Correlation between in-Situ small punch and conventional test result in evaluating hydrogen embrittlement susceptibility of API X70 base and weld under hydrogen gas mixture

The presence of microstructural heterogeneity within welded joints, combined with environmental factors, can have detrimental effects on the service life of engineering structural components. This study explores the effect of hydrogen/methane (H2/CH4) gas mixture under various volume fractions of hydrogen on the deformation and fracture behavior of a welded API X70 pipeline material employing the small punch test method. The welded metal showed more vulnerable to hydrogen-assisted fracture in comparison to the base metal, as evinced by a notable reduction in the volume fraction of hydrogen required for the transition from ductile to brittle behavior. The hydrogen embrittlement indexes obtained through the small punch test method are more severe than those obtained through conventional test methods. The direct utilization of small punch test results for the assessment of hydrogen compatibility in structural materials may yield overly conservative estimations. Quantitative parameters derived from the small punch test of both materials, along with fracture morphologies, were analyzed to establish a correlation between the hydrogen embrittlement indexes obtained through the small punch test and those obtained through conventional test methods. The small punch test method appears capable of detecting hydrogen embrittlement sensitivity resulting from adverse microstructural features existing in the metal matrix.