Conversion between non-isothermal and isothermal transformation kinetics of γ to α for C–Mn and Nb microalloyed steels

Abstract

In the present paper, austenite γ to ferrite α transformation kinetics of steels at different cooling rates has been measured by thermal dilatation. The relationship between lnln[1/(1 − X)] and ln(|CR|) (CR: cooling rate) was plotted using the data at early stage of cooling, which well fitted a linear relationship for the calculation of exponential values of n in the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation for isothermal kinetics using the method developed by Rios. The values of k in the JMAK equation obtained with the Rios method, however, have led to big discrepancies when the isothermal equations were used to predict the transformation kinetics during cooling. By assuming a Gaussian function of temperature, k was calculated using an optimisation method based on the rule of additivity. The isothermal transformation model was used to predict the transformation kinetics during cooling, showing good agreement with the measured data. It has been proved that even though the rule of additivity has to be relaxed to take into account the effects of cooling rates, precise conversion between non-isothermal and isothermal kinetics can still be realised.