Role of grain boundary engineering in the SCC behavior of ferritic–martensitic alloy HT-9

This paper focuses on the role of grain boundary engineering (GBE) in stress corrosion cracking (SCC) of ferritic–martensitic (F–M) alloy HT-9 in supercritical water (SCW) at 400 °C and 500 °C. Constant extension rate tensile (CERT) tests were conducted on HT-9 in as-received (AR) and coincident site lattice enhanced (CSLE) condition. Both unirradiated and irradiated specimens (irradiated with 2 MeV protons at 400 °C and 500 °C to a dose of 7 dpa) were tested. Ferritic–martensitic steel HT-9 exhibited intergranular stress corrosion cracking when subjected to CERT tests in an environment of supercritical water at 400 °C and 500 °C and also in an inert environment of argon at 500 °C. CSL-enhancement reduces grain boundary carbide coarsening and cracking susceptibility in both the unirradiated and irradiated condition. Irradiation enhanced coarsening of grain boundary carbides and cracking susceptibility of HT-9 for both the AR and CSLE conditions. Intergranular (IG) cracking of HT-9 results likely from fracture of IG carbides and seems consistent with the mechanism that coarser carbides worsen cracking susceptibility. Oxidation in combination with wedging stresses is the likely cause of the observed environmental enhancement of high temperature IG cracking in HT-9.