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Abstract
The mechanical properties of modified heat treated silicon modified 4330 steel have been studied over the temperature range 203–293 K with the aim of developing a modified heat treated steelfor ultrahigh strength applications. The microstructure of the modified heat treated steel consists of triple phases of martensite, bainite, and retained austenite. This is produced by partial isothermal transformation at 593 Kfor 300 s, followed by oil quenching and subsequent tempering at 473 K (after austenitisation at 1173 K). Compared with the conventionally quenched and tempered steel, the modified heat treated steel exhibited improved plane strain fracture toughness at a similar tensile strength level over the temperature range 203–293 K. Compared with the conventional quench and temper, the modified heat treatment also decreased the ductile to brittle transition temperature in the Charpy impact test. Compared with the austemper traditionally employed to obtain the mixed structure of bainite and retained austenite, the modified heat treatment significantly improved the lower temperature strength and the plane strain fracture toughness of the steel in addition to the ductile to brittle transition temperature. The results are discussed.
MST/2077