ENVIRONMENTALLY ASSISTED CRACKING OF ALLOY X-750 IN SIMULATED PWR COOLANT

Environmentally assisted cracking (EAC) of different heat treatments of alloy X-750 has been studied in simulated pressurized water reactor (PWR) primary water from 50 to 340-degrees-C. The alloy, with single- (704-degrees-C/20 h) and two-stage (885-degrees-C/24 h + 704-degrees-C/20 h) aging treatments, was increasingly susceptible to EAC with decreasing temperature below -250-degrees-C. This susceptibility was due to hydrogen embrittlement, as indicated by the sensitivity to the hydrogen content of the water and behavior in hydrogen gas. At higher testing temperatures, a second mechanism, possibly anodic dissolution, was dominant for alloys with either a lower solution heat treatment temperature (980-degrees-C/1 h) or two-stage aging treatment. This second mechanism was suppressed by a higher temperature solution treatment (approximately 1100-degrees-C/1 h) and a single-stage aging treatment, which improved alloy performance.