Finite element simulation of the diffusion and distribution of hydrogen at pore defects in X65 steel circumferential weld seam

Abstract

Utilizing long-distance pipelines for large-scale hydrogen transportation is one of the most economical and effective ways. However, hydrogen embrittlement (HE) may be induced during the pipeline service process. Welded joints require particular attention as critical parts of the pipeline for stable and sustainable operation. In this paper, two-dimensional models of circumferential pipeline weld seam containing porosity defects were established, which analyzed the influences of the distribution and dimension of the pores on the hydrogen diffusion characteristics at the joints based on the microstructure-defect-stress interactions. In addition, the effect of post-weld heat treatment (PWHT) on the diffusion and distribution of hydrogen atoms was discussed. The results showed that porosity defects caused an increase in local stress, and the magnitude of hydrogen enrichment at the welding seam was associated with the distribution of pores. Compared with the base value, hydrogen concentration at the cover layer is relatively low, with a maximum concentration of 0.0206×10⁻⁶ ppm. When the porosity exists in the high-stress region, the critical size of porosity for HE susceptibility is 0.4 mm. In the case of hydrogen pipelines, hydrogen enrichment reduces the acceptability of the pores’ dimension in the welding seam. The post-weld heat treatment has a significant stress relief effect, with a reduction of the maximum hydrogen concentration in the porosity region by approximately 50%.