Stress Corrosion Cracking and Crack Tip Characterization of Alloy X-750 in Boiling Water Reactor Environments

Abstract

The stress corrosion cracking susceptibility of Inconel alloy X-750 in the HTH heat-treated condition has been evaluated in high-purity water @ 93 and 288°C under Normal Water Chemistry (NWC) and Hydrogen Water Chemistry (HWC) conditions. SCC crack growth rates of approximately 1.2x10-7 mm/s (K=28 MPa√m) under NWC conditions and 1.4x10-8 mm/s (K=28 MPa√m) under HWC conditions in 288°C high-purity water were observed. The alloy was also tested in HWC at 93 °C. No SCC crack growth was observed at K= 35 MPa√m for the 525 h of testing at 93°C. At 288°C the fracture mode transitioned from predominantly transgranular cracking under fatigue conditions to intergranular cracking under constant stress intensity for both environments. Atom Probe Tomography (APT) was used to characterize the crack tip and ahead of the crack tip. The results suggest that oxygen diffuses ahead of the crack tip along specific crystallographic directions.