Insights into stress corrosion crack growth of 316NG heat-affected zone in simulated PWR primary water

Abstract

The SCC propagation behavior in the HAZ of 316NG remains insufficiently characterized. This study systematically evaluates SCC growth in HAZ specimens under simulated PWR primary water conditions (325 °C, 1200 mg/L B, 2 mg/L Li, both hydrogenated [30 mL (STP)/kg H₂] and oxygenated [0.5 ppm O₂] environments) at constant K = 30 MPa·m¹/². Compact tension (CT) specimens that contained artificial cracks positioned 1 mm and 4 mm from the weld fusion line were employed to asses SCC growth in the HAZ, where the crack propagation region extended to a normalized distance of 0.41–0.45 from the inner wall surface. Surprisingly, the HAZ exhibited crack growth rates marginally lower than or essentially comparable to those of the parent metal in both oxygenated and hydrogenated water environments. This suggests that compressive residual stresses in the SCC propagation region may mitigate the crack growth acceleration typically induced by strain hardening (up to 20 % hardness increase relative to the parent metal). High-resolution transmission electron microscopy (HRTEM) analysis demonstrated nickel (Ni) enrichment at grain boundaries ahead of advancing crack tips, attributed to rapid iron (Fe) diffusion along grain boundaries toward the crack tip and selective oxidation of Fe and chromium (Cr) at the crack tip. The oxide film formed a distinct bilayer structure, with an outer Fe₃O₄ magnetite layer and an inner FeCr₂O₄ spinel layer.