Transformation kinetics in controlled-power and controlled-temperature cycle testing

On-heating transformation kinetics were investigated for several steels by using a Gleeble capable of programmable power input as well as programmable temperature cycling. Transformation kinetics determined in both modes are reported. The temperature cycles are significantly different between the two modes due to the latent heat associated with the phase transformations. Both diffusion rates and transformation driving force increase with temperature above the eutectoid temperature, therefore the latent heat can potentially have a significant impact on the transformation kinetics. Experiments with plain carbon steels illustrate that the latent heat of austenite formation causes an appreciable temperature arrest during transformation, and the dilatation response is similarly altered. A kinetic transformation model, based on the decomposition of pearlite and the diffusional growth of austenite, reproduced the transient dilatation data obtained from both control modes reasonably well using the same kinetic parameter values.