Microstructure and Hardness Evolution during Deformation near Ae₃ in a Cr–Mn–Ti Gear Steel

Developing appropriate thermomechanical processing routes for decreasing macrohardness of gear steels without heat treatment has a numerous futuristic applications in rolling industry. The effect of different deformation processes applied to Cr–Mn–Ti gear steel on the evolution of microstructure and hardness are studied by using a thermal simulator. It is found that imposing second-pass deformation near Ae₃ temperature inhibits occurrence of dynamic recrystallization (DRX), and increases stored deformation energy in austenite, which significantly promotes ferrite and pearlite transformation. As a result, the maximum volume fraction of ferrite and pearlite approaches ≈90%, and the corresponding macrohardness decreases to ≈215 HV. The difference of ferrite transformation between different deformation processes is analyzed using a thermodynamic equation by taking into account the stored deformation energy. The present calculations indicate that interval between Ae₃ and deformation temperature can only be avoided at lower deformation temperature of second-pass. This restricts static recovery or recrystallization, and quantities of dislocations that act as nucleation sites are retained before ferrite transformation.