Jie Hou, Bo Gao, Cuirong Liu, Zhiquan Huang
• The 9Ni Steel/S30408 SS composite plates were prepared by two-pass hot rolling, and Quenching−Tempering and Quenching-Intercritical Quenching-Tempering heat treatment were carried out respectively. • Quenching-Intercritical Quenching–Tempering treated composite plate exhibits optimal strength-ductility balance, maintaining higher interfacial bonding strength. • Characterized that reversed austenite can improve the low temperature toughness of 9Ni steel by rotating deformation and absorbing dislocations. Due to the corrosion resistance and cost limitations of 9Ni steel and the inadequate yield strength of S30408 stainless steel (S30408 SS), developing high-performance 9Ni Steel/S30408 SS composite plates is crucial. This study fabricated such plates via a two-pass hot rolling process and systematically compared the effects of Quenching − Tempering (QT) and Quenching- Intercritical Quenching − Tempering (QLT) heat treatments on their mechanical properties and microstructure. The QLT-treated specimens exhibited optimal performance: shear strength reached 465.98 MPa (surpassing QT’s 413.99 MPa), ultimate tensile strength increased to 654.9 MPa (7.8 % improvement over QT), while elongation and fracture strain doubled compared to the as-rolled state, reaching 53.41 % and 49.53 % respectively. Microstructural analysis revealed that QLT significantly enhanced homogeneous recrystallization, increased intragranular misorientation, and reduced internal stress, achieving an exceptional strength-ductility balance correlated with elongation. Crucially, QLT generated unique slender, willow-leaf-shaped reversed austenite in the 9Ni steel layer and promoted denser Cr 23 C 6 precipitation at S30408 SS grain boundaries. These microstructural modifications demonstrate that QLT successfully coordinated matrix refinement and interface optimization, enabling synergistic enhancement of strength and plasticity in the composite plates.