Effect of Electrochemical Hydrogen Charging Time on Hydrogen Embrittlement of the Hot-Rolled and Accelerated Cooling Treated API X70 Steel

This study investigated the effect of the hydrogen charging time on the hydrogen embrittlement of hot-rolled and accelerated-cooling treated API X70 steel by using the electrochemical hydrogen charging method. Tensile and V-notch Charpy impact test were completed for the specimens of hydrogen charged for 0, 2, 4, and 10 h, and then microstructural observation were performed. The tensile and yield strength tended to decrease slightly and the total elongation started to decrease rapidly up to 2 h charging time and then these gradually decreased with the hydrogen charging time. The post-uniform elongation increased slightly after the holding time of 2 h due to the occurrence of two-step stress reduction phenomenon and its mechanism has suggested. The effect of the hydrogen embrittlement on impact absorption energy according to the hydrogen charging time were also investigated. The fracture surface of the hydrogen charged ones are generally composed of cleavage fracture surfaces. It was demonstrated that the external cracks were formed on the outside of tensile tested specimen and the external shape of the fracture portion was fractured without large elongation at hydrogen charged specimens.