Research on Hydrogen-Induced Crack Propagation Behavior in the Girth Weld Zone of X80 Hydrogen-Enriched Pipelines Based on the Phase Field Method

Abstract

Hydrogen energy is increasingly becoming a critical component of China's energy structure due to its cleanliness, zero carbon emissions, high energy efficiency, and wide availability. Mixing hydrogen with natural gas in specific proportions and transporting it through the existing natural gas pipeline network is widely regarded as an economical and effective method of hydrogen utilization. However, pipeline failure due to hydrogen embrittlement (HE), especially in the girth weld zone, is a major challenge for hydrogen-mixed transportation. This paper investigates the hydrogen permeation behavior in various zones of the girth weld through electrochemical hydrogen permeation tests, elucidating the reasons for differences in hydrogen permeability coefficients and adsorbed hydrogen concentrations in each zone. A compact tension (CT) specimen model based on the phase field method (PFM) was developed to simulate and fit the critical energy release rate of the X80 pipeline girth weld zone in a hydrogen environment. From the obtained force-displacement curves, the critical J-integral for each zone in a hydrogen environment was calculated, examining the fracture toughness variations of X80 pipeline steel base metal (BM) and weld metal (WM) under different hydrogen concentration conditions. Additionally, a quarter-pipe model of an X80 pipeline with a crack was developed using a phase field (PF) fracture model coupled with hydrogen diffusion, simulating the hydrogen-induced cracking phenomenon of the pipeline under actual working conditions. The study investigated the effects of internal pipeline pressure, initial hydrogen concentration, crack geometry, and defect types on the hydrogen concentration distribution at the crack tip and the PF value. Results indicated that before crack propagation, increasing internal pipeline pressure raised the hydrogen concentration at the crack tip, whereas after crack initiation, the hydrogen concentration at the crack tip decreased; increasing initial hydrogen concentration exacerbated the performance degradation of the girth weld zone; the sharper the crack geometry, the higher the hydrogen concentration at the crack tip and the more severe the damage at the crack tip. The models and analytical methods established in this study provide a theoretical basis and technical support for predicting and assessing the safety of pipelines under actual operating conditions. The research findings can guide and inform the design of safer hydrogen-mixed transportation systems.

Keywords: Phase field methodX80 pipeline steelHydrogen embrittlementGirth weldFracture toughness