Hook crack in electric resistance welding line pipe steel

One of the production methods for API line pipe steel is electric resistance welding (ERW). This kind of pipes is encountered with hook crack which is one of the defects that is observed in the upset zone of ERW pipes, detecting by ultrasonic inspection. Hook crack is a small crack following the weld flow lines and if severe enough, opened up at the pipe surface. These defects were commonly associated with sulphide stringers and non metallic oxide inclusions. In this research, welds containing hook cracks were obtained from Ahvaz pipe making factory. Scanning electron microscope (SEM) examination showed that all hook cracks were associated with small non metallic inclusions and X-ray microprobe analysis (EDX) was performed to detect the main elements associated with the inclusion. Hook cracks were found to be associated with Al2O3, CaO, CaS and MgO. This work revealed that these defects can be eliminated using; calcium treatment and Argon stirring improvement in ladle furnace, also using tundish refractory with higher quality and control of melt flow in ladle furnace and tundish.