Some aspects of thermal fatigue in stainless steel

Abstract

This paper is concerned with the analysis of failures in the moderator circuit branch piping of the ATUCHA-1 pressurized heavy water reactor (PHWR), which is made of austenitic steel to DIN 1·4550 specification (similar to AISI 347). These failures are considered to result from thermal fatigue processes induced by fluctuations in a zone where stratified temperature layers occurred, the fluctuations being associated with variations in moderator flow. The first section evaluates the possibility of cracking due to thermal fatigue phenomena and concludes that under service conditions a crack may initiate and grow through 7 mm thickness of the branch pipe. In laboratory thermal fatigue tests that simulated the thermomechanical conditions for such a component, the number of cycles required to initiate a thermal fatigue crack in a notched modified standard fatigue specimen was about 10 3. This value may be used to give a conservative prediction of the number of thermal cycles for crack initiation in actual branch pipes, including those subject to the cold plug condition which is produced in some emergency shut-down and valve testing situations. It was also demonstrated that beyond a crack depth of 7 mm stress corrosion cracking is the main process in further crack propagation. The relevance of this prediction is confirmed by microfractographic observations, since the brittle nature of the fracture surfaces under service conditions appears very different from the transgranular ductile striations found in both thermal and mechanical fatigue test specimens as a result of interacting environmental effects.