Saving energy by modification of Tomita heat treatment method for AISI 4340 steel

Abstract

In this paper a modified up-quenching heat treatment method (Tomita method) for 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 and modified up-quenching austempering heat treatment on the mechanical properties. The specimens, were cut from a bar with 25mm diameter and after achieving the best temperature and time of austenitizing, austenitized at 800oC for 60min and followed by quenching at 430oC for the high austempering temperature to achieve the upper bainite morphology and at 360oC for the lower austempering temperature to achieve the lower bainite morphology. The specimens selected for up-quenching, after austenitization were quenched to below Ms (martensite start temperature) (320oC) for 120sec, followed by heating at 500oC to achieve the mixed structure of tempered martensite and upper bainite for 1000sec. The specimens selected for modified up-quenching, after austenitization were quenched to below Ms for 120sec, followed by heating at 400oC to achieve the mixed structure of tempered martensite and lower bainite for 1000sec. It is also shown that the best combination of strength and ductility can be achieved by the modified heat treatment that has been suggested in this investigation by the decrement of heat energy for the austenitization temperature, up-quenching temperature and time.