フラッシュ溶接部靭性向上に関する研究（第１報）―高強度鋼板のフラッシュ溶接性の検討―

Abstract

The development of welding technologies with high productivity is very important in the field of welding. Flash welding is essentially very fast welding and can meet this requirement. However, their weld properties, especially fracture toughness, have not been acceptable and thus the application has been limited to the field that low temperature toughness is not required. In this study, factors governing flash weld toughness have been studied using API X80 grade high strength steel. Results obtained have been summarized as follows ; (1)Weld defect at the weld line has been minimized with the increase of upsetting length. The use of pre-heating process is also effective. (2)Despite of such defect delimitation, the weld toughness has still shown lower values. (3)Welded HAZ showed coarse-grained upper bainite in the as-welded condition, whose vTrs showed worse values when the notch position become closer to the weld line. Good correlation between vTrs and the prior γ grain size has been observed in the synthetic HAZ. In case of actual FBW-HAZ, while the vTrs at 3 mm or 4 mm apart from the weld line showed similar correlation with that of the synthetic HAZ, the vTrs at the weld line was much higher than this correlation line, which is caused by the upsetting deformation. (5)According to the upsetting simulation based on the hot forging, its toughness sharply declined as the increase of upsetting length especially in the steel containing large number of inclusions. It has been considered that the weld toughness degrade due to the existence of the inclusions at the weld interface and this degradation is multiplied by the micro-structural factors, such as HAZ microstructure, grain size and localized softening region.