Stress corrosion cracking of austenitic alloys in supercritical water

Tapered uniaxial tensile samples under constant load and bent ring samples under constant strain were used to investigate the stress corrosion cracking (SCC) of Alloy 600 and 690 in supercritical water. To verify the effectiveness of the tapered tensile sample design, the SCC of two 304 stainless steels with high and low carbon content were tested in supercritical water at 400°C, 25MPa and 10-15ppb dissolved oxygen. Both testing methods are effective to initiate SCC in 304SS, Alloy 600 and 690. Based on the bent ring SCC testing, the SCC resistance of Alloy 690 in supercritical water is lower than Alloy 600. This unexpected lower SCC resistance is probably due to the much lower tensile ductility (less than 10%) of the tested Alloy 690 as compared with the standard 40-50% ductility and the severe plastic deformation and strain in the bent ring samples. The large TiN particles and their inhomogeneous distribution in the alloy may promote the SCC initiation in this specific Alloy 690 in the bent ring samples. SCC is more likely to initiate from the Electrical Discharge Machined (EDM) or end mill machined surfaces than from ground and polished surfaces.