Effect of hot rolling conditions on the microstructure and mechanical properties of Incoloy 825/API X65 clad plates

In this study, two Incoloy 825/API X65 clad plates were produced by hot rolling: one at a finish rolling temperature of 1050 °C, above the non-recrystallization temperature (T nr = ∼970 °C) of API X65 steel (FT1050) and the other at 825 °C, below T nr (FT825). Compared to the FT1050 plate, the FT825 plate exhibited a thicker decarburized layer and finer grains in API X65 steel, as well as larger γ grains and a thicker carburized layer in Incoloy 825. Both plates showed ∼7 μm-thick interfacial diffusion layers containing oxides, with Cr diffusing farther than Ni from Incoloy 825 into API X65 steel, due to its higher diffusivity, despite steeper Ni concentration gradients. The FT825 plate exhibited superior shear strength and displacement, with fracture propagating across both the interfacial and decarburized layers, whereas the FT1050 plate fractured primarily along the interfacial layer. This difference in fracture behavior is attributed to variations in interfacial oxides. In the FT1050 plate, interfacial oxides were larger and more elongated, while in the FT825 plate, they were finer and more numerous. The finer oxides likely enhanced interfacial strength and promoted competitive cracking between the interface and the decarburized layer, thereby improving the shear properties.