Effect of Ti content on the inclusions, microstructure and fracture mechanism of X100 pipeline steel laser-MAG hybrid welds

TLDR: In this paper, the microstructure evolution, impact toughness and fracture mechanism of X100 steel welds with different Ti content using laser-MAG hybrid welding were investigated. And the results illustrated that with increasing Ti content, the outer layer inclusion composition evolved from mostly Al2O3 to Ti2O 3 and finally to TiC.

Abstract: Laser-MAG hybrid welding has a wide range of application prospects for pipeline steel welding, because it combines the advantages of arc welding and laser welding. However, the brittle microstructure of bainite or martensite in hybrid welding results in low impact toughness, and thus it cannot be used in pipeline laying. Hence, in this study, we induced acicular ferrite (AF) into laser-MAG hybrid welds, because of its good impact toughness. We prepared X100 steel welds with different Ti content using laser-MAG hybrid welding. Then, the microstructure evolution, impact toughness and fracture mechanism of the welds with different Ti content were investigated. The results illustrated that with increasing Ti content, the outer layer inclusion composition evolved from mostly Al2O3 to Ti2O3 and finally to TiC. Thus, Ti2O3 induced AF nucleation by forming an Mn-depleted zone, which caused the volume fraction of AF to initially increase and then decrease. The impact toughness also changed accordingly (16.3 → 21.9→8.7J) and the cracks nucleated and propagated in granular bainite during impact fracture. When they encountered AF, the cracks deflected and even stopped propagating. Therefore, by controlling the Ti content in the laser-MAG hybrid weld, the impact energy at −40 °C was higher than in the base metal. In addition, the higher quantity of AF with a high effective grain boundary density (0.57/μm) and low kernel average misorientation value could effectively hinder crack propagation and consequently improve impact toughness.