Partitioning and the growth of bainite

This communication presents a viewpoint for the kinetics of bainite formation in Fe-C and Fe-C-M steels, where M is a substitutional alloying element(s). Bainite is defined microstructurally as a non-lamellar aggregate of ferrite (*a) and carbide(s) that forms during the diffusional eutectoid decomposition of supersaturated single-phase austenite (*g) [1]. The carbide(s) can be cementite (M3C) and/or alloy carbides (e.g., M2C, M6C). The focus of past research has been on ferrite, since it is the dominant product phase in terms of free energy change and volume fraction. We feel that the neglect of carbide formation (especially in high hardenability alloy steels) has prevented the development of a fuller, more robust understanding of bainite valid across all temperatures and alloy compositions. In essence, bainite is defined to include all eutectoid products that are not pearlite or martensite. This covers a huge variety of product morphologies, and their grouping under a single term has blurred the very distinctions that are essential for understanding bainite. The many bainite morphologies found in different alloys and temperatures arise due to the following: (1) ferrite exhibits several morphologies: allotriomorphs or Widmanstatten rods, laths or plates, (2) the carbides may be significantly separated from the ferrite in both space and in time, and (3) the carbides themselves may exhibit different morphologies. Both early [2] and more recent work [1,3,4] have emphasized that the relative rates of ferrite and carbide precipitation determine these product geometries.