Partitioning of carbon from supersaturated plates of ferrite, with application to steel processing and fundamentals of the bainite transformation

A model is reviewed, that describes the endpoint of carbon partitioning between supersaturated ferrite and retained austenite. A new process, quenching and partitioning (Q&P), has been developed recently to intentionally employ such partitioning in creating useful ferrous microstructures containing retained austenite. The process involves quenching austenite below the martensite-start temperature, followed by a partitioning treatment to enrich the remaining austenite with carbon, thereby stabilizing it to room temperature. Recent experimental studies have confirmed that Q&P provides a viable means to create microstructures containing carbon-enriched retained austenite, and attractive property combinations have been achieved in a variety of materials, while opportunities remain for further optimization. Furthermore, some implications of the partitioning model with respect to fundamentals of the bainite transformation are discussed, including the possibility of displacive growth under carbon diffusion control, with an austenite composition at the α/γ interface represented by the (adjusted) T0 composition. It is suggested that individual movements of iron atoms are likely during growth of Widmanstätten ferrite, and that there may be a need for further consideration of thermally activated iron-related processes in general.