Cracking analysis of nuclear class 316L stainless steel pipe with screwed hole for radiographic test

In order to find out the reason of cracking on one nuclear class 316L stainless steel pipe with screwed hole for radiographic test, the analyses of macrostructure/microstructure, chemical composition, mechanical properties, Vickers-hardness, morphology of fracture, corrosion products and stress distribution of the screwed hole on the cracking pipe were done. The results show that the crack propagates along the grain boundary and the material in the fracture exhibits a rock candy pattern and a large amount of corrosion products with polygonal crystals, which indicates that the cause of failure is attributed to inter-granular stress corrosion cracking (IGSCC) induced by interaction of stagnant water of pressured water reactor and material. The main reason of strain-hardening is the interference fit between the plug and screwed hole which suffering higher welding residual stress. More non-destructive tests on the similar structure in the in-service NPPs and less uses in new NPPs are strongly suggested. (authors)