Pipe welding is about 100 years old, and at this writing the manufacture of welded wrought-iron pipe consumes a large percentage of all the wrought iron made. The first pipe was heated over a coke fire and lap-welded in sections. After the Napoleonic wars gun barrels were a drug on the market and came to be used for water piping. This supply seemed to stimulate the demand, and soon after important inventions began to assist and simplify the original primitive methods. James Russell, who substituted the butt for the lap-welded pipe in 1825, may be called the father of the pipe industry.

Wrought-iron pipes are made in a number of ways. The original lap-weld is still used for its strength. Most pipe is butt-welded from long scarfs of iron, heated in a gas or coke oven, bent round, reheated, and then welded by being drawn over a mandrel and through a die of slightly lower caliber than the pipe. In this way the edges are squeezed together on the outside, while the mandrel pressing on the inside completes the weld. A recent invention is pipe made from a spirally wound and welded strip of iron. Weldless drawn-steel tubing is also finding a market, but so far welded pipe has proved to be cheaper. The process is now a continuous one; and electric welded pipe is now on the market.

Welded pipe is made in sizes of from 1/8 inch to about 30 inches internal diameter. Larger than this, it is generally riveted. The iron must be of very good quality, quite pure, and low in sulphur and carbon. Fifty thousand pounds per square inch tensile strength is as high as it is safe to try to use, as stronger irons than this give weaker welds. High-carbon irons and steels should never be used for pipe, though their initial strength is great. Such metal welds poorly and may easily pull apart at the weld under great pressure.