Mash seam welders are not applied in continuous cold rolling lines, because rolling fracture occurs under conditions of high reduction owing to the presence of residual steps at the edge of the lap seam. The flattening mechanism of residual steps has mainly been investigated. The shear force of crossing the upper and lower swaging wheels smoothens the steps without forming double-lapped defects. However, the theory of preventing double-lapped deformation remains unclear. On the basis of experimental analysis and FEM, in this report the effect of cross swaging on the flattening of mash seam welds is discussed. Double-lapped defects occur parallel to the sheet surface in the early stage of swaging, because bulging deformation at the edge of the lap seam cannot be prevented. The double-lapped deformation is prevented as follows: (1) Because of the high-temperature zone within the welded sheets after welding, shear deformation due to cross swaging occurs easier than bulging. (2) As tensile force in the longitudinal direction owing to thermal shrinkage occurs within the sheets immediately after welding, plastic flow in the longitudinal and thickness directions by cross swaging is enhanced.
