Tingshu Chen

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.

In this study, the hydrogen embrittlement (HE) susceptibility and hydrogen-induced cracking behavior of X70 pipeline steel were investigated in CH₄–H₂ mixed-gas environments containing 5–20% H₂ at a total pressure of 7 MPa. Slow strain rate tensile tests, thermal desorption spectroscopy, fractography, and electron backscatter diffraction (EBSD) were employed to correlate hydrogen uptake, mechanical degradation, fracture morphology, and crack evolution. The results showed that increasing the hydrogen fraction progressively deteriorated the mechanical performance of X70 pipeline steel, as evidenced by reductions in tensile strength and elongation. This degradation was accompanied by a fracture-mode transition from micro-void coalescence to quasi-cleavage fracture. More importantly, EBSD analysis revealed a hydrogen-content-dependent change in the role of the heterogeneous microstructure. At relatively low hydrogen fractions, fine-grained regions and microstructural heterogeneity acted as effective barriers to hydrogen-induced crack propagation. However, at higher hydrogen fractions, this intrinsic crack-arresting capability was substantially weakened. Newly formed cracks were observed to initiate ahead of pre-existing cracks and propagate preferentially along grain boundaries, with limited local plastic deformation. These findings demonstrate that the increased HE susceptibility of X70 pipeline steel under high-H₂ mixed-gas conditions is associated with the progressive loss of the crack-arresting resistance provided by its heterogeneous microstructure.

This work proposes an intelligent grey-wolf-optimizer-improved Apriori algorithm (GWO-Apriori) to mine the association rules of hidden dangers in hydrogen pipeline transmission stations. The optimal minimum support and minimum confidence are determined by GWO instead of the time-consuming trial approach. Experiments show that the average support and average confidence of association rules using GWO-Apriori increase by 29.8% and 21.3%, respectively, when compared with traditional Apriori. Overall, 59 ineffective association rules out of the total 105 rules are filtered by GWO, which dramatically improves data mining effectiveness. Moreover, 23 illogical association rules are excluded, and 12 new strong association rules ignored by the traditional Apriori are successfully mined. Compared with the inefficient and labor-intensive manual investigation, the intelligent GWO-Apriori algorithm dramatically improves pertinency and efficiency of hidden danger identification in hydrogen pipeline transmission stations.