Effects of Hydrogen Environment on the Mechanical Property and Structural Integrity of X80 Pipeline Girth Weld

Abstract Transporting hydrogen-blended natural gas with a high hydrogen blending ratio in high-grade steel pipelines presents a promising solution for long-distance hydrogen energy transportation in the future. However, the performance degradation and cracking risk of girth welds in hydrogen environments pose significant technical challenges, limiting the development of hydrogen energy transportation technology. There are differences between the circumferential and longitudinal mechanical properties of welds in pipelines. Therefore, the currently widespread practice of using circumferential girth weld round bar to determine girth weld mechanical properties and assess its cracking risk is inaccurate. Moreover, there is limited research on how hydrogen environments influence these mechanical property differences. To address this gap, this study designs and produces longitudinal notched cross-weld specimens and circumferential full-weld standard specimens of X80 pipeline girth weld metal. A slow strain rate tensile test is conducted in both typical hydrogen-blended environments (with a 20% hydrogen blend) and high-pressure (10 MPa) air environments, using X80 pipeline girth weld metal. A novel method, combining finite element modeling and optimization algorithms, is proposed to reverse the constitutive relationship of X80 pipeline girth weld metal in pipe longitudinal direction under hydrogen environments. The Kernel-Based Extreme Learning Machine (KELM) and Improved Grey Wolf Optimization (I-GWO) techniques are utilized to obtain the true stress-strain constitutive relationships under different environmental conditions. This study compares the longitudinal and circumferential stress-strain constitutive relationships in air and hydrogen environments. Based on fracture toughness tests of X80 pipeline welds and the Engineering Critical Assessment (ECA) method, effects of hydrogen on the mechanical properties and structural integrity of cracked defects in girth welds were preliminary investigated. The findings provide valuable insights for fracture assessments of pipeline girth welds, particularly in hydrogen environments, using failure assessment diagrams.