Environment hydrogen embrittlement of pipeline steel X70 under various gas mixture conditions with in situ small punch tests

A series of small punch (SP) tests was conducted to investigate the effect of natural/hydrogen gas mixture conditions on the mechanical properties of X70 pipeline steel. The various gas mixture conditions included 0.1, 0.5, 1, 3, and 5 vol% hydrogen blends and three typical pipeline pressure levels (5, 7, and 10 MPa). The hydrogen embrittlement (HE) susceptibility grades depend on the hydrogen concentration and tend to increase with hydrogen concentration. The hydrogen gas mixture conditions have an insignificant effect on the yield load of the elastic bending region, whereas the maximal load decreases significantly when the hydrogen concentrations increase. Moreover, the quantitative parameters obtained from the SP load displacement curves and fracture morphologies were analyzed to characterize the HE behavior. By evaluating the SP absorption energy as a function of the hydrogen pressure, the ductile–brittle transition and saturated hydrogen pressures of the HE were determined. According to the results, the strength degradation grades of the mechanical properties of X70 pipeline steel in the gaseous hydrogen environment were not affected by the exposure time. •Effect of CH4/H2 gas mixture conditions on X70 pipeline steel was investigated.•HE grades of the SP results are more severe than the results obtained with conventional tensile specimens.•Long-term exposure to gas mixture had no remarkable effect.•Ductile–brittle transition and saturated hydrogen pressure for the HE occur at very low-pressure.