Failure analysis of primary manway studs removed from two PWR plants

During 1980, on-site inspections performed at two pressurized water reactor plants revealed cracks in several of the studs used to retain the steam generator primary manway closures. The stud material used for this application was AISI 4340 steel with normal chemistry and tempered martensite microstructure at a nominal yield strength range of 900-1000 MPa. An examination performed on selected studs revealed that failure resulted from stress corrosion cracking. The studs examined had been exposed to primary water and compounds containing S and Cl. The presence of compounds containing S and high concentrations of MoO3 appear to be a result of decomposition of the MoS2 base lubricant applied during stud tightening. These conditions apparently caused crack initiation at low temperatures during plant shutdown at locations in the studs with high stress intensities