Analysis of microstructure and local softening of heat-affected zone of submerged-arc welded X80 joint

X80 pipeline steel was welded by submerged arc welding process. The local softening phenomenon and failure behavior of heat-affected zone (HAZ) of the welding joint were characterized by digital image correlation (DIC) tensile tests using mini-sized flat specimens. The relationship between strength degradation and microstructural evolution was discussed carefully. The DIC results show that the primary fine-grain heat-affected zone (FGHAZ) and the secondary heat-affected zone (UAFZHAZ and ICFGHAZ) are both the softening areas in HAZ of joint. The microstructural characterization shows that the softening is mainly attributed to the dislocation strengthening weakening resulting from the decreasing of dislocation tangles and dislocation walls and the precipitation strengthening weakening due to the changes in form and quantity of M/A. But, the softening of secondary heat affected zone can be effectively alleviated by other surrounding regions which is favorable to joint. On the contrary, the strength degradation of the primary heat affected zone has a greater threat to the service safety of welded joints.