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

The transformation and stabilization behaviors of the austenite (gamma) 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 alpha and gamma phases in TBF steel during rapid cooling and austempering treatment. The gamma phase with a low carbon concentration (gamma((low-C))-Fe) immediately disappeared before the austempering time of 300 s. This indicates a remarkable increase in the lattice constant of the gamma with a high carbon concentration (gamma((high-C))-Fe) because of the bainite transformation of gamma((low-C))-Fe and carbon partitioning to gamma((high-C))-Fe. When the austempering time increased from 300 s to 3600 s, the volume fraction of gamma((high-C))-Fe monotonically decreased and the lattice constant was hardly changed.

Automated summary

The transformation and stabilization behaviors of the austenite phase during bainite transformation in the early stages of austempering treatment of low-alloy TBF steel were studied. It was found that the low-carbon austenite phase disappeared within 300 seconds, while the lattice constant of the high-carbon austenite phase increased significantly. The volume fraction of the high-carbon austenite phase decreased monotonically as the austempering time increased, while the lattice constant remained almost unchanged.