Effect of tempering time on microstructure and mechanical properties of a low carbon stainless bearing steel

Experiments were conducted to evaluate the effects of tempering time range from 2 to 12 h at 540 degrees C on the microstructure and tensile properties of a low-carbon high-alloy stainless bearing steel after quenching, cryogenic treatment and tempering. The results show that with tempering time increase, martensite laths refine slightly, the volume fraction of austenite increases through reversed one formation, although residual one decomposes. Furthermore, nano-carbides form after tempering for 4 h and further coarsen with time extending. Yield strength increases significantly with increasing tempering time from 2 h to 4 h primarily contributed by the presence of numerous carbides, but further increasing the time to 8 h and 12 h, it increases quite slightly. But ultimate tensile strength continues increasing with the time from 4 to 12 h, probably affected by higher TRIP effect from higher volume fraction of reversed austenite in the samples after tempering for 8 and 12 h. An excellent combination of strength and ductility are obtained after tempering for 8-12 h.