Failure analysis of leakage at welded joints of duplex stainless steel in offshore gas field wellhead pipelines

Failure analysis was carried out for the leakage problem caused by cracks in the welding joint of duplex stainless steel tee in the wellhead pipeline of offshore gas fields. The failure causes were comprehensively analyzed from macroscopic and microscopic fracture morphology, crack propagation path, microstructural characteristics,phase proportion and hardness of the welding joint. The results show that the crack initiates from the weld toe of the welding joint on the inner wall of the pipeline, passes through the heat affected zone and extends to the base metal,leading to pipeline leakage. The main failure reasons are the stress concentration caused by poor weld formation of the welding joint on the inner wall of the pipeline and the corrosion environment, which induces stress corrosion cracking. A large number of secondary austenite are formed in the weld metal structure, resulting in the increase of austenite content and the ferrite content is only 20%, which reduces the resistance of the weld metal to stress corrosion cracking. A large number of blocky sigma（σ） phase intermetallic compounds distributed along the grain appear in the base metal structure, leading to brittleness of the base metal, accelerating crack propagation and increasing the risk of local corrosion leakage.