Recent observations on the cracking of 304L stainless steel in cyclic moist environments within CANDU nuclear power plants

摘要

The stress corrosion cracking (SCC) of stainless steel type 304L has been observed in a CANDU nuclear plant. The cracking occurred on the inside surface of a piping structure, transgranular in nature, and present in all regions of the pipe, including sections adjacent to a weld, along the straight section of the pipe, and at bends in the pipe. While it is generally understood that such cracking mechanism is governed by specific intrinsic parameters associated with stress, environment, and material factors, the environmental factors in this particular case are not typical. This paper discusses the results of the failure analysis conducted on the affected component material. In addition, the results of efforts towards generating an improved understanding of stainless steel SCC in a specific operating environment are presented. Moreover, the assessment of the observed mechanism includes the investigation of residual stresses within the affected piping using Orientation Imaging Microscopy (OIM) to assess the relative measure of residual plastic strain present in the crack locations and in the general tube microstructure. Measurements were performed on sections of undamaged test welds, bends, and around the crack locations. Additionally, secondary ion mass spectroscopy (SIMS) was used to examine metal surface oxides buried beneath deposits and at strained regions of the pipe in order to elucidate the operating environment. Some experimental work, initiated to confirm some of the important environmental and material factors, was carried out to obtain information on the threshold levels of contaminants for a given range of temperatures.