Influence of Imposed Strain Rate on Stress Corrosion Damage Mechanisms in Synthetic Seawater of Austenitic–Ferritic Stainless Steel X6CrNiMoCu25-6

Austenitic–ferritic stainless steels are frequently selected for components in seawater-cooling pumps in power plants and nuclear stations due to their generally superior resistance to localized and stress corrosion cracking (SCC) in comparison with austenitic grades. However, during operation, these cast components are exposed to combined applied/residual stresses and chloride-rich seawater. This work addresses the research gap by investigating the SCC susceptibility of X6CrNiMoCu25-6 in the hyper-quenched state (45% ferrite–55% austenite) using slow strain-rate tensile (SSRT) tests in synthetic seawater at 70 °C at five imposed strain rates ranging from 2.38 × 10−7 s−1 to 10−4 s−1. The work is complemented by macrographic examination of crack initiation and SEM microfractographic analysis of the fracture surfaces. The findings indicate that the grade demonstrates only negligible susceptibility to SCC with respect to permissible stress, exhibiting a maximum reduction in tensile strength that is constrained to 5% relative to a neutral reference medium. However, a distinct critical strain rate of approximately 10−6 s−1 has been identified. Further fractographic and microstructural analysis demonstrates that the susceptibility at this critical rate is governed by a localized mechanism. This mechanism is characterized by repeated pitting nucleation at slip-step emergence sites in the ferrite, selective ferrite dissolution, and progressive α/γ interfacial decohesion. These processes result in mixed intergranular/transgranular crack propagation, rather than by bulk mechanical softening. These findings address a particular lacuna in the duplex-steel SCC literature by providing a quantitative strain-rate/damage criterion, in conjunction with elongation-based rather than stress-based susceptibility indicators, for the assessment and management of the SCC risk of this cast duplex grade in real seawater-cooling pump service. From a fundamental standpoint, these findings indicate that the susceptibility of SCC in this duplex grade is governed by a competition between plastic-deformation kinetics and electrochemical dissolution kinetics. This competition reaches a maximum at an intermediate critical strain rate, thereby providing further experimental support for a general strain-rate/dissolution-competition model of SCC applicable to duplex steel.