Pipe Welding Process
Pipe Welding : General
The most commonly used processes for joining pipe are the manual oxyacetylene process and manual shielded metal-arc process. Automatic and semiautomatic submerged arc, inert gas metal-arc, and atomic hydrogen welding are also used. particularly in shop operations. The manual shielded metal-arc process may be used for welding all metals used in piping systems, whereas manual oxyacetylene welding is generally limited to small size piping or to welding operations where clearances around the joints are small. The equipment required for the oxyacetylene process is also much less expensive and more portable than that required for shielded metal-arc welding.
Shielded Metal-Arc Pipe Welding
(1) The shielded metal-arc process can be used for welding pipe materials such as aluminum, magnesium, and high chromium-nickel alloys that are difficult to weld by other processes. In shielded metal-arc welding, the number of passes required for welding ferrous metal piping varies with the pipe thickness, the welding position, the size of the electrode, and the welding current used.
(2) The number of passes required for welding low alloy and low carbon steel pipe depends on the thickness of the pipe, the welding position, the size of the electrode, and the current used but, in general, is approximately one pass for each 1/8 in. (3.2 mm) of pipe thickness. When welding in the horizontal or rolled position, the number of layers is usually increased 25 to 30 percent. Smaller electrodes are used to lessen the heat concentration and to ensure complete grain refinement of the weld metal.
(3) The electrodes used vary from 1/8 to 5/32 in. (3.2 to 4.0 mm) diameter for the first pass, 5/32 in. (4.0 mm) diameter for the intermediate passes, and up to 3/16 in. (4.8 mm) for the top passes and reinforcement.
Manual Oxyacetylene Pipe Welding
The number of passes required for pipe welding with the oxyacetylene flame depends on the thickness of the pipe, the position of the pipe, and the size of the welding rod used. The thickness of the deposited layer is somewhat more than that deposited by the shielded metal-arc process.
Direction of Welding Pipe
(1) In manual shielded metal-arc welding, as much welding as possible is done in the flat or downhand position using suitable power driven equipment for rotating the pipe at a speed consistent with the speed of welding. When the pipe is in a fixed horizontal position, the weld is usually made from the bottom upward. With thin or medium thickness pipe, the welding is done downward. More metal is deposited when welding upward. Complete grain refinement is easier to achieve, and welding downward requires a much higher degree of manual skill.
(2) When the pipe is in a fixed vertical position, it is customary to deposit the filler metal in a series of overlapping string beads, using 1/8 in. (3.2 mm) maximum electrodes, and allowing 25 to 30 beads per square inch of weld area.
(3) When welding by the oxyacetylene process, the directions of welding as described above will, in general, apply. Backhand welding is used when welding downward, and forehand welding is used when welding upward.