Impacts of near-Ms austempering treatment on microstructure evolution and bainitic transformation kinetics of a medium Mn steel

The microstructure evolution and bainitic transformation of an Fe–0.19C–4.03Mn–1.48Si steel subjected to near-M_s austempering treatment were systematically investigated by combining dilatometer, X-ray diffraction, and electron microscopy. Three additional austempering treatments with isothermal temperatures above M_s were used as benchmarks. Results show that the incubation period for the bainitic transformation occurs when the medium Mn steel is treated with the austempering temperature above M_s. However, when subjected to near-M_s isothermal treatment, the medium Mn steel does not show an incubation period and has the fastest bainitic transformation rate. Moreover, the largest volume fraction of bainite with a value of 74.7% is obtained on the condition of near-M_s austempering treatment after cooling to room temperature. Dilatometer and microstructure evolution analysis indicates that the elimination of the incubation period and the fastest rate of bainitic transformation are related to the preformed martensite. The advent of preformed martensite allows the specimen to generate more bainite in a limited time. Considering bainitic ferrite nucleation at austenite grain boundaries and through autocatalysis at ferrite/austenite interfaces, a model is established to understand the kinetics of bainite formation and it can describe the nucleation rate of bainitic transformation well when compared to the experimental results.