This investigation deals with the mechanism on the formation of white line in welded joints of ERW steel pipes. In order to demonstrate the formation mechanism, the difference in carbon contents between weld line and extruded melting bead in simulated resistance welded joints was investigated in detail, using hypoeutectics, eutectics and hypereutectics in Fe-C alloys.
In hypoeutectics, the carbon content in weld line is lower than that in base metal, while the carbon content in extruded melting bead is higher. On the other hand, the carbon content in weld line is higher than that in base metal and the carbon content in extruded melting bead is lower in hypereutectics. No difference in carbon content occurs between weld line and extruded melting bead in eutectics.
It can be deduced from these results that two phase region where solid and liquid phase coexist exists in weld line, and the liquid phase coexisting with solid phase is extruded outward and the solid phase remains in the weld line at upsetting. Thus, it can be concluded that the difference in the content of alloying elements between solid phase and liquid phase leads to the formation of white line in resistance welded joints such as ERW steel pipes.