Self-shielded flux-cored wire is a convenient and efficient consumable for pipeline field girth welding because of its self-protection characteristic and high deposition rate,especially for remote construction sites in rugged terrain. From the perspective of pipeline safety,the impact toughness of the girth welds is an important factor in pipeline integrity,which determines the crack arrest behavior in the girth welds. Therefore,improving the girth weld impact toughness is of primary importance in the field of pipeline girth welding. Three self-shielded flux-cored wires comprising different chemical composition systems have been applied to large diameter X80 UOE( U-ing-O-ingExpanding) pipeline semi-automatic girth welding,and the impact toughness of the welds has been evaluated by girth weld chemical composition analysis and microstructural analysis using scanning electron microscopy( SEM) and energy dispersive spectrometry( EDS) to investigate pipeline girth weld impact toughness and find ways to improve it.This helps in determining the main factors that influence girth weld impact toughness. Pipeline girth weld impact toughness is mainly determined by the final microstructure produced in the solid-state phase transition. In the as-weld state,acicular ferrite( AF) and fine bainite( FB) are a benefit to the impact toughness. For multilayer semiautomatic self-shielded flux-cored wire welding,the normalizing and tempering function of the latter beads to the initial beads plays an important role in the transition of girth weld microstructure,which affects the impact toughness. The original AF and FB and the corresponding heat treatment microstructure of the fine and uniform block ferrite and pearlite result in very good impact toughness. The following two mechanisms are found to promote the production of AF and FB in the girth weld. First,elements promoting the broadening of the austenitic region,such as Ni,C,Cu,and Mn,induce low temperature phase transitions and restrain the opposing function of Al,which is a benefit to the production of AF and FB. Second,dispersed high-melting-point inclusions,especially Al2O3,induce the nucleated production of AF. The advantageous function of inclusions is determined by their shape,distribution,and dimension. Dispersed spherical inclusions of small dimension are a benefit to the production of AF,and result in good impact toughness.
