Modeling of Bainite Transformation During Partitioning Process and Atomic-Scale Characterization of Bainite

This paper presents a detailed investigation of the interaction between bainite transformation and carbon partitioning during isothermal holding of intercritical heating, quenching, and partitioning (I&Q&P) steel. The kinetics of isothermal bainite transformation is modeled based on the assumption of displacive mechanism. A systematic analysis of the autocatalytic parameters extracted from the fitting curves reveals that autocatalysis plays only a minor role in bainite transformation due to quite small size of parent austenite grains transforming into bainite, independent from each other. Atom probe tomography (APT) is used to investigate the localized composition changes at the interface between bainitic ferrite and retained austenite. The results indicate that carbon enrichment of austenite occurs and Mn atoms segregate at phase grain boundary. Furthermore, in terms of the thermodynamic analysis, bainitic ferrite forms under diffusionless growth followed by tempering, and in present work, the growth of bainitic ferrite can be divided into two parts: bainite transformation quickly (BTQ) and bainite transformation slowly (BTS).