Evolution of microstructure and mechanical properties of 0.34C ultrafine bainitic steel with increasing tempering temperature and holding time

The effects of rapid tempering on the microstructure and mechanical properties of 0.34C ultrafine bainitic steel were studied. Results indicate that the carbon atoms in bainitic ferrite gradually segregated to dislocations when tempered at 450°C. Cluster strengthening improved the yield strength of the steel and stabilized the retained austenite. At 650°C, most of the retained austenite was decomposed, and numerous carbides were precipitated. The yield strength of the steel was not reduced, but the plasticity and impact toughness were greatly reduced. The optimum temperature for rapid tempering was 550°C, and its microstructure and mechanical properties gradually changed with the extension of tempering time. The duration of tempering can be determined on the basis of specific requirements.