Quantitative analysis of Mo solute drag effect on ferrite and bainite transformations in Fe-0.4C-0.5Mo alloy

Mo addition is widely used to retard ferrite transformation and increase hardenability of steels. It has been well recognized that Mo segregation at ferrite/austenite interface induces solute drag effect (SDE) and reduces migration rate of ferrite/austenite interface while retardation by Mo is less obvious in bainite transformation. However, element segregation, solute drag effects and its interface character dependence have not been clarified quantitatively. Therefore, in the present study, amount of Mo segregation, energy dissipation during interface migration and interface orientation relationship (OR) in ferrite and bainite transformations in Fe-0.4%C-0.5%Mo (mass%) alloy have been investigated quantitatively. Mo segregation at interfaces with non K-S OR is more significant than that at interface with near K-S OR. Amount of Mo segregation at interface with non K-S OR increases with increase in transformation time investigated while energy dissipation decreases. The amount of Mo segregation and energy dissipation measured at non K-S interface coincides well with SDE model using optimized segregation energy and interface thickness. On the other hand, energy dissipation at bainite/austenite interface is large in spite of negligibly small Mo segregation, which is inconsistent with the SDE model.