Hydrogen induced stress cracking of super duplex stainless steel UNS S32760 - A root cause failure investigation

Abstract

© 2019 by NACE International. A recent subsea failure of two coupling assembly flanges made of UNS S32760, a 25 wt% Cr super duplex stainless steel, lead to an extensive root cause failure analysis. The components showed a single longitudinal crack along the swaged ferrule, which arrested towards its thicker section. The brittle nature of the fracture surface, calcareous deposits on the ferrule and exposure to cathodic protection suggested hydrogen induced stress cracking-a form of environmentally assisted cracking-as a plausible failure mechanism. Thus, the three causative factors promoting hydrogen induced stress cracking, namely, the microstructure, the environment and applied and residual stresses in the material were the focus of this investigation. This paper discusses the material characterization work, which includes optical and scanning electron microscopy, X-ray spectroscopy, electron backscatter diffraction analysis, tensile testing, microhardness mapping, and corrosion examinations. The results have shown that the failure resulted from a combination of factors, typical for hydrogen induced stress cracking. The measured hydrogen content in parts of the material exceeded 40 ppm, more than an order of magnitude higher than what is normally expected in super duplex stainless steels. Additionally, a highly anisotropic, coarse microstructure was observed, which in combination with the introduced cold work from the swaging process and potential stress raisers from the ferrule design and machining could have facilitated crack initiation, ultimately leading to the failure of the component. This hypothesis was reinforced by the presence of secondary cracks along the main, brittle fracture surface. Further, mechanical testing results showed a detrimental effect on the material's properties due to the presence of residual hydrogen and the swaging operation.