Failure analysis of a gas transport pipe made of API 5L X60 steel

Pipelines used in the oil and gas transport usually contain some sulfur dioxide, which absorbs hydrogen from the pipe wall and accelerates hydrogen damage of steels. Also, the welding process and the type of manufacturing process can facilitate the entry of hydrogen into the steel. In this research, API 5L X60 pipes used in the refinery of Tang-e-Bijar region of Kermanshah, Iran, were studied. Microstructural studies showed that the primary microstructure of the steel consisted of 55% ferrite and 45% pearlite. The microstructure in the welding zone could include Widmanstatten ferrite, based on its morphology shown in the optical etched microstructure. The observed cracks in the microstructure were also of step-wise cracking and represent the possibility of hydrogen induced cracking. Pre-etching observations using light microscopy shows MnS impurities in the structure. Also, SEM/EDS analysis revealed that the cracks were formed from the accumulation of MnS impurities and these impurities were preferred sites for nucleation and growth of the cracks. The results of the mechanical testing including hardness, impact and tensile test showed that the steel had a ductile behavior for all the sections and no hydrogen embrittlement has been probable.