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Lap-Welded Tubes
Lap-welded tubes are made from heated skelp, which has its edges beveled (scarfed), and is passed through bending rolls. This process curls the skelp into the shape of a pipe; it is then reheated to a welding temperature and is passed through a pair of welding rolls, between which is fixed a mandrel on the end of a long rod. The roll presses the two edges of the scarfed plate over the mandrel, welding them together. Lap-welded pipe is made up to 30 inches in diameter inclusive. All tubes, after being welded, are run through sizing rolls to give them true outside dimensions; they are then straightened in cross rolls, cooled, cut to length and tested under hydraulic pressure.
Welded tubes larger than 30 inches in diameter are made from bent plates, the edges of which are raised to a welding heat by a special burner; the plates then are welded together by a machine constructed for this purpose.
Seamless Tubes
A seamless tube is one in which the walls have never been separated from the time the metal was in a molten condition to the time of the completion of the manufactured from solid steel blooms or as to produce a standard length of tube, esses for manufacturing seamless tubing, to heat the billet and to pass it through a mandrel. This process pierces a hole through the center of the billet, after which the tube is rolled successively between rolls and over a mandrel, until the proper diameter and thickness of wall is obtained. Tubes thus produced either may be hot or cold-drawn over a mandrel to final size. In cold-drawing, the tube is first pickled in a bath of dilute sulphuric acid to remove all scale; it is rinsed in water and drawn without further treat-ment. The tube must be annealed and pickled after each draw-ing.
Cold drawn tubes, from 0.375 to 1.25inches outside diameter and from No. 16 to No. 23 B. W. G. are generally known as bicycle tubing. Tubes, 1 to 4 inches in diameter and No. 13 to No. 6 B. W. G.,are used for boiler purposes. Tubes of other thicknesses than those enumerated, generally are termed mechan-ical tubes and are used for parts of many classes of machinery such as bushings, hollow shafts, spindles, axles, collars, rings, ferrules, pump barrels, etc.
Seamless tubes over 5.5 inches in diameter and up to about 20 inches are, as a rule, of shorter length and are manufactured from plates. The process consists in first pressing the form of a cup from a heated, circular plate which, after reheating, is forced through a succession of dies by a punch or mandrel insert-ed in the cup, until the thickness of the walls of the now hollow vessel have been reduced materially. Further reduction in thick-ness is obtained by subsequent drawing similar to that of seam-less tubes. Cylinders for compressed gases, which are subjected to great pressure, are made in this manner, the open end being swaged down and is provided with a suitable connection. Tube works operate, in addition to their tube mills, large departments for finishing the pipe after it leaves the testing bench. This work consists in threading, upsetting, bending, flanging, etc.
As pipes are commercially not over 20 feet in length, they must be coupled, in other words, they must be provided with means for connecting them to form pipe lines. This is accom-plished by various forms of couplings. These either may be screwed couplings or flanged connections, both types being fabri-cated in departments containing special machinery for this pur-pose. Most gas and water pipe is galvanized and this process is carried on in a separate shop with special mechanical means for galvanizing.
Commercial pipe is sold at a list price per foot or bundle and not by weight, except hot-drawn seamless tubing above 5.5 inches outside diameter, which is sold at a net price per pound. Stand-ard weight is shipped with threads and couplings, while standard extra strong pipe is shipped with plain ends, unless otherwise ordered. Pipe will vary 5 per cent above and 5 per cent below the weight per foot ordered and stock lengths are 16 to 18 and 20 feet. In the United States over 3,500,000 tons of pipe and tubular goods are produced each year.