Simulation study of carbide formation and dissolution in upper/lower bainite

Abstract

The distribution of carbon in bainitic microstructures is one of the most important factors governing the bainite transformation. Depending on the location and state of carbon, the resulting microstructure is either lower or upper bainite. Both configurations can occur in the form of carbide free bainite or classical bainite with carbides between the ferritic subunits instead of austenite. To improve the understanding of the mechanisms leading to these microstructures, a simulation study is carried out using the thermokinetic software package MatCalc 6. The results of this computational analysis suggest that carbides nearly always form within the ferritic subunits, due to the low solubility of carbon in ferrite. However, at high bainite transformation temperatures, these carbides rapidly dissolve due to a redistribution of carbon from the subunit into the surrounding austenite. This indicates that even if initially lower bainite forms, carbides are no longer found within the ferritic subunits after some time. The process of carbide formation and dissolution observed in the simulations has a major impact on the kinetics of carbon redistribution, i.e., how long it takes for the carbon to diffuse out of a subunit and is often neglected when bainite transformation is discussed.