Contribution of Localized Deformation to IGSCC and IASCC in Austenitic Stainless Steels

Abstract

It is known that the irradiation-induced microstructrual and microchemical changes in austenitic alloys may affect the irradiation-assisted stress corrosion (IASCC) susceptibility. However, a recent study [1] shows that IASCC susceptibility is more likely to be linked to the mechanical properties such as yield stress and hardness that arise from the microstructrual change. A higher yield stress generally corresponds to a lower level of uniform elongation but a higher degree of localized deformation. Localized deformation, induced by low stacking fault energy (SFE) and/or irradiation damage, may play a key role in IGSCC and IASCC susceptibility of austenitic stainless steels and nickel-base alloys. Proton irradiation and constant extension rate tensile testing (CERT) were used to examine the potential impact of SFE and irradiation on deformation mode and IASCC. Three model alloys (E : UHP-304, H : 304+Si and L : 304+Cr+Ni) having a spread in SFE were selected for this study. Two batches of samples were irradiated with 3.2 MeV protons at 360°C to 1.0 and 5.5 dpa respectively, and then incrementally strained in 288°C Ar atmosphere to 3%, 7% and 12%. After each strain level, the degree of strain localization, as determined by channel height, width and spacing were quantified using SEM and AFM on replicas of the surfaces of the deformed samples. Results were compared to those from SCC tests in simulated LWR environments to determine if localized deformation is a controlling factor in IASCC.