Constant load tests of age-hardened nickel-based alloys in Oskarshamn 2 BWR

Tensile and creviced tensile specimens of high strength nickel-base alloys X-750 and 718 were loaded between 0.5 to 1.3 times yield and placed in the residual heat removal (RHR) system in Oskarshamn 2, an operating Swedish boiling water reactor (BWR). Two heats of alloy X-750, representing six different heat treatments and one heat of alloy 718 given two heat treatments, were used to make the test specimens. The specimens were removed and examined at each yearly refueling outage to determine whether failure had occurred. The results span a 6-year exposure period. The failures were all intergranular. In the case of alloy X-750, a low solution-annealing temperature, 985 degree C (1800 degree F), was associated with a high failure rate. This rate was somewhat lower for single-aged versus double-aged material. No failures occurred for specimens solution-annealed at a higher temperature, 1150 degree C (2100 degree F), regardless of single or double aging. Specimens fitted with tubes to create a crevice condition did not perform as poorly as expected because the crevice formed may not have been sufficient to create a corrosion crevice. Some specimens failed in either the transition zone at shank-to-specimen head or in the threaded section. These types of failures show the importance of controlling stress risers during the design of components. The data indicated that alloy 718 is more resistant than alloy X-750 but not immune