Welding two pieces of pipe together to form a pipe joint is a somewhat more challenging task than welding pieces of sheet metal, and preparation plays a crucial role in determining the quality of your product. In fact, insufficient preparation is often the culprit behind faulty welds. Preparation for a pipe joint weld basically consists of four key steps.
The first step is to prepare the edges of the two pipes for welding. The edges should be beveled at about 37 1/2 degrees so that when the two pipe ends fit together, they form a V that describes a total angle of 75 degrees. The width of the "face" at the tip of the bevel on each pipe end should be between 3/32 and 1/8 inch thick. Pipe ends are typically beveled beforehand by a shop with the proper equipment; otherwise, they must be beveled with an oxyacetylene torch. The oxyacetylene torch leaves an oxide film that can have a highly detrimental effect on the quality of the weld, so welders must remove this film with a hand grinder.
Cleaning & Fitting
The surfaces on the pipe bevel absolutely must be clean before you begin welding. Oil, scale, rust and grease can all lead to a poor weld. Vigorous wire brushing helps remove these contaminants and also get rid of any oxide film. Fitting the two pipe ends together is the next step, and this one too is crucial. Aligning the two pipe ends incorrectly could create a fatal flaw. The pipe ends can be aligned with the aid of a spacer wire, a piece of wire bent into a V-shape.
Finally, the pipe ends are ready to be tack welded. Tack welds are a series of welds evenly spaced around the pipe, each of them about 3/4 inch in length. The tack welds will hold the pipe ends in the proper position with respect to each other while the remainder of the joint is welded. It's important that they be strong. They are often ground before welding the joint, although this too depends on the nature of the job.
Once the tack welds have been made, the pipe ends can be welded together. It's especially important to prevent the molten metal from dripping, so welders generally "whip" the arc away from the puddle in the direction they are welding then back into the puddle. The movement has to be very short so the weld remains shielded by the inert gas; if the metal is no longer shielded, the metal can combine with oxygen in the surrounding air to create weak oxides in the weld. The skill required in both preparation and welding explains why pipe welders are often in high demand -- and why the certification for pipe welding is considered especially desirable.
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