Irradiation-assisted stress corrosion cracking of Type 347 and Type 316 steels irradiated in commercial pressurized water reactors

Investigations of Type 347 and Type 316 austenitic stainless steel baffle-former bolt failures at 4-loop downflow pressurized water reactors (PWRs) in the US have identified significant amounts of transgranular cracking on the fracture surfaces along with intergranular cracking and ductile rupture. The expected failure mode for baffle-former bolts is irradiation-assisted stress corrosion cracking (IASCC), which typically results in an intergranular cracking mode. The presence of transgranular cracking raised the question of whether IASCC was responsible for the failures, or if a fatigue component was contributory. Detailed fractography and metallography of bolts with and without cracks confirmed that an environmentally-assisted cracking mechanism was responsible. Reviews of the literature identified numerous cases where stress corrosion cracks (including irradiation-assisted stress corrosion cracks) in austenitic stainless steels transitioned from intergranular to transgranular, typically as a more severe stress state developed. The potential cracking mechanisms are discussed. This allowed the conclusion that IASCC was responsible for the degradation observed in the baffle-former bolts, though a component of alternating loading cannot be excluded. The downflow plant configuration results in increased stress on neighboring intact bolts after bolt failure, resulting in the development of ‘clusters’ of failed bolts, which grew as additional bolts failed. This cluster growth contributed to the severe stress state imposed on the bolts.