Improving the Cryogenic Strength-Ductility-Toughness of Multi-Alloy Steel by Optimizing the Feature of Retained Austenite

There is a general inverse relationship between strength and toughness in low-carbon steels. The transformation-induced plasticity (TRIP) effect of retained austenite (RA) can help overcome this contradiction by providing strain hardening and transformation toughening during service. In this study, the effect of RA on the microstructure evolution, cryogenic mechanical properties, and fracture behavior of multi-alloyed Ni-containing steel is explored. Two well-established techniques are employed to tailor the RA: quenching-intercritical tempering (QIT) and quenching-lamellarizing-intercritical tempering (QLIT). QLIT steel has a 25.1 vol% RA fraction, which is twice that of QIT steel, and offers exceptional stability at -196 degrees C. As a result, QLIT steel exhibits superior cryogenic mechanical properties in comparison to QIT steel, including a 6.9% increase in yield strength up to 1228 MPa, a 36.6% increase in elongation up to 33.2%, and a 147.4% increase in impact energy up to 193 J. These improvements are due to the higher fraction of RA transformed into the martensite during tensile deformation, resulting in an enhanced TRIP effect and increased work-hardening rate. The exceptional stability of the RA also provides more sustainable TRIP toughening during cryogenic impact deformation, which helps to arrest crack propagation. In this study, a new heat-treatment process is proposed to optimize the volume fraction and stability of retained austenite (RA) in multi-alloyed steel. The resulting excellent mechanical properties are due to the enhanced transformation-induced plasticity (TRIP) effect and increased work-hardening rate. The stability of RA also provides sustainable TRIP toughening during cryogenic impact deformation, helping to arrest crack propagation.image (c) 2024 WILEY-VCH GmbH