Effect of Thermal Aging on the Intergranular Stress Corrosion Cracking Susceptibility of Type 310S Stainless Steel

The effect of thermal aging on the intergranular stress corrosion cracking (IGSCC) susceptibility of Type 310S stainless steel was investigated. Samples were solution-annealed (baseline), sensitized (short-term aging), and thermally-treated (long-term aging) to form Cr-rich M₂₃C₆ carbides and sigma (σ) phase precipitate particles, respectively, on the grain boundaries. The thermal-aging heat treatments represent two limiting exposure conditions expected in-service for a fuel cladding in the Canadian Generation IV (supercritical water-cooled reactor) design concept and were considered to better understand the risk factors associated with an in-service IGSCC damage mode. The sensitized treatment exhibited the highest relative degree of sensitization (measured by double loop electrochemical potentiokinetic reactivation testing in 2 M H₂SO₄ + 0.01 M KSCN) and IGSCC susceptibility (measured by slow strain rate testing [SSRT] in a hot alkaline solution). The relative susceptibility reflects the controlling role played by Cr-depleted zones, which were only observed in the sensitized (short-term aging) material. The relatively large σ phase grain boundary particles in the thermally-treated (long-term aging) material preferentially cracked during SSRT, suggesting an apparent IGSCC susceptibility at the sample surface. However, the absence of Cr-depleted zones in this material prevented IGSCC from occurring, despite preferential cracking in the σ phase grain boundary particles. The dominant fracture mode in the hot alkaline solution was transgranular mixed-mode cracking. The implications of the results are discussed within the context of risk factors associated with an in-service IGSCC susceptibility in supercritical water.