatometric analysis. Based on length change measurements according to the temperature at different heating rates, the critical transformation temperatures were determined using the extrapolation method, intersection of lines, and first derivative criterion for each of the stages. The austenite volume fraction was calculated using the lever rule from the starting to the ending temperature of the austenitic transformation, and the coefficient of linear thermal expansion was calculated for the transition temperature between transformation phases using the mixing rule and the coefficients before and after austenite formation. The kinetics of the austenite formation were estimated using the Johnson–Mehl–Avrami–Kolmogorov diffusive model for both cases. In the first case, the formation kinetics were calculated by separating the transformation stages, which showed that the value of the n parameter changes by approximately one unit when changing from the first to the second phase, indicating changes in the site and geometry of nucleation, whereas the value of the k parameter remains practically constant regardless of the phase transformation. Lastly, the total austenite volume fraction and the kinetic parameters were compared considering the transformation in a single phase for both cases, demonstrating that the austenite volume fraction is overestimated when the transformation phases are not separated, causing the values of n to stay above 3, while the magnitude of k increases slightly.