The Effect of Hydrogen on Fracture Toughness of High Strength Linepipe Steel

As the use of hydrogen is attracting attention towards the realization of a low-carbon society, pipelines for transporting hydrogen gas are being considered as a method of transporting hydrogen in large quantities and at low cost.However, the effects of hydrogen embrittlement on pipeline steel are not clear.In this study, the effect of hydrogen on ductile crack initiation and growth resistance was investigated in high-strength linepipe steel.Fracture toughness tests showed that the steel exhibited stable ductile crack growth behavior under hydrogen environment without unstable brittle fracture, but the ductile crack growth resistance was significantly lower than that in the absence of hydrogen.In order to evaluate the effect of hydrogen on the mechanical properties controlling ductile crack growth resistance, tensile tests were conducted using smooth and circumferential notched round-bar specimens (R1 and R2 specimens) with and without hydrogen charging.The hydrogen-charged specimens exhibited a smaller reduction in area, indicating that hydrogen reduced the ductility of the high-strength linepipe steel.The effect of hydrogen on a stress triaxiality dependent critical strain for ductile failure was evaluated from the test results and elastic-plastic FEM analysis, and it was found that a stress triaxiality dependent critical strain for ductile failure were reduced by hydrogen.Fracture surface and cross-section of the round-bar specimens were observed to investigate the effects of hydrogen on critical strain for ductile fracture and damage behavior, and it was considered that the hydrogen decreased critical strain for void initiation.Consequently, degradation of ductile crack growth resistance due to hydrogen was caused by decrease critical strain for void initiation.