Effect of the Electro-Chemical Hydrogen Charging lime on Hydrogen Embrittlement of X70 Steel Using for Gas Pipeline

This study investigated the effect of the hydrogen charging time under the electro-chemical hydrogen charging condition on hydrogen embrittlement of hot-rolled and then air-cooled API X70 steel. Tensile and V-notch Charpy impact test specimens were held for 0 h, 2 h, 4 h, and 10 h in the electrolyte pot during electro-chemical hydrogen charging processes and then both of tensile and impact tests were completed at room temperature. The microstructure consisted of fine acicular ferrite, coarse polygonal ferrite and pearlite. The yield and tensile strength decreased slightly and elongation decreased rapidly up to 2 h holding time and then decreased slowly with the holding time. The V-notch Charpy impact value decreased continuously up to 4 h holding time and then increased slightly up to 10 h. The morphology of the fracture surface changed from a ductile type to a brittle type with the hydrogen charging time. Secondary cracks were observed in the hydrogen charged specimens. No external cracks were formed at the specimen surface of the tensile tested with 0 h holding time, but many external cracks were observed at the surface of the hydrogen charged specimens. Cracks were observed in the nominal and parallel directions to the tensile direction at the longitudinal cross-section region near the tensile fractured surface with the 2 h held specimen.