Numerical Simulation of Hydrogen Diffusion in X80 Welded Joint Under the Combined Effect of Residual Stress and Microstructure In homogeneity

Welded joints of hydrogen-containing coal gas transmission pipelines are prone to hydrogen enrichment due to their severe microstructure inhomogeneity and residual stress in them, and thus lead to the decrease of plasticity and toughness. In order to investigate the effect of local hydrogen enrichment on the safety of hydrogen-containing coal gas transport pipelines, a three dimensional numerical simulation model was established to investigate the hydrogen diffusion behaviour considering the combined effect of microstructure inhomogeneity and residual stress in X80 spiral welded pipeline by using ABAQUS software. Results showed that both microstructure inhomogeneity and residual stress could lead to hydrogen diffusion. The distribution of hydrogen concentration in the pipeline was similar to that of hydrostatic stress distribution. That is, the higher the hydrostatic stress value, the higher the corresponding hydrogen concentration, indicating that the influence of residual stress on the hydrogen diffusion behaviour is greater than that of microstructure inhomogeneity. The enriched hydrogen concentration at the center region of the welded joint with the highest residual stress was 2.7 times higher than that without considering residual stress. Equivalent charging hydrogen pressure was put forward to reflect the degree of hydrogen enrichment in weld metal. Slow strain rate tension (SSRT) tests were subsequently performed on weld metal specimen at equivalent charging hydrogen pressure to investigate the effect of hydrogen enrichment on hydrogen embrittlement (HE) susceptibility. The SSRT tests performed in nitrogen gas and simulated coal gas were used for comparison. The HE index increased from 18.56% in simulated coal gas to 32.53% in equivalent charging hydrogen pressure, increasing by 75.27%. Therefore, the residual stress is a non-ignorable factor, because it could lead to hydrogen enrichment and could significantly influence HE susceptibility in welded joint. The determination of hydrogen enrichment in welded joint by using numerical simulation method is the basis to evaluate the safety of coal gas transmission pipeline.