Role of initial microstructures in governing austenite reversion kinetics and morphology in medium manganese steel

The influence of two distinct initial microstructures, fully martensitic (M) and bainitic ferrite with retained austenite (BF+RA), on austenite reversion kinetics and microstructural evolution was systematically examined in a Fe–0.19C–4.03Mn–1.48Si medium-manganese steel. These microstructures were obtained through direct quenching and austempering, respectively, followed by intercritical annealing. During annealing at 943 K, the BF+RA microstructure attained the equilibrium austenite fraction within 1 hour, whereas the M microstructure required approximately 3 hours, indicating significantly accelerated austenite reversion kinetics in the BF+RA condition. At equilibrium, the M microstructure contained 16.7% globular austenite and 23.5% acicular austenite, while the BF+RA microstructure exhibited a lower fraction of globular austenite (7.8%) but a substantially higher fraction of acicular austenite (35.4%). Similar trends were observed during annealing at 963 K. Theoretical analysis suggests that coarse cementite particles serve as preferential nucleation sites for globular austenite; therefore, carbide coarsening during intercritical annealing promotes globular austenite formation. In contrast, in the BF+RA initial microstructure, pre-existing austenite suppresses cementite precipitation prior to reversion, thereby inhibiting globular austenite development. Experimental observations combined with thermodynamic calculations further demonstrate that pre-existing austenite provides energetically favorable growth sites during recovery, eliminating the requirement for new nucleation events and significantly accelerating the early stages of austenite reversion. Additionally, manganese enrichment within carbides was more pronounced in the M microstructure than in the BF+RA microstructure, resulting in stronger solute drag effects on austenite nucleation and growth and consequently contributing to the comparatively slower reversion kinetics observed in the martensitic condition.