Failure analysis of 316L austenitic stainless steel bellows used under dynamic sodium in a creep testing chamber

Abstract

Failure analysis has been carried out on 316L austenitic stainless steel seamless bellows served as mechanical and environmental isolator in a creep chamber for carrying out creep test under dynamic sodium. A set of 4 seamless bellows welded with a supporting ring by TIG welding process has been used. Visual examination and liquid penetration test revealed that the failure reveal as holes occurring at the bellow convolution which was welded with the supporting ring. Severe loss of material at the failed region due to reaction of leakage liquid sodium with residual oxygen and moisture in argon cover gas was observed. Further, deposition of chromium- and iron-rich oxides was observed in and around the failure location. Microstructure studies revealed the presence of interdendritic porosity in the weld fusion zone and evidences of sensitized grain boundary with extensive precipitation of chromium-rich carbides in the coarse heat affected zone in the bellow. The leakage of sodium through the interdendritic pores and micro-cracks might have facilitated the reaction of sodium with bellows. Improper method for welding of the thin-walled bellow convolution with a supporting ring is considered as the root cause of the failure. Welding technique and method facilitating the formation of equi-axed microstructure with least formation of dendritic structure in the fusion zone and the use of proper cooling arrangement during welding to avoid sensitization have been recommended to avoid the bellow failure during service.