On the mechanism of bainitic transformation and the quality of heat treatment in engineering steels with a bainitic structure

Traditional approaches to analyzing bainitic structures often rely on outdated methods, which pose challenges for the accurate identification and quantitative characterization of the features of bainitic transformation. In recent years, therefore, high-resolution scanning electron microscopy and electron backscatter diffraction (EBSD) techniques have been increasingly employed to study bainite structures. These advanced methods enable precise characterization of various types of bainitic microstructures. This paper presents a brief review of recent studies on the mechanism of bainitic transformation, along with the authors’ own experimental findings from bainitic hardening of engineering steels. It is emphasized that, despite numerous investigations, the bainitic transformation mechanism remains a subject of ongoing debate. Unresolved issues include the transformation of the face-centered cubic (FCC) lattice into body-centered cubic (BCC), the role of carbon diffusion leading to its redistribution and the formation of carbon-supersaturated ferrite, as well as the processes of carbide precipitation. Bainite provides a high level of mechanical performance in terms of strength, ductility, and toughness. Compared to pearlite, sorbite, and troostite, lower bainite exhibits higher hardness and strength while maintaining excellent ductility and toughness. These properties are attributed to the higher carbon content in ferrite, increased dislocation density in bainite, and the formation of fine carbide precipitates.