Effect of austempering conditions on the microstructure and mechanical properties of AISI 4340 and AISI 4140 steels

AISI 4340 and AISI 4140 are normally used for different structural applications in quenched and tempered state due to their high strength, high abrasion and fatigue resistance but they lack toughness. Presence of dual phase microstructure such as ferrite-bainite and ferrite-martensite in steels have been reported to improve toughness but at the cost of strength. It is believed that development of bainite-martensite dual phase microstructure may help in obtaining better combination of mechanical properties. In this research work, development of bainite-martensite dual phase microstructure has been carried out at different austempering temperatures and times to study microstructure evolution and its effect on mechanical properties. Microstructural characterization of samples was carried out by scanning electron microscope (SEM) whereas mechanical characterization was done by performing hardness and impact testing. Development of dual phase bainite-martensite microstructure has shown to impart better combination of strength and toughness. In comparison with AISI 4140 steel, AISI 4340 provided better combination of hardness and toughness in the austempered state. It has been observed that decrease in the austempering temperature helped in obtaining better combination of hardness and impact strength in both grades of steels due to formation of lower bainite martensite dual phase microstructure at lower temperature.