S. M. safi, islamic azad university of ahvaz, ahvaz, Iran
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 25mm diameter and after achieving the best temperature and time of austenitizing, austenitized at 800C for 60 min and followed by quenching at 430C for the high austempering temperature to achieve the upper bainite morphology and at 360C for the lower austempering temperature to achieve the lower bainite morphology.
In the case of successive austempering, the specimens were first austempered at 430C for different periods (500sec. and 60sec.) and then austempered at 360C 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(320C) for 120 sec. followed by heating at 400C to achieve the mixed structure of tempered martensite and lower bainite and 500C to achieve the mixed structure of tempered martensite and upper bainite for 1000 sec.
The all of processes were performed in the salt bath furnaces. Experimental results are presented and the advantages of the modified method are discussed.
As well, it is shown that the best combination of strength and ductility can be achieved by the proposed heat treatment method. This modified method, can offer techniques that simultaneously improve not only strength 12 %,but also ductility 38 %( compare with results of ductility after other heat treatment methods). While, conventional heat treatment of ultra high strength steels (UHSS) cannot always meet the strict engineering requirements for improved strength and ductility simultaneously. It has been shown that the mixed structure of tempered martensite and lower bainite that has been suggested in this investigation offers a good combination of strength and ductility.
Summary: In this paper, a modified up-quenching heat treatment method to the ASSAB 705M steel (ultra high strength steel) is proposed.
As well, it is shown that the best combination of strength and ductility can be achieved by the proposed heat treatment method. This modified method, can offer techniques that simultaneously improve not only strength 12 %( compare with results of strength after other heat treatment methods), but also ductility 38 %( compare with results of ductility after other heat treatment methods).
