The tale of upper and lower bainite: A computational analysis of concurrent C-diffusion and precipitation

Carbon partitioning in bainite is a central mechanism controlling this transformation. Depending on whether C atoms form precipitates in the ferrite platelets or remain dissolved in a solid solution, the resulting microstructure is denoted as either lower or upper bainite. In the present analysis, a simulation study is conducted with long-range diffusion of C and simultaneous precipitation of carbides. The computational analysis suggests that carbides always form within the ferritic subunits before carbon redistribution into the surrounding austenite commences, independent of temperature. Since these carbides can rapidly dissolve again at higher bainite transformation temperatures, they are commonly not experimentally observed in upper bainite. At lower temperatures, carbide re-dissolution takes significantly more time, and carbides are, therefore, commonly identified, with the corresponding microstructure denoted as lower-bainite. Since carbides shall always form in the ferrite platelets, a classification of upper and lower bainite based on the existence of carbides should be reconsidered.