The Influence of Microstructural Characteristics on Austenite Formation Kinetics in a Plain Carbon Steel

Abstract

Since the condition of austenite phase formed during intercritical annealing treatment has a crucial impact on the final microstructure and mechanical properties of dual-phase (DP) steels, detailed investigations with regard to austenite formation in this heat treatment process need to be done. In this study, the effects of different microstructural features, such as ferrite grain size, cementite particle size, and pearlite morphology, on austenite formation in a plain carbon steel (0.165 wt pct C, 1.15 wt pct Mn) during isothermal intercritical annealing treatment have been evaluated. The Johnson–Mehl–Avrami–Kolmogorov (JMAK) model was used for modeling the kinetics of austenite formation in this steel during isothermal annealing treatment. The volume fraction of austenite (martensite at room temperature) at different intercritical annealing holding times was calculated by this model using the corresponding results obtained from the experiments. It was found that the starting steel microstructure from which austenite phase is formed has a significant effect on both austenite nucleation and growth. The effect of microstructural parameters on the kinetics of austenite formation in ferrite-cementite steel microstructures was more significant than that in ferrite-pearlite (F-P) steels. An increase in the average cementite particle size or ferrite grain size in ferrite-cementite steels caused a significant decrease in the rate of austenite formation. In F-P steels, on the other hand, pearlite morphology exhibited a small effect on the kinetics of austenite formation while ferrite grain size had a pronounced effect on the rate of austenite formation at the later stage of intercritical annealing, i.e., ferrite to austenite transformation stage.