Austenite stabilization kinetics in early stages of austempering treatment in TRIP-aided steel using in-situ neutron diffraction technique

The transformation and stabilization behaviors of the austenite (γ) phase during bainite transformation in the early stages of austempering treatment of low-alloy transformation-induced plasticity-aided bainitic ferrite (TBF) steel were investigated by in-situ neutron diffraction measurement and microstructure observation. Neutron diffraction peak profile analysis with a short time range resolution showed the changes in the phase fractions and the lattice constants of the α and γ phases in TBF steel during rapid cooling and austempering treatment. The γ phase with a low carbon concentration (γ(low-C)-Fe) immediately disappeared before the austempering time of 300 s. This indicates a remarkable increase in the lattice constant of the γ with a high carbon concentration (γ(high-C)-Fe) because of the bainite transformation of γ(low-C)-Fe and carbon partitioning to γ(high-C)-Fe. When the austempering time increased from 300 s to 3600 s, the volume fraction of γ(high-C)-Fe monotonically decreased and the lattice constant was hardly changed.