Improvement of circumferential ductility by reducing discontinuities in a high-Mn TWIP steel weldment

Abstract

Recently, high-frequency electrical resistance welded (HF-ERW) pipes have been introduced as an application of high-Mn twinning induced plasticity (TWIP) steels. However, the excellent ductility of the TWIP steels has been underutilized in expandable pipe applications due to discontinuities in the weldment. This work presents two key ways to improve the pipe expandability or circumferential ductility. In-depth microstructural analysis and flaring tests were conducted to identify bond line discontinuities and to evaluate the circumferential ductility. These results indicate that ductility formed to be strongly related to the welding atmosphere and post-weld heat treatment (PWHT) conditions. The welding atmosphere was related to oxide defects, while the PWHT conditions were related to intrinsic discontinuities. The distribution and amount of penetrator defects along the bond line are the main factors related to the deterioration of the circumferential ductility, which is affected by the sheet metal compositions and decreased remarkably at low PO2 content. We also suggest that PWHT is necessary for improving the ductility by inducing changes in the bond line texture and hardness distribution. The results were verified based on changes in the crack path propagation.