Synergistic enhancement of cryogenic tensile properties and toughness in HSLA steel through intercritical heat treatment

This study elucidates the microstructural evolution during intercritical heat treatment of a high-strength low-alloy (HSLA) marine steel and its impact on cryogenic performance. Thermo-Calc and DICTRA simulations, combined with multiscale characterization, reveal austenite reversion and elemental partitioning. The resulting hierarchical microstructure comprises annealed martensite, lath-shaped intercritical ferrite, and minor retained austenite (RA), with notable C, Mn, and Ni enrichment in the reversed austenite. Compared to the quenched and tempered (QT) condition, the intercritically quenched and tempered (QIT) specimen exhibits a comprehensive enhancement in strength, ductility, and toughness at both room and cryogenic temperatures. At −196 °C, its ultimate tensile strength, yield strength, total elongation, and impact energy reach 1153 MPa, 1038 MPa, 25.1%, and 116.5 J, respectively. In contrast, the QT specimen achieves corresponding values of 1112 MPa, 1101 MPa, 19.8%, and 9.3 J. The pronounced increase in yield strength at −196 °C for the QIT specimen is driven primarily by its elevated screw dislocation density, which governs the low-temperature flow stress, and augmented by substantial solute segregation at interfaces and within hard domains, which raises the threshold stress for dislocation motion through static pinning. Furthermore, the heterogeneous lamellar microstructure, with its alternating hard and soft domains, facilitates internal stress/strain accommodation during cryogenic deformation, thereby enhancing the strain hardening capacity and ultimately improving ductility. This combined effect, together with microstructural refinement and a low-temperature solid-solution softening, results in a significantly higher J -integral and superior cryogenic toughness in the QIT specimen.