Analysis of impact toughness variation for flat position multi-pass weld joint

A multi-pass weld metal in flat position was studied, which was welded by a flux cored wire used in X70 pipeline. Analysis included observation of the fracture surfaces to determine initiation location (optical and scanning electron microscopy) and metallographic examination of the weld microstructures. It was found that the position of impact specimen is different, which will cause different impact energy; the proportion of the columnar crystal or coarse crystal area,which was near the position of impact specimen notch root, was a main reason for the impact toughness variation but is not the main reason of low values. The weld defects and coarse grain were the main reason for the variation and low values of impact toughness. When the impact energy is 54.2 J, the impact sample was cleavage fracture. Crack source were welding defects such as inclusions, the defects of incomplete fusion and so on, and the nearer the position of these defects to the notch, the smaller the impact energy.