Effect of highly pressurized hydrogen gas charging on the hydrogen embrittlement of API X70 steel

During the use of API X70 steel as a pipeline structural material for the transportation of natural gas, hydrogen embrittlement can occur due to the hydrogen contained in natural gas. The aim of this study is to investigate the effects of the hydrogen content under high-pressure hydrogen gas conditions on the hydrogen embrittlement of air-cooled API X70 steel. The air-cooled API X70 steel was manufactured by hot rolling and was then air-cooled to room temperature. Tensile test specimens were held for 0 h, 1000 h, and 2000 h within a pressure vessel filled with 100% hydrogen gas at a gas pressure of 10 MPa, with the tensile tests then performed at room temperature. The microstructure of the API X70 steel consists of coarse polygonal ferrite, coarse pearlite, and fine acicular ferrite. The yield and tensile strength increased and elongation decreased considerably after a holding time of 2000 h compared to those of 0 h and 1000 h within the pressure vessel. The morphology of the fracture surface changed from ductile to brittle upon hydrogen gas charging. Secondary cracks were observed in both of the hydrogen-gas-charged specimens. No external cracks were formed on the surface of the tensile-tested specimen with a 0 h holding time; however, many external cracks were observed on the specimen surface subjected to hydrogen gas charging.