Effect of post-weld heat treatment on microstructure evolution in dissimilar joints for subsea oil and gas systems

In subsea oil and gas systems, microalloyed steel linepipes are welded to low alloy steel (e.g. 8630) forgings using dissimilar fillers, such as alloy 625. Post-weld heat treatment (PWHT) is necessary to temper the heat affected zone (HAZ) and relieve residual stresses. During PWHT, carbon diffuses from the forging into the weld, where it is able to form critical new phases while creating a carbon depleted region within the HAZ. Although the phases that form within the weld metal are well documented, the extent of carbon diffusion remained unexplored. To tackle the problem, a combined experimental and modelling approach has been adopted. DICTRA simulations were employed in calculation of elemental diffusion, while electron microscopy has been used to verify phase formation and the extent of carbon diffusion. The results reported herein indicate that it is possible to modify the susceptibility of these joints to hydrogen embrittlement.