Plastic Strain, Environmental, and Crevice Effects on SCC Initiation and Propagation in Types 316NG and 304 Stainless Steel

Initiation of stress corrosion cracking (SCC) in constant-extension-rate (CERT) test specimens of type 316NG, solution-annealed type 304, and sensitized type 304 stainless steel (SS) has been studied in a water environment containing sulfate impurity at a temperature of 289°C and a strain rate of 2 × 10^-7 s^-1. To improve the sensitivity of the experimental technique, a small-diameter through-hole was drilled in the center of the gauge length of the CERT specimens. This geometry localizes the region of crack initiation in the specimen and allows the detection of very small cracks by scanning electron microscopy. The tests were interrupted after various strains for observation of crack initiation. Estimation of strain concentration by means of Neuber’s rule, and elastic-plastic finite-element calculations in conjunction with crack initiation observations, show that crack initiation in CERT specimens occurs at plastic strains lower than 2 to 3% even for 316NG SS in an impurity environment; also, there exists a trend to suggest that solution-annealed type 304 SS is more resistant to crack initiation than type 316NG SS and sensitized type 304 SS. The SCC initiation studies performed on solution-annealed type 304 SS suggest that the lowest plastic strains at which SCC initiation occurs are not significantly altered by the impurity environment, even when the concentration of dissolved oxygen in the water or the open-circuit electrochemical potential (ECP) is changed over a wide range. The results obtained under creviced conditions (i.e., with SS pins inserted in the holes) show that this newly developed technique is useful in studying the severity of crevice effects on SCC susceptibility. Also, CERT test results obtained from specimens fabricated from 28-in.-diameter welded 316NG SS piping suggest that the weld-induced plastic strains, if any, have a negligible effect on crack initiation and growth. Finally, crack growth results for type 316NG SS, solution-annealed type 304 SS, and sensitized type 304 SS, tested in the same environments in which crack initiation has been investigated, are compared to evaluate differences in the crack initiation and crack growth behavior.