Effect of Carbon Content on Bainite Transformation Start Temperature in Middle–High Carbon Fe–9Ni–C Alloys

Bainite in steel is an industrially useful structure. However, the controlling factor of its transformation start point is not clearly known. In this study, we measured the bainite transformation start temperature (Bs) in Fe–9Ni–C alloys containing 0.3–0.9 mass%C via microstructure observation of the specimens held isothermally between 600 K and 798 K. Bs existed between 758 K and 773 K in all alloys used, and was independent of carbon content. Especially, Bs was higher than T0, at which fcc and bcc of the same composition have the same free energy, at more than 0.3 mass%C. This result was completely different from that of our previous study on low carbon Fe–9Ni–C alloys, in which Bs decreased with the increase in carbon and kept the certain driving force of partitionless transformation from fcc to bcc. Bs in middle–high carbon alloys corresponded to the temperature of the intersection point between T0′, at which the driving force is 400 J/mol, and the γ/(γ + θ) phase boundary. This suggests that the nucleation and growth of bainitic ferrite in austenite containing solute carbon higher than T0′ is promoted by the precipitation of cementite in austenite.