Pipe welding positions are dictated by the job. Pipe welds are made under many different requirements and in different welding situations. In general, the position is fixed, but in same cases can be rolled for flat-position work. Positions and procedures for welding pipe are outlined below.
Horizontal pipe rolled Weld
(1) Align the joint and tack weld or hold in position with steel bridge clamps with the pipe mounted on suitable rollers (fig. 6-33). Start welding at point C, figure 6-33, progressing upward to point B. When point B is reached, rotate the pipe clockwise until the stopping point of the weld is at point C and again weld upward to point B. When the pipe is being rotated, the torch should be held between points B and C and the pipe rotated past it.
(2) The position of the torch at point A (fig. 6-33) is similar to that for a vertical weld. As point B is approached, the weld assumes a nearly flat position and the angles of application of the torch and rod are altered slightly to compensate for this change.
(3) The weld should be stopped just before the root of the starting point so that a small opening remains. The starting point is then reheated, so that the area surrounding the junction point is at a uniform temperature. This will insure a complete fusion of the advancing weld with the starting point.
(4) If the side wall of the pipe is more than 1/4 in. (0.64 cm) in thickness, a multipass weld should be made.
Horizontal Pipe Fixed Position Weld.
(1) After tack welding, the pipe is set up so that the tack welds are oriented approximately as shown in figure 6-34. After welding has been started, the pipe must not be moved in any direction.
(2) When welding in the horizontal fixed position, the pipe is welded in four steps as described below.
Step 1. Starting at the bottom or 6 o’clock position, weld upward to the 3 o’clock position.
Step 2. Starting back at the bottom, weld upward to the 9 o’clock position.
Step 3. Starting back at the 3 o’clock position, weld to the top.
Step 4. Starting back at the 9 o’clock position, weld upward to the top overlapping the bead.
(3) When welding downward, the weld is made in two stages. Start at the top (fig. 6-35) and work down one side (1, fig. 6-35) to the bottom, then return to the top and work down the other side (2, fig. 6-35) to join with the previous weld at the bottom. The welding downward method is particularly effective with arc welding, since the higher temperature of the electric arc makes possible the use of greater welding speeds. With arc welding, the speed is approximately three times that of the upward welding method.
(4) Welding by the backhand method is used for joints in low carbon or low alloy steel piping that can be rolled or are in horizontal position. One pass is used for wall thicknesses not exceeding 3/8 in. (0.95 cm), two passes for wall thicknesses 3/8 to 5/8 in. (0.95 to 1.59 cm), three passes for wall thicknesses 5/8 to 7/8 in. (1.59 to 2.22 cm), and four passes for wall thicknesses 7/8 to 1-1/8 in. (2.22 to 2.87 cm).
Vertical Pipe Fixed Position Weld. Pipe in this position, wherein the joint is horizontal, is most frequently welded by the backhand method (fig. 6-36). The weld is started at the tack and carried continuously around the pipe.
Multipass Arc Welding.
Root beads. If a lineup clamp is used, the root bead (view A, fig. 6-37) is started at the bottom of the groove while the clamp is in position. When no backing ring is used, care should be taken to build up a slight bead on the inside of the pipe. If a backing ring is used, the root bead should be carefully fused to it. As much root bead as the bars of the lineup clamp will permit should be applied before the clamp is removed. Complete the bead after the clamp is removed.
Care should be taken that the filler beads (view B, fig. 6-37) are fused into the root bead, in order to remove any undercut causal by the deposition of the root bead. One or more filler beads around the pipe usually will be required.
The finish beads (view C, fig. 6-37) are applied over the filler beads to complete the joint. Usually, this is a weave bead about 5/8 in. (1.59 cm) wide and approximately 1/16 in. (0.16 cm) above the outside surface of the pipe when complete. The finished weld is shown in view D, figure 6-37.
Aluminum pipe welding.
For aluminum pipe, special joint details have been developed and are normally associated with combination-type procedures. A backing ring is not used in most cases. The rectangular backing ring is rarely used when fluids are transmitted through the piping system. It may be used for structural applications in which pipe and tubular members are used to transmit loads rather than materials.