Bond Formation Mechanism for Resistance Welding of X70 Pipeline Steel

A Gleeble^® thermo-mechanical simulator combined with microstructure characterization using a field emission scanning electron microscope was used to provide insights into the seam weld formation during resistance welding (RW). Gleeble^® was used to physically/microstructurally simulate the seam weld formation during RW for the first time. It was found that a peak temperature of 1500°C and 10-mm stroke produced a microstructure in the solid-state bondline, the flash, and the heat-affected zone similar to the resistance welded pipe manufactured in an industrial scale. Using the force response obtained during seam weld formation in Gleeble^®, microstructure characterization of the seam weld, and thermodynamic calculations, it is proposed the seam weld in a resistance weld consists of a mushy zone with delta ferrite and solute-enriched liquid, which solidifies into austenite, and on post welding cooling, transforms into ferrite and stringers of M/A, respectively. The presence of a mushy zone in the weld joints provides a physical explanation for the “decarburization” phenomenon observed in the seam of resistance welds.