Hydrogen-Induced Damage Evolution and Quasi-Cleavage Cracking in API X70 Pipeline Girth Welds Under Blended CH4-H2 Environments

Hydrogen blending in natural gas pipelines poses an embrittlement risk to girth welds. This study investigated an API X70 girth weld in CH4-H2 mixtures containing 5–20% H2 at 4–10 MPa using slow strain rate tensile testing, fractography, and electron backscatter diffraction (EBSD). Relative to pure CH4, the maximum reductions in yield and ultimate tensile strengths were 7.2% and 3.9%, respectively, whereas elongation decreased from 16.46% to 13.04% under the 10 MPa-20% H2 condition. Fractography revealed increasingly extensive edge-localized quasi-cleavage (QC) regions under higher hydrogen exposure conditions. Post-fracture EBSD revealed condition-dependent differences in micro-void morphology and secondary cracking. At 7 MPa-20% H2, intersecting high KAM bands in two regions near a representative secondary crack exhibited local peak values of 2.65°–3.87° and were geometrically compatible with distinct {110} plane traces. However, the examined crack path did not consistently follow a single {110} trace. These findings indicate a close association between QC cracking and localized intragranular plastic deformation and were consistent with a contribution from hydrogen-enhanced localized plasticity (HELP), although vacancy-mediated damage and hydrogen-enhanced decohesion could not be excluded.