Development of Three Wires Submerged Arc Welding Technique for the Production of Large Diameter and Heavy Wall Thickness Line Pipes (Report 1) : Factors on Bead Shape and Sound Weld Metal

Abstract

The development of three wires submerged arc welding techniques was carried out to produce line-pipes in the large-diameter and heavy-wall-thickness from 20 to 50 mm by UOE process. Some factors on a sound weld metal and a bead shape have been clarified as follows.(1) The good correlation between the penetration depth of the weld bead and the sum of 3√ I4/v⋅V2a at the leading and the middle wire could be obtained. The bead width had the good correlation with the heat input, and it was spread by an increase of the welding current and/or the arc voltage at the middle and the trail wire or by a decreasing of the welding speed.(2) The stable arc voltage and the good bead appearance can be obtained in the use of a large diameter welding wire because of lowering down the current density in it. The wire of 4.0 mm in diameter is too small to get a good weld bead, but the wire of 6.4 mm in diameter is so rigid that undercuts in the weld bead are sometimes caused by the unstable wire feeding. As a result the optimum wire may be 4.8 mm in diameter.(3) As lengthening a wire spacing, nitrogen and oxygen content in a weld metal are increased and they deteriorate the Charpy value, so that each wire spacing is desired to set within about 30 mm.(4) To get the high welding speed due to increasing a melting rate may be to weld at a high welding-current, but in this case, weld defects of the bead appearance such as undercuts are caused because of the hunting of the arc voltage happened by higher welding-current. Therefore sums of the welding current in three wires are desired to be about less than 3000 Amp. and the welding current at each wire is limited in the use of the fused flux.