Effect of Thermal Aging on Stress Corrosion Cracking in High Temperature Water for Cast Duplex Stainless Steel Used in Primary Coolant Pipe

The Z3CN20.09M cast duplex stainless steel (CDSS) for primary circuit main pipelines in second generation nuclear plants was subjected to accelerated thermal aging at 450°C for 5000 h (~64.7 equivalent service years at 290°C cold‑leg temperature), and its stress corrosion cracking (SCC) behavior was investigated. Slow strain rate tensile (SSRT) tests were employed to study the influence of thermal aging on SCC susceptibility and crack initiation mechanisms. The results showed that thermal aging induced spinodal decomposition in the ferrite phase of Z3CN20.09M CDSS, leading to severe embrittlement and a significant reduction in corrosion resistance. SCC results revealed a modest increase in susceptibility that did not scale linearly with prolonged thermal aging. The fracture morphology exhibited an increased tendency for brittle cleavage, and cracks were more likely to initiate along the ferrite/austenite phase boundaries with prolongation of aging time. The degradation of both mechanical and corrosion properties of the ferrite phase, caused by thermal aging, was identified as the primary mechanism accelerating SCC under multi‐factor coupling.