In this study, the stress corrosion cracking (SCC) of an Inconel Alloy 690 (UNS N06690) with a high percentage (26%) of cold work in two different deaerated high temperature waters, supercritical (above critical point) water (SCW) at 400oC and subcritical (below the critical point) water at 360oC, was studied by using tapered uniaxial tensile samples under constant load and U-bend samples under constant strain. The highly cold worked Alloy 690 tested is not representative and has a high density of TiN particles. This Alloy 690 is susceptible to stress corrosion cracking in SCW. The Alloy 690 tensile sample under 500MPa stress cracked significantly after exposure to SCW (400oC, 25MPa) for 696 hrs, but it did not crack after exposure to 360oC subcritical water under similar exposure duration and pressure. This shows supercritical water is more aggressive than subcritical water to initiate stress corrosion cracking. Generally, Alloy 690 has outstanding SCC performance in high temperature water. This unexpected poor SCC resistance in supercritical water is probably due to the much lower tensile ductility of this particular highly cold worked Alloy 690 and the unusual high density of TiN particles and their inhomogeneous distribution in the alloy. The strain level of the U-bend samples has significant effects on the stress corrosion cracking response. The U-bends with 10% strain cracked very severely in supercritical water, but the U-bends with a lower 6% strain did not crack in either supercritical or subcritical water.
Because Alloy 600 (nominal composition Ni-15.5Cr-9Fe) is susceptible to stress corrosion cracking (SCC) in both pressurized water reactors (PWRs) and Boiling Water Reactors (BWRs)[1], the higherchromium Alloy 690 (nominal composition Ni-30Cr-8Fe) with better stress corrosion cracking performance has been increasingly used for fabricating thin-walled steam generator tubing for pressured water reactors (PWRs).
