Effect of heat input on microstructure and impact toughness of X80 FCAW-S weld joint

This study investigated the effect of welding heat input on the impact toughness of X80 pipeline steel joints produced by self-shielded flux-cored arc welding (FCAW-S). Low heat input (LHI), medium heat input (MHI) and high heat input (HHI) joints were prepared by adjusting welding current and travel speed. Through systematic characterization using scanning electron microscope (SEM), electron backscatter diffraction (EBSD), transmission electron microscope (TME) and instrumented Charpy impact testing, the correlation between weld-metal microstructural and crack initiation/propagation behavior was established. The results show that the microstructure is significantly affected by heat input, and the HHI specimen contains coarse lath bainite (LB) and blocky M-A constituent in the weld center, while the LHI specimen exhibits a complex structure of fine-grains with island-like and elongated M-A. The grain coarsening of HHI specimen results in the lowest kernel average misorientation (KAM=1.04°), indicating reduced geometrically necessary dislocation density and increased deformation incompatibility, which together enhanced the brittle fracture susceptibility in coarse bainite regions. Both LHI and MHI specimen showing ductile fracture characteristics with high shear fracture percentages (70% and 69%, respectively) and high crack-propagation energies (Wp = 122 J and 75J). In contrast, the HHI specimen exhibited predominantly brittle fracture with 27% shear rate and significantly reduced Wp (5 J), while its crack-initiation energy (Wi = 62 J) decreased by only 9.1% compared with the LHI (Wi = 67 J). The deterioration in crack propagation resistance primarily originated from reduced grain boundary density due to grain coarsening under high heat input, which weakened grain boundary-induced crack blocking effects. Additionally, secondary crack nucleation and rapid propagation at coarse M-A constituents, coupled with microstructure heterogeneity induced by elevated heat input, further diminished crack propagation resistance. This research establishes the multi-scale microstructure-property relationship for the influence of heat input regulation on crack growth resistance, providing theoretical guidance for toughness optimization of high-strength steel welded joints.