A new modified austempering to increase simultaneously strength and ductility for UHS steels

Abstract In this paper, a modified up‐quenching heat treatment method for ASSAB 705M (AISI 4340) steel (ultra high strength steel) is proposed. A low alloy steel (0.33% C), was used to study the effect of isothermal austempering, successive austempering and modified up‐quenching austempering heat treatment on the mechanical properties. The specimens, were cut from a bar with 25 mm diameter and after achieving the best temperature and time of austenitizing, austenitized at 800°C for 60 min and followed by quenching at 430°C for the high austempering temperature to achieve the upper bainite morphology and at 360°C for the lower austempering temperature to achieve the lower bainite morphology. In the case of successive austempering, the specimens were first austempered at 430°C for different periods (500 sec and 60 sec) and then austempered at 360°C for 1000 sec to achieve the mixed structure of upper bainite and lower bainite morphology. The specimens selected for up‐quenching, after austenitization were quenched to below MS (martensite start temperature) (320°C) for 120 sec, followed by heating at 400°C to achieve the mixed structure of tempered martensite and lower bainite and 500°C to achieve the mixed structure of tempered martensite and upper bainite for 1000 sec. It is also shown that the best combination of strength and ductility can be achieved by the mixed structure of tempered martensite and lower bainite that has been suggested in this investigation. This modified method, can offer techniques that simultaneously improve not only strength by 12% (comparing with results of strength after other heat treatment methods), but also ductility by 38% (comparing with results of ductility after other heat treatment methods).