Effect of microstructure homogeneity on the impact fracture mechanism of X100 pipeline steel laser–MAG hybrid welds with an alternating magnetic field

Abstract

Laser–metal active gas (MAG) hybrid welding has broad application prospects in pipeline steel welding because of the advantages of high penetration and efficiency. Because very little of the wire and shielding gas will reach the bottom of the molten pool, inhomogeneous microstructures will appear in the thickness direction of laser-MAG hybrid welds. In this study, we introduced an alternating magnetic field (AMF) into laser-MAG hybrid welding to study the effect of microstructure homogeneity on the impact fracture mechanism of X100 pipeline steel laser-MAG hybrid welds. The results illustrated that the arc and laser zones of the welds without an alternating magnetic field (WAMF) were composed mainly of acicular ferrite (AF) and lath martensite (LM), respectively. The distribution of inclusions in the thickness direction of the weld became homogeneous, and a homogeneous microstructure composed of AF and LM was obtained in the AMF weld. Among the WAMF and AMF welds, the laser zone of the WAMF weld had the lowest impact toughness due to the low density of the effective grain boundaries and high kernel average misorientation value. The difference in the impact toughness between the laser and arc zones of the WAMF weld led to a difference in the fracture mode. In addition, the impact fracture mechanism of the AF and LM mixed microstructures was investigated, which showed that AF provided a basis for crack deflection and void formation because of its highly effective grain boundary density. The large-angle deflection of the main crack, secondary cracks in the prior propagation direction, and the combined action of plastic deformation and voids improved the impact energy. The results of this study provide guidance for the study of fracture processes in heterogeneous microstructures.