Softening mechanism and fatigue performance analysis of heat affected zone in welding of microalloyed submarine flexible pipes

Subsea flexible pipelines are key equipment in deep-sea resource development, serving as the "lifeline" connecting underwater production systems with surface treatment facilities. Its core function is to safely and efficiently transport various marine resources. The welding Heat Affected Zone (HAZ) is a weak link in the welding joints of submarine flexible pipelines. Under the action of welding thermal cycles, its microstructure and mechanical properties will undergo significant changes. This study investigated the nature and mechanism of the softening zone in the heat affected zone of micro alloy steel flexible pipelines after three welding processes (bottom welding, gap welding, and continuous welding) using characterization techniques such as Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). Based on this, the influence of the softening zone on fatigue performance was analyzed. The results showed that after welding processes, the proportion of recrystallized grains increased, the high strain area decreased, the small angle grain boundaries in the softening zone significantly decreased, and the dislocation density decreased. The formation mechanism of the softening zone is mainly due to the recovery and partial recrystallization of the strengthened structure under the action of welding thermal cycling, as well as the reduction of strain strengthening and fine grain strengthening effects. When the width of the softening zone is 3mm, the strain fatigue life decreases by about 2 times. Therefore, the width of the softening zone of the welding HAZ should be controlled below 3mm.