Understanding the effect of strain localization on corrosion fatigue of type 304 austenitic stainless steels in high temperature water

Austenitic stainless steels are widely used in pressurized water reactors (PWRs) because of their good corrosion resistance. However, they are susceptible to environmentally-assisted cracking (e.g. stress corrosion cracking and corrosion fatigue) when exposed to high temperature reactor coolant. In the present paper, two heats of low sulphur type 304 austenitic stainless steels that exhibited different corrosion fatigue behaviors in simulated PWR primary coolant have been investigated. Both heats showed enhanced crack growth, with the degree of enhancement increasing with decreasing loading frequency; however, whilst one remained enhanced, the second heat retarded to rates close to those observed in air when rise time increased to 510 s and 1500 s. Deformation behaviors of both heats were quantitatively studied at ambient temperature and 300 ℃ via high resolution digital image correlation (HRDIC). Particular attention was paid to strain localization and the occurrence of planar slip vs. alternate slip. HRDIC analysis showed that alternate slip was more prominent on the heat that retarded whilst the sample that was enhanced displayed higher strain localization clusters, especially at elevated temperature. It is therefore postulated that the limited alternate slip and the enhanced strain localization at elevated temperature are accountable for the greater environmental enhancement because alternate slip can act as a crack deflection mechanism and slow down crack propagation when the cracking is crystallographic.