X65/Ni825 bimetallic composite pipes combine the load-bearing capacity of pipeline steel with the corrosion resistance of nickel-based alloys, making them promising candidates for harsh oil and gas transportation environments. However, their welded joints usually exhibit significant microstructural and compositional heterogeneities, especially in fusion zones and interpass transition regions, which can strongly affect local mechanical properties. In this study, X65/Ni825 bimetallic composite pipe welded joints were investigated, and the microstructure, elemental transition behavior, microhardness, and local mechanical properties of different weld passes and fusion zones in full high-alloy filler metal welded joints and hybrid filler metal welded joints were compared. The results show that all weld passes in the full high-alloy filler metal welded joint are mainly composed of γ-Ni cellular/columnar dendrites, showing good microstructural and mechanical uniformity. In contrast, the hybrid filler metal welded joint exhibits obvious microstructural passing. A compositional transition zone is formed between the transition and filler passes due to local remelting, dilution, and metallurgical mixing, accompanied by sharp changes in hardness and local strength. Overall, the full high-alloy filler metal system is more effective in reducing microstructural, compositional, and mechanical discontinuities within the weld, providing guidance for welding process optimization of X65/Ni825 bimetallic composite pipes.
Fu et al. (Thu,) studied this question.