Strengthening 42CrMo4 steel by isothermal transformation below martensite start temperature

Ultrahigh-strength low alloy 42CrMo4 steels, widely used in the automobile industry, were heat treated by two different cycles, namely: conventional austempering above the martensite start temperature MS and low-temperature isothermal transformation below MS, 604 K (331 °C). Metallurgical and mechanical properties of the heat-treated alloys were investigated carefully by various analysation techniques. The results of tensile testing indicated that the proof strength and ultimate tensile strength could be significantly improved by isothermal transformation below MS at 583 K (310 °C) for 3.6 ks without degrading the ductility. The ultrafine-grained/nanostructured mixed lower bainite/martensite microstructure created by quenching and partitioning martensite, and the unique fracture morphology were found to be responsible for this improvement. To further confirm the experimental results, a micromechanical model was employed to analyse the strengthening mechanisms.