Failure analysis of an AISI 316 steel pipe elbow exposed to the weather for three years after 16 years of operating at 515 °C

The failure of an AISI 316 austenitic stainless-steel pipe used in a hydrotreatment plant was investigated. Circumferential cracks starting from outside the pipe and parallel to the weld were identified in the pipe elbow. The failure occurred after three years of plant inactivity and exposure to the atmosphere. The pipe was operated regularly for 16 years at a temperature of 515 °C before failure, having undergone phase transformations that made the steel susceptible to intergranular attack, followed by stress corrosion cracking at room temperature. The preferential precipitation of chromium-rich M23C6 carbides at the grain boundaries allowed the occurrence of sensitization, which, associated with the residual tensile stresses caused by the welding process and the presence of chlorine from industrial atmosphere concentrated under insulation, were responsible for the failure by stress corrosion cracking.