The correlation between hydrogen permeation behavior and microstructure in L360QS pipeline steel welded joints

In this research, the microstructure, electrochemical hydrogen permeation properties, thermal desorption spectroscopy (TDS) characteristics, hydrogen diffusion and trapping behavior in different micro-regions of L360QS pipeline welded joints were systematically investigated. The results prove that the heat-affected zone (HAZ) of the L360QS pipeline steel welded joints exhibits the highest effective hydrogen diffusion coefficient and the largest amount of diffusible hydrogen compared with those in the weld metal and base metal. The tendency of hydrogen diffusion and accumulation in the HAZ is closely related to the presence of more straight grain boundaries. The straight grain boundaries permit an increase in effective hydrogen diffusion and equilibrium hydrogen concentration. The high grain densities of base metal in fine grains would hinder hydrogen diffusion，resulting in a lower effective hydrogen diffusion coefficient. Due to the presence of polygonal ferrite, pearlite, and granular bainite with a significant number of irreversible hydrogen traps for the weld metal，it hinders hydrogen diffusion and lead to the decrease of diffusible hydrogen content. This research provides a fundamental understanding of hydrogen diffusion behavior which will support the optimization of welding processes for L360QS pipeline in hydrogen-rich environments.