Microstructural Characterizations and Kinetics Model of Isothermal Bainite Formation with the Presence of Prior Martensite in High‐Carbon Nanostructured Bainitic Steel

Abstract

Quenching and subsequent isothermal bainite transformation (QBT) and direct isothermal transformation (DIT) procedures are performed to comparatively evaluate the impact of the introduced prior martensite on bainite transformation behaviors and microstructural characterizations in a high‐carbon nanostructured bainitic steel. A thermodynamic model under a QBT process based on displacive theory is also developed to expound the bainite formation kinetics. The results indicate that the modified kinetics model, by considering austenite/martensite interface nucleation, austenite grain boundary nucleation, and autocatalytic nucleation, evolved here can be used to accurately describe the bainite formation behavior in the presence of prior martensite. A certain fraction of prior austenite/martensite interface can serve as the preferential nucleation site for the bainite transformation and assist in an acceleration of transformation rate. The bainite nucleation event initially takes place at prior martensite plate boundary in the QBT process instead of austenite grain boundary in the DIT process and subsequently continues through autocatalytic nucleation at austenite/bainite interface generated at either austenite/martensite interface or austenite/austenite interface. The introduced prior martensite slightly refines bainite sheaves and lessens the blocky retained austenite size, presenting a feasible approach to shorten the required time for producing a nanosized bainitic microstructure.