Insights into gaseous hydrogen permeation behavior in welds of X65 pipeline steel

Abstract

This study compares the gaseous hydrogen permeation behavior of API 5L X65 pipeline steel welds produced using hybrid laser–arc welding (HLAW) and gas metal arc welding (GMAW). Electron backscatter diffraction (EBSD) and high-pressure hydrogen permeation tests were used to evaluate the grain boundary characteristics and hydrogen diffusion in the welds. The HLAW weld displayed a finer microstructure with a higher fraction of high-angle grain boundaries (HAGBs) compared to the GMAW weld. This resulted in a lower effective hydrogen diffusion coefficient (3.13×10⁻6 cm2/s) and a higher surface hydrogen concentration (6.63×10⁻12 mol/cm3) in the HLAW specimen. Thermal desorption spectroscopy (TDS) further revealed higher hydrogen accumulation due to stronger hydrogen trapping associated with HAGBs in the HLAW welds, which are formed by the rapid cooling inherent to this welding technique. These results show that enrichment of HAGBs effectively hinders hydrogen diffusion. Therefore, optimizing weld design using HLAW can reduce hydrogen permeation in pipeline steels intended for hydrogen transport applications.