Abstract: In a hydrocracking process, the connecting pipeline at the bottom of the buffer of the hydrogen compressor fractured. A comprehensive study on the causes of the failure fracture was carried out through methods such as macroscopic inspection, chemical composition analysis, hardness testing, metallographic analysis, microscopic morphology of the fracture surface, and composition analysis of the scale on the fractured pipeline. The fracture surface showed a typical cleavage morphology, indicating that the failure cracking was caused by sulfide stress corrosion cracking. The failure fracture occurred because the cooling rate during the welding of the weld was too fast, resulting in the presence of martensite and bainite in the weld and heat - affected zone tissues. Since martensite and bainite tissues can generate crack sources during the process of bearing dynamic loads, cracks are most likely to occur at stress - concentrated positions such as lack of penetration. Therefore, the crack originated from the lack - of - penetration position of the pipe and propagated towards the outer wall. At the same time, corrosive media such as H₂ and H₂S in the medium formed a corrosive environment, accelerating the cracking and fracture of the pipe.