Unusually High Fracture Toughness of ASTM A723 Steel from a Mixed Martensite/Bainite Microstructure.

摘要

A mixed martensite/bainite microstructure in the high strength-low alloy steel (ASTM A723 pressure vessel steel) can be obtained by quenching from the austenitizing temperature to below M sub s, but above M sub f, and isothermally holding. As the supercooled austenite cools below M sub s, some martensite is formed and the isothermal hold results in the remainder of the untransformed austenite transforming to bainite. The resulting microstructure may be very fine depending on the prior austenite grain size. Since the martensite is formed first, the supercooled austenite between the martensite needles transforms to bainite during the isothermal hold. The resultant lamellar structure can result in remarkable fracture toughness with quite high strength (K sub Ic of 255 MP a sq m and ultimate tensile strength of 1200 MPa). In this report, the results of varying prior austenite grain size on strength and toughness and the strength-toughness relationship for the martensite/bainite mixture that may produce the highest toughness are presented.