Degradation Heterogeneity in Active X70 Pipeline Welds Microstructure-Property Coupling Under Multiphysics Environments of Hydrogen-Blended Natural Gas

This study investigates the performance degradation of X70 steel weld material in high-pressure natural gas pipelines in the Sichuan-Chongqing region and its impact on pipeline safety by investigating their behavior under multiphysics environments, including varying gas media (nitrogen, methane, hydrogen-blended), pressure conditions (0.1–10 MPa), and material regions (base metal vs. weld). A key novelty of this work is the introduction of a “degradation rate” metric to quantitatively assess the deterioration of weld mechanical properties. A key novelty of this work is the explicit introduction of a “degradation rate” metric to quantitatively assess the deterioration of weld mechanical properties. Slow strain rate tensile tests, combined with fracture morphology and microstructure analysis, reveal that welds exhibit inferior mechanical properties due to microstructural inhomogeneity and residual stresses, including a yield stress reduction of 15.2–18.7%. The risk of brittle fracture was highest in the hydrogen-blended environment, while nitrogen exhibited the most benign effect. Material region changes were identified as the most significant factor affecting degradation. This research provides crucial data and theoretical support for pipeline safety design and material performance optimization.