Characterization of bond line discontinuities in a high-Mn TWIP steel pipe welded by HF-ERW

• This study focused on applying the HF-ERW process to the seam welding of expandable pipe using TWIP steels.
• For improvement of the circumferential ductility, deterioration factors were characterized.
• Penetrator which would mainly deteriorate the circumferential ductility consisted of round MnO and Mn2SiO4.
• Metallurgical evidence of existing theory regarding the mechanism of defect formation during the HF-ERW was characterized.

In this work, the microstructure and defects in a high-frequency electrical resistance welded (HF-ERW) pipe of high-Mn twinning-induced plasticity (TWIP) steel were characterized. The microstructure of the base metal and the bond line were examined using both optical microscopy and scanning electron microscopy. The features of the bond line were similar to those of conventional steel. Simultaneously, the circumferential ductility was evaluated via a flaring test. It was concluded that the deterioration of the circumferential ductility in a high-Mn TWIP steel pipe was caused by irregular shaped oxide defects and a penetrator that had been formed during welding. Specifically, the penetrator, which is composed of MnO and Mn2SiO4, was found to be the most influential on the circumferential ductility of the welded pipe. The penetrator was analyzed using both an electron probe micro analyzer and transmission electron microscopy, and the formation sequence of the penetrator was evaluated.