Objective  Clarify the failure cause of the flange bolt fracture at the wellhead of a high sulfur gas well in Sichuan.

Methods  The failure process and causes were analyzed by the methods of macroscopic observation, physical and chemical testing of materials, fracture morphology analysis, microscopic corrosion morphology and corrosion product analysis, combined with the characteristics of the service environment.

Results  The bolt material exhibits hardness exceeding 35 HRC, elongation after tensile fracture of less than 16% and percentage reduction of area of less than 50%, indicating a high susceptibility to stress corrosion cracking. The bolt displays brittle fracture, with the fracture surface showing a high S element mass fraction of 9.67%, indicative of sulfide stress corrosion cracking. The most severely corroded areas of the bolt are concentrated in the middle thread region, with corrosion paths of the thread extending to the upper and lower sides. The main corrosion product is Fe₃O₄, and severe oxygen corrosion occurs.

Conclusions Due to long-term atmospheric oxygen corrosion and accelerated crevice corrosion, the threads from the middle to the both ends of the bolt are progressively corroded and damaged, weakening the flange's sealing capabilities. Microleakage sulfur gas diffuses and accumulates along the corrosion paths, and the combined effect with the bolt's pre-tightening force ultimately leads to sulfide stress corrosion cracking, resulting in bolt failure.