Stress corrosion cracking of low alloy steel weldments in LWR environments

Results obtained at VTT in the ICG-EAC Group bolt-loaded WOL specimen testing round robin for pressure vessel steel weldments in simulated PWR primary water (typical of Loviisa VVER-440 power plant) are presented. Two low alloy type A508 Cl.2 steels were tested; one steel had a very low S-content (0.002% S) and the other steel had normal medium S-content (0.008/0.009% S). The low sulphur steel did not show any indication of stress corrosion crack growth, while in one specimen of normal A508 Cl.2 steel minor crack extension (less than 1 mm) was locally observed at the precrack tip in the HAZ of the weldment. Crack extension was explained by a water chemistry transient in one phase of the test and the phase of cracking could be related to this occurrence by detailed fractographic examination. Because of difficulties in locating exactly the precrack tip in the HAZ of the weldment, it was observed in the end of the test that precrack tips in various specimens were located either in the weld metal or in the HAZ or in some cases also very close to the fusion line. Thus, this study was also testing the stress corrosion cracking susceptibilities of the various zones of the weldments of low alloy pressure vessel steel A508 Cl.2. Stress corrosion crack extension in small amounts was observed only in one HAZ specimen of the normal medium S-content steel.