Microstructures and their distribution within HAZ of X80 pipeline steel welded using hybrid laser-MIG welding

Abstract

Large microstructure gradient in the heat-affected zone (HAZ) in the joints of large thick material welded by the hybrid laser-MIG welding technology reduces the in-service reliability and durability of the welded structure. It is of great importance to characterize and analyze how the microstructure distribution and evolution occur in the HAZ of the laser-MIG hybrid welded joints fabricated with X80 pipeline steel. In this article, the HAZ is found to comprise these characteristic zones, namely banded microstructure HAZ (BMHAZ), fine-grained HAZ (FGHAZ), transitional microstructure HAZ (TMHAZ), and coarse-grained HAZ (CGHAZ). The zone of the HAZ contains quasi-polygonal ferrite (QF), M-A component, polygonal ferrite (PF), and bainite ferrite (BF). From the base metal side towards the weld center with the decrease in the distance, the size of the M-A components decreases and its distribution is more dispersed, while the content of QF decreases in the HAZ. The average diameter of the original austenite grain increases gradually and so does the content of lath microstructures.