Unravelling interfacial bonding mechanism under heat treatment in hot compression bonding of heterogeneous stainless steel and mild steel

Hot compression bonding (HCB) is a widely used metal-joining technique, particularly for dissimilar materials; however, the interfacial bonding mechanisms remain insufficiently understood. In this study, stainless steel and mild steel were joined by HCB under compressive strains ranging from 5% to 40%, followed by annealing. The effects of compressive strain and post-bonding heat treatment on the interfacial microstructure and mechanical properties were systematically investigated. The hot compression process generated significant stored deformation energy, which promoted the formation of a recrystallized fine-grained layer (RGL) during subsequent annealing. The thickness of the RGL increased with increasing compressive strain. Microstructural analysis revealed that recrystallization and phase transformations on both sides of the interface, together with elemental diffusion, resulted in substantial grain refinement near the bonded region. Nanoindentation measurements confirmed that the refined grains contributed to the higher hardness at the interface compared with the adjacent base metals. Annealing not only relieved residual stresses but also enhanced interfacial bonding through increased atomic diffusion. The diffusion of Ni and Cr across the interface promoted the formation of Ni-rich and Cr-rich intermetallic precipitates, which, in combination with the RGL, contributed to improved interfacial bonding. These findings provide new insights into the microstructural evolution and bonding mechanisms of HCB-joined dissimilar steels and demonstrate the beneficial role of post-bonding heat treatment in improving joint performance.