INFLUENCE OF WELD PUDDLE SHAPES AND THE PRIMARY STRUCTURES ON A BEAD SOLIDIFICATION DEFECT IN HIGH SPEED WELDING: SOLIDIFICATION PROCESS OF SUBMERGED ARC WELD METALS (REPORT 4).

When high speed submerged arc welding (SAW) was carried out using highly basic fluxes to decrease the oxygen content of the weld metal of large-diameter linepipes, a continuous shrinkage cavity is formed to the bead surface and obstructed the high speed of SAW in the past. To raise the welding speed of SAW through solidification process improvement, findings concerning the weld puddle shape and the liquid metal flow in the fundamental studies of Reports NOs. 1 and 3 have been effectively applied to the present investigation. The method of preventing the shrinkage cavity has been experimentally examined from the aspects of weld puddle shapes and solidification structures. It was elucidated that the shrinkage cavity occurred when the healing action of the liquid at the rear part of the tear-drop-type weld puddle was insufficient, and would be prevented by positively forming the equiaxed dendrites at the bead center, which permits the rear part of the weld puddle to approximate to an elliptical shape.