Ferrite-Bainite Dual Phase Structure and Mechanical Characterization of AISI 4340 Steel

AISI 4340 steel is a medium hardness and moderate strength alloy, finds application in different heat treatment conditions. Commercially available AISI 4340 steel can be subjected to special heat treatment to obtain duplex structures (ferrite-martensite or ferrite-bainite) with different intercritical austenitisation temperatures for mechanical property enhancement and serves as a platform to discuss the property deviation. In the present work dual phase structure (ferrite-bainite) is odtained by partial austenitization of room temperature structure followed by isothermal quenching (austempering) in the lower bainite temperature range. Since it is medium carbon steel, the dual phase contains well dispersed bainite phase in the continuous ferrite matrix. Accordingly, specimen are heated to intercritical temperatures (750, 770, 790°C) for 2 hours, rapid quenching in the salt bath containing sodium nitrate and nitrite mixture maintained in the lower bainite temperature range (350°C) for two time durations 24 minutes (optimum) and 24 hours (prolonged holding) followed by air cooling. Dual phase obtained in optimum soaking duration shows moderate hardness, strength, ductility and toughness values whereas prolonged duration shows improved values of ductility and toughness if the austenitising temperature is selected suitably so that the data may be employed for machinability improvement.