Comparison of Microstructure and Residual Stress Affecting Stress Corrosion Cracking Susceptibility of Austenitic Stainless Steel Tubes

Abstract

Three 304L austenitic stainless steel feedwater heater (FWH) tubes used in pressurized water reactors (PWRs) were investigated in this study. By comparing metallurgical differences, such as microstructural features, micro-hardness, and circumferential residual stress of the failed and non-failed FWH tubes by stress corrosion cracking (SCC), factors which could affect SCC susceptibility were assessed. Considering the impurity controlled and hydrogenated water chemistry, it is likely the mechanism of sudden SCC failures under PWR operation would be associated with material aspects rather than water chemistry. In tubes which experienced a large number of SCC failures, significant tensile circumferential residual stresses were measured. Such residual stress could be generated by work hardening during straightening of the rolling process, which directly applies stress on the surface of the tubes. The non-failed tube showed very low residual stress, by using straightening or stretching process. The rolling process induces a large amount of dislocations and an increasing tendency in micro-hardness values toward the outer region. Although the hardness values satisfied the material limitation requirement, the presence of large residual stress is thought to increase the SCC susceptibility of FWH tubes. Thus, this study indicates that straightening by stretching process has benefits in terms of low residual stress, assuring the long-term integrity of FWH tubes against SCC failure. Meanwhile, when the rolling process is used, additional heat treatment could be a practical method to reduce residual stress in FHW tubes. Key words: austenitic stainless steel, work hardening, micro-hardness, residual stress, split-ring test