Effect of Mo microalloying on impact toughness of X80 pipeline steel in SCGHAZ

The microstructure and impact toughness of subcritically reheated coarse-grained heat-affected zone (SCGHAZ) of X80 pipeline steels with different Mo content (0Mo and 0.11 wt%Mo) were investigated. Gleeble 3500 thermal simulation experiment was used to simulate the heat-affected zone (HAZ) of X80 pipeline circumferential weld, and the two samples were characterized by a series of material testing techniques such as SEM, TEM, EBSD and XRD. The research showed that: Appropriate addition of Mo element can improve the hardenability of the matrix, increase the content of bainitic ferrite (BF) in the microstructure, refine the BF lath, cause plastic deformation, and reduce local stress concentration. The increase of BF content greatly increases the proportion of high angle grain boundary (HAGB) in order to effectively inhibit crack propagation. On the other hand, Mo element influences precipitation with solid solution, reduces the early energy barrier for nucleation, delays the disappearance of dislocation lines at high temperature, forms high-density dislocation grid, so that it can increase the nucleation site for NbC precipitation, prevent the coarsing of NbC, increases the threshold of external stress required for the generation of micropores induced by precipitation, and hinder crack initiation. The impact toughness of SCGHAZ was significantly improved by adjusting the microstructure and second phase morphology of HAZ by using Mo element microalloying.