Examination of phase transformation kinetics during step quenching of dual phase steels

Employing step quenching method for producing dual phase (DP) steels, kinetics of austenite to ferrite phase transformation was studied. In this regard, a series of samples were austenitized at 960 °C for 600 s. These specimens were then annealed at different intercritical temperatures, for different times between 0 and 7200 s, followed by water quenching. Detailed microstructural analysis was employed to study the variation of volume fraction as well as morphology of ferrite phase with intercritical annealing time. Moreover, mechanism of ferrite formation, by intercritical annealing treatment, was studied. It was seen that at initial annealing times a thin layer of allotriomorphic ferrite was formed at the primary austenite grain boundaries. Increasing the annealing time led to growth of this layer and formation of DP steel incorporating martensite islands distributed within polygonal ferrite matrix, after water quenching. Employing Johnson-Mehl-Avrami-Kolmogorov approach, a model was then successfully developed to predict formation of ferrite during intercritical annealing treatment. The Avrami exponent was found to be 0.56, indicating that the austenite to ferrite phase transformation, within the intercritical temperature region, was diffusion controlled, governed by one-dimensional growth.