Failure analysis of 2205 duplex stainless steel connecting pipe for composite plate pressure vessel in a high pressure gas field

Pitting and cracking failure of 2205 duplex stainless steel connecting pipe in a high-pressure gas field occurred. Macro inspection, non-destructive testing, physical and chemical tests, corrosion tests and heat treatment were carried out for the pipe in order to identify the failure cause and mechanism. The results showed that large-size non-metallic inclusions composed of MnS and (Si, Mn, Ca, Al, Cr, Mg) oxides and existence of tiny precipitates didn't meet the requirements of Order Technical Agreement, which resulted in a serious reduction in the impact toughness and pitting resistance. The main reason for the failure is the existence of the harmful precipitates produced by stress relief annealing and large-size non-metallic inclusions. The rapid deterioration of the corrosion environment in the past three years has played a role in inducing and promoting the failure. Fine precipitates mainly reduce the impact toughness, while large-size non-metallic inclusions mainly reduce the pitting resistance. The formation of pits and the large-size non-metallic inclusions are the key factors for the generation of micro cracks, and the fine precipitates accelerate the crack growth.