Research on Weld Microstructure and Properties of Self-shielded Flux Cored Wire

Adopting E71T8-Ni1J self-shielded flux cored wire to conduct semi-automatic welding for X70 line pipe,the weld microstructure characteristics and mechanical properties of different welding layer and interlayer HAZ of self-shielded flux cored wire were investigated by spectrum analysis, metallographic microscope, scanning electron microscopy(SEM) and mechanical properties test. The experimental results showed that the microstructure of root welding mainly consists of fine equiaxed grains, a small amount of granular bainite(GB) and lath martensite; obvious austenite grain boundaries can be observed on filling welding layer, and the microstructure is mainly composed of quasi-polygonal ferrite, granular bainite and M/A constituents; the heat affected zone(HAZ) microstructure of the welding layer is composed of quasi-polygonal ferrite, granular bainite, M/A constituents and the chain-like M/A constituents distributing continuously along the austensite boundaries. The tensile strength of the weld metal is between 720 MPa and 750 MPa, and charpy impact energy is 78~128.5 J at-20 ℃. The chain-like M/A distributing along the grain boundaries, the parallel arrangement of strip M/A between transgranular granular bainite lath in the weld layer HAZ, and rough and uneven weld metal microstructure will reduce the toughness of weld metal.