Machinery's Handbook, 31st Edition
1616 Pipe Welding On pipes over 6 inches outside diameter use 4 tack welds. Locate tack welds at 12, 3, 6, and 9 o’clock. Root Welding Procedures for FCAW 5G Pipe Welds: Root weld MIG vertical-down. Weld sequence: 12 to 3, 9 to 6, 3 to 6, and 12 to 9 o’clock positions. Start and finish MIG root welds at tack centers. Use slight weave oscillation over tacks. No weave necessary if 1 ∕ 8 - to 5 ∕ 32 -inch root gap is maintained. Weaving may be required if root gap is less than 1 ∕ 8 inch (3 mm). Weaving is also beneficial for root welds between 7 and 6 o’clock, and between 5 and 6 o’clock. After each root pass, blend the starts and stops back to the original tack thickness. To complete the root, ensure that the weld stops and starts on the last tack, and that the root weld center is ground flat or slightly concave. Remove any slag islands. FCAW 5G (Non-rotated) MIG Welding of Thick-Walled, Carbon-steel Pipes, Proce dure for Fill and Cover Welds.— This procedure can be applied to most common pipe sizes, and should be given special consideration for 5G (non-rotated) welds on carbon steel pipe with 3 ∕ 8 inch (10 mm) wall thickness and thicker. Pipe and Weld Data Electrode for fill and cover passes = 0.035 inch (0.9 mm) diameter, 71T-1 flux-cored Gas = argon with 15–25% CO 2 Gas flow rates = 30 to 40 cubic ft/hr Set an initial wire feed rate of 350 to 450 in/min (12 to 1 o’clock position on typical wire feed unit), 135–165 amps, 25–28 volts. Alternatively, use a wire feed setting of 350 in/min (12 o’clock on wire feed unit), which should result in about 135–145 amps, 25–26 volts. If the weld pool and weld heat build up permit, increase the wire feed rate to 380 in/min (between the 12 and 1 o’clock positions), 150 amps, 27 volts. Try also a wire feed setting of 420 in/min (1 o’clock on the wire feeder), 165 amps, 28 volts. Determine the low and maximum wire feed rates to be used by examination of the weld fusion obtained in sec tioned test samples. Wire extension: Adjust contact tip so it is recessed 1 ∕ 2 inch within the nozzle to provide a total wire stick out from the contact tip of 0.7 to 1 inch (18 to 25 mm). Fill and Cover Pass Procedures for FCAW 5G Pipe Welds: Weld vertical-up. If the pipe diameter allows the fill pass to be made in two passes, start at the 7 o’clock position and weld to the 1 o’clock position. This approach is preferable to starting and finishing on the root tacks. Starting at the 7 o’clock position will ensure that optimum weld energy is achieved as the first pass welds over the initial 6 o’clock root tack location. Use the grinder to feather the first 1 inch (25 mm) of the weld start and stop of the first pass, before applying the second vertical-up weld pass. Use a slight weave action for the fill pass. Remove all flux-cored slag between weld passes. Make sure no fill pass is greater in depth than 1 ∕ 8 inch (3 mm). Use a straight weave across the root face. At the bevel edge use a slight upward motion with the gun. The motion should be no greater than the wire diam eter. Then use a slight back step for added bevel fusion and to avoid undercuts. Leave 1 ∕ 32 to 1 ∕ 16 inch (0.8 to 1.6 mm) of the groove depth to provide for the optimum cover pass profile. The bevel edge will act as a guide for the cover pass weld. If more weld fusion is required for pipe thicker than 3 ∕ 8 inch (10 mm), after the root weld is complete, preheat the pipe to between 400 and 600 ° F (200–300 ° C) before welding. Preheating is typically not necessary for a cover pass. For pipe diameters on which the welder needs more than two passes for the vertical-up welds, the recommended sequence for vertical-up welding is:
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