EFFECTS OF DISSOLVED HYDROGEN ON THE ENVIRONMENTALLY ASSISTED CRACKING OF 316 STAINLESS STEEL IN PWR PRIMARY WATER AT 325 °C

摘要

Slow strain rate tensile (SSRT) tests were performed on hollow cylindrical specimens of 316 stainless steel (316 SS) in a simulated pressure water reactor (PWR) environment. The effects of dissolved hydrogen (DH) in the environment and surface finish condition of an inner surface of the hollowed specimen on stress corrosion crack (SCC) initiation and small crack growth behavior of 316 SS steel were investigated in PWR primary water at 325 °C with various DH levels of 5, 15, 30 and 50 cc (STP) H_2/kg H_2O. SCC initiation and small crack growth behavior of specimens with inner surface finish by a drilled and honed showed a similar trend in the water chemistry with different DH levels. Drill finished surfaces have higher hardness than those of honed surface and were more detrimental on SCC initiation and propagation at a fix hydrogen level. The elongation to the maximum load of the specimen at 50 cc (STP) H_2/kg H_2O was the largest, while for 15 cc (STP) H_2/kg H_2O it was the smallest. Fractographic examinations on the failed samples revealed a mixed mode of transgranular and intergranular cracking. The cracks seem transgranular at the initial stage and then change to intergranular as cracking progress. The crack depth and crack numbers were measured by cross sectional inspections on four different planes along the gauge length by optical microscopy (OM) and scanning electron microscopy (SEM). Most cracks were found to be less than 5 μm. The maximum crack growth rate (CGR) exhibits a peak at a DH level of 15 cc (STP) H_2/kg. The maximum CGRs of drilled finished specimen are higher than those of hone finished specimen in all DH levels. The total crack numbers for the drilled specimens are more than those of honed specimens. This means that the propensity of crack initiation is higher in drilled specimens than honed specimens. The influence of DH on the SCC behavior is discussed.