Machinery's Handbook, 31st Edition
Pipe Welding
1613
fusion. With flux-cored electrodes, the minimum amount of wire stick out is 3 ∕ 4 inch (19 mm). Weld fusion can be improved in welding pipe of 0.4 inch (10 mm) wall thickness and thicker by preheating the work to a temperature between 400 and 500 ° F (204 and 260 ° C). Horizontal Pipe Welding: In 1G welds (see Fig. 1a), the pipe should be rotated in the direction that moves the solidifying area away from the wire tip, to minimize penetration and resulting breakthrough. Welding of pipe in the 2G, horizontal position is made more difficult by the tendency for the molten metal to drip from the weld pool. Such dripping may cause an excessively large keyhole to form during the root-welding pass, and in sub sequent passes electrode metal may be lost. Metal may also be lost from the edge of the upper pipe, causing an undercut at that side of the weld. Vertical-down Welding: With the pipe axis horizontal (as in the 5G position in Fig. 1f), vertical-down welding is usually started at the top or 12 o’clock location, and proceeds until the 6 o’clock location is reached. Welding then starts again at the 12 o’clock location and continues in the opposite direction until the 6 o’clock location is reached. Vertical- down welding is mainly used for thin-walled, low-carbon steel pipe of 1 ∕ 8 to 5 ∕ 16 inch (3.2 to 7.9 mm) wall thickness, which has low heat-retaining capacity so that the weld metal cools slowly, producing a soft and ductile structure. The slow rate of cooling also permits faster weld deposition, and, when several beads are deposited, causes an annealing effect that may refine the entire weld structure. Vertical-up Welding: In the 5G position, vertical-up welding normally begins at the 6 o’clock location and continues up to the 12 o’clock location, the weld then being com pleted by starting at the 6 o’clock location on the other side of the pipe and traversing up to the 12 o’clock location again. Vertical-up welding is more suited to pipe with thick walls and to alloy steels. However, the greater heat sink effect of the heavy-walled pipe may result in a faster cooling rate and embrittlement of the material, especially in alloy steels. The cooling rate can be reduced by slowing the rate of traverse and depositing a heavier bead of metal, both facilitated by welding in the vertical-up direction. Using a thicker electrode and higher current for thicker-walled pipe to reduce the number of beads required may result in dripping from the molten puddle of metal. Defects such as pin holes, lack of fusion, and cold lap, may then appear in the weld. Vertical-up welding of pipe in the 5G, fixed, horizontal position, Fig. 1f, used for thick-walled pipe, is probably the most difficult for a welder, but once mastered will form the basis for other methods of pipe welding. Starting at the 6 o’clock location, the arc for the root pass is struck overhead, with the electrode at an angle of 5 to 10 ° from the vertical, on the joint, not on the tack weld. A long arc should be maintained for a short-period while weaving the electrode to pre-heat the area ahead of the weld. Only small amounts of filler metal will be transferred while this long arc is maintained in the overhead position. The electrode tip is then advanced to establish the correct arc length and held in position long enough for the keyhole to form before starting to lay down the root bead, moving up toward the 12 o’clock location. Thin-wall Pipe: The optimum globular/spray parameters for welding rotated, (1G posi tion) thin-wall pipe of less than 12 inch (305 mm) diameter are 0.035-inch (1 mm) elec trode wire fed at 380–420 in/min (9.7–10.7 m/min) with a protective gas mixture of argon 80 to 85, CO 2 15 to 20 percent, and current of 190 to 210 amps. These conditions will provide deposition rates of about 6 lb/hr (3 kg/hr). Use of Flux-Cored Electrodes.— Small diameter, flux cored electrodes developed in the eighties are still a rarity in many pipe welding shops, but flux cored welding can produce
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online