Billets are rolled into skelp (which is similar to sheet-bar) of the width and thickness required for the pipe. Each strip of rolled skelp is cut into lengths of about 20 feet, and each length is made into a pipe.
Welded pipe is made either by lap or butt welding of skelp strips. Pipes of 1 1/4 inches diameter and under are butt welded, as shown in Fig. 62 and those over 1 1/4 inches diameter are lap welded as shown in Fig. 63.
The methods of welding are as follows, viz.:
For Butt Welds. - In a long flat-bottomed heating furnace with a door at one end for receiving strips and a door at the other end for removing them, a number of strips are kept side by side at various degrees of heat up to welding heat. When a strip has reached welding heat, a man reaches in at the removing door with a long pair of tongs and curls over the edges for a few inches along the end of the strip. Working quickly, he pulls the end from the furnace, points it through a tapered ring known as a bell (B, Fig. 64) and grips this end with the nippers R. Just the instant the nippers grip the strip, the hook H is caught by a moving endless chain under the bench and the strip is pulled through the bell. The bell is so shaped that it rolls the strip into cylindrical form and forces the edges together firmly enough to make the weld. The bell rests against a shoulder 8 on the bench while the strip is passing through, but when through, the bell falls into a basin of cooling water and another bell is held in place by tongs ready for another strip to start through.
Fig. 63. Butt and Lap-Welded Pipe Joints.
Fig. 64. - Equipment for Making Butt-Welded Pipe.
For Lap Welds. - The shaping and the welding are accomplished in two heatings. The first heating is merely a red heat for shaping the strip into cylindrical form, a process similar to that for butt welding. The second heating is for welding the pipe.
It is apparent that the bell of Fig. 64 will not bring the lap firmly together for a lap weld, hence after the pipe has been given a cylindrical shape, it is reheated to welding heat in another furnace and run through the welding rolls RR, Fig. 65. A view of the actual machine is shown in Fig. 66. The rolls are two grooved
Fig. 65. - Rolls and Mandrel for Making Lap-Welded Pipe, wheels which press the lapped edges together against a mandrel M placed in the end of the pipe as it enters the rolls. The mandrel is held directly between the two rolls by the long rod 8. The pressure of the rolls forces the pipe against this mandrel and makes the weld.
Fig. 66. - Machine for Lap-Welding Pipe.
After welding, both butt and lap-welded pipes are run cold through sizing rolls, which gives them correct outside diameter, and through cross rolls, which straighten them and give the surface a clean finish. The pipes then go to the inspection table, ends are sawed off, and the short pieces are crushed cold to show the effectiveness of the weld. The pipe is carefully inspected outside, is tested by hydraulic pressure of at least 600 lbs. per square inch, and, if passed, it is annealed to reduce the size of the crystals and increase the elasticity.
Pipes are threaded on the ends after annealing and are then ready to be made up in bundles for shipping. Each length of pipe is shipped with a short threaded sleeve called a coupling screwed on one end of the pipe. This is used in joining lengths of pipe together. Some grades of boiler tubes are produced by the lap-welding process.