Study on Stress Corrosion Performance of Z3CN20.09M Stainless Steel in High-temperature and High-pressure Water Environments Using Micro Four-point Bending Specimens

To explore the stress corrosion performance of Z3CN20.09M stainless steel under simulated PWR primary water conditions and analyze its crack initiation mechanism, this study was designed to determine the crack initiation behavior and threshold stress in a high-temperature, high-pressure aqueous environment relevant to nuclear power plant service. The steel was subjected to 20% cold working to accelerate stress corrosion cracking first in the experiment. Micro four-point bending specimens and tensile specimens were prepared, and the surfaces of the test specimens were polished. The phase composition of the specimens was then quantitatively analyzed using electron backscatter diffraction (EBSD) technology. Slow strain rate tensile tests were conducted at 330 ℃ in air to determine the yield strength (YS) and ultimate tensile strength of the cold-worked materials. Constant-load stress corrosion cracking initiation tests were performed on four-point bending specimens in an autoclave under simulated PWR primary water conditions. Stress levels were set at 1.0, 0.9, 0.8 and 0.7 times the yield strength (corresponding to 525, 472.5, 420 and 367.5 MPa, respectively). Specimens were periodically retrieved every 360 hours for microstructural analysis. Crack initiation was observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), while energy-dispersive X-ray spectroscopy (EDS) analyzed corrosion products and elemental distribution near the cracks. Results indicate the material primarily consisted of an austenitic matrix containing approximately 10% ferritic island phases. Cold working generates numerous parallel slip bands within austenite grains, originating from the multiple slip systems inherent to the face-centered cubic structure. These slip bands clustere at phase boundaries and grain boundaries, creating significant localized strain concentrations that promoted crack initiation in these regions. At the highest stress levels (1.0YS and 0.9YS), cracks form within the first 360 hours after exposure. At 0.8YS stress, crack initiation is delayed until 1 080 hours. It is noteworthy that no cracks are detected even after 2 520 hours of testing at 0.7YS stress. The relationship between applied stress and crack initiation time exhibites an approximate parabolic trend, indicating the existence of a stress-initiation threshold below 0.8YS (420 MPa). Microstructural analysis reveals that slip bands extending to grain boundaries created stress concentrations, leading to accelerated oxidation at these locations. EDS confirms significant oxygen enrichment at crack edges, verifying that boundary oxidation precedes crack initiation. Furthermore, significant differences in oxidation severity were observed on opposite sides of the crack, correlating with slip band density in neighboring grains. This differential oxidation creates local voltage differences, with the more severely deformed side acting as an anode to accelerate dissolution along the interface and crack propagation. At phase boundary intersections with the most severe dislocation stacking, the crack path exhibited a transition from intergranular to transgranular cracking. As cracks propagate into the material interior, oxygen rapidly diffuses along grain boundaries, generating corrosion products like Cr₂O₃ near the boundaries. Volume expansion due to oxidation induces a wedging effect at the crack tip, promoting continued crack growth along the oxidized grain boundary direction. When cracks propagate to the austenite/ferrite phase boundary, ferrite exhibits a clear resistance to crack growth. Consequently, cracks tend to turn along the high-angle grain boundary-phase boundary direction rather than directly penetrating ferrite, continuing to grow along the oxidation direction of the austenite-ferrite phase boundary. In summary, the crack initiation time-stress relationship for Z3CN20.09M stainless steel was obtained, elucidating the stress corrosion cracking initiation mechanism of the material. These findings provide crucial reference for investigating the stress corrosion behavior and managing the service life of this material.