Effect of Grain-Boundary Discontinuous Precipitation Evolution on the Transition of Creep-Rupture Failure Modes in Ni–Fe-Based Alloy/Inconel 617 Dissimilar Welded Joints

The creep-rupture behavior and microstructural evolution of Ni–Fe-based alloy/Inconel 617 dissimilar welded joints were investigated by creep-rupture testing, interrupted creep testing, and microstructural characterization. Tests were conducted at 630–750 °C under stresses of 130–375 MPa. All joint specimens fractured in the heat-affected zone (HAZ) or base metal (BM) on the Ni–Fe-based alloy side, indicating that the Ni–Fe-based alloy side was the creep-critical region of the joint. Three failure regimes were identified according to the relative rupture lives of the joints and the Ni–Fe-based alloy BM. Fractographic observations revealed predominantly intergranular fracture, with creep cavities and cracks preferentially associated with grain-boundary discontinuous precipitation (DP) regions containing coarsened rod-like γ′ precipitates and precipitate-free zones (PFZs). Differences in DP evolution between the HAZ and BM were closely related to creep-damage localization and failure behavior. Interrupted creep tests showed that temperature was the dominant factor affecting DP formation and growth, while applied stress accelerated its evolution. DP developed rapidly during the initial exposure stage and subsequently grew more slowly. These findings clarify the relationship between grain-boundary DP evolution and creep-rupture failure in Ni–Fe-based alloy/Inconel 617 welded joints, and also provide microstructural guidance for assessing and mitigating creep degradation in dissimilar welded components used in high-temperature A-USC systems. Future work should combine longer-term creep testing, phase-resolved microstructural characterization, and predictive modeling to establish quantitative relationships among DP evolution, creep-damage accumulation, and rupture life.