Mechanisms of Stress-corrosion cracking of alloy X-750 in high-purity water

Abstract

The high-strength, nickel-base alloy X-750 is susceptible to stress-corrosion cracking (SCC) in high-purity, deaerated water. Crack initiation occurs at high temperatures [>400°F (204 °C)]. Crack propagation occurs in both high-temperature and low-temperature [<300 °F (149 °C)] water. High-temperature crack growth rates are on the order of mils per day. Low-temperature cracking is extremely rapid [≈100 in. (250 cm) per day]. Three heat treatments of Alloy X-750 are investigated: (1) stress equalized—1625 °F (885 °C)/24 hours + 1300 °F (704 °C)/20 hours, (2) direct aged—1300 °F (704 °C)/24 hours, and (3) solution annealed and direct aged—2000 °F (1093 °C)/ 1 hour + 1300 °F (704 °C)/20 hours. Stress-equalized Alloy X-750 is most susceptible to SCC; solution-annealed and direct-aged Alloy X-750 is least susceptible. A hydrogen embrittlement model of SCC is developed that predicts SCC performance on the basis of grain boundary chemistry and grain boundary carbides. Phosphorus segregates to the grain boundary in concentrations of up to ≈700 times the bulk concentration during processing and heat treatment. Phosphorus at the grain boundaries increases susceptibility to high- and low-temperature SCC. The presence of M23C6-type carbides and /or the absence of MC-type carbides at the grain boundaries improves SCC performance in high-temperature water.