Degradation and crack initiation behavior at SA508/Alloy 52M interface of electro-slag strip cladding in simulated PWR primary water

The SA508/Alloy 52M nickel-based alloy overlay fabricated by electro-slag strip cladding (ESSC) is the core protective structure for pressurized water reactor (PWR) steam generator tube sheets. Aging and cracking at its fusion boundary (FB) interface directly threaten the long-term safe operation of nuclear power equipment. In this study, the correlation laws between microstructure, mechanical properties, and degradation and crack initiation behavior of the ESSC SA508/Alloy 52M interface were systematically investigated through multi-scale microstructural characterization, residual stress and hardness measurements, combined with slow strain rate tensile (SSRT) tests in simulated PWR primary water at 325°C. Results show that two typical morphologies exist at the interface: straight FB and partially melted zone (PMltZ), with PMltZ accounting for 23±2% of the total interface length, significantly higher than that in gas tungsten arc welding (GTAW) joints. The internal feathery martensite region formed within the PMltZ exhibits the highest hardness and residual compressive stress across the entire interface. The cracking susceptibility of the three types of PMltZs follows the order: island-shaped PMltZ > hook-shaped PMltZ > martensite region, which is consistent with the variation trends of oxide film thickness and crack aspect ratio. Crack propagation shows obvious interface selectivity, and the martensite region exhibits lower-than-expected cracking susceptibility due to the pinning effect of high-density Cr 23 C 6 carbides, which offsets the adverse impact of low Cr content. Further analysis indicates that microstructural inhomogeneity is the dominant factor for interfacial cracking, mechanical property mismatch is the direct driving force, and the corrosive environment is a necessary condition. Compared with GTAW joints, ESSC overlays have higher PMltZ coverage, larger compositional fluctuations, and their SCC susceptibility is less affected by the tensile direction. This study reveals the unique SCC failure mechanism of ESSC dissimilar metal interfaces, providing a theoretical basis for welding process optimization and full-life cycle safety assessment of nuclear steam generator tube sheet overlays.