CONSTRUCTION OF BUILT-UP STEEL CANNON. The material of which modern guns are made is 'low steel' eontaining about one-half of one per cent. of carbon. In the United States it is made by the open-hearth process. (See IRON AND STEEL, MET, L:1.1;6y or.) In Germany crucible steel is used. Bessemer steel is practically de barred. lweanse it is trot pure or sound enough for use in cannon. The steel is east into ingots roughly approximating in shape the pieces to be made Inn)] then], but larger. United States Army specifications require about 3(1 pet cent. at top and 0 per cent. at bottom of the ingot to be discarded. The metal is generally compressed by Whitworth's fluid compresshin.
After the ingot is cast, it is cooled very slowly to avoid strains, and then the surplus portions are cut off and specimens taken for chemical analysis and tensile test. The balance of the ingot is now bored in a lathe, then heated and forged on a mandrel.
After forging, the specifications require that the strains due to this operation be removed by annealing. The forging is then turned and bored in a lathe to nearly its finished size and speci mens taken for test. It is then oil-tempered to give toughness. hut this process is alit to induce strains in the metal, to remove which the forging is generally required to be again annealed. Tubes are bored in a lathe with a long boring bar fed into the tube, which rotates while the bar remains stationary. The tool carried at the end of the bar is a 'hog-nose' or semi cylinder of cast iron, which, by its pressure on the bore, already cut, steadies and supports a cutting tool at its forward end. The bore thus produced is straight (within a small fraction of an inch in the whole length of the tithe). but rough, and is smoothed by a reamer, a cylinder of wood fitting the bore tightly and carrying long side cutters. During or after the boring, the outside of the tube is turned to diameters greater by the calculated shrinkages (about 0.0003 of the diameter) than the inner diameter of the hoop to encircle it. (The operations upon jackets and hoops are in general similar to those upon tubes.) When ready for assemblage the hoop or jacket to be put on is heated in an oven (generally in a vertical position, in an oil furnace, to 500 to 800 degrees F.), and expanded to 0.03 inch
to 0.08 inch larger diameter than the part it is to surround. It is then lowered over the latter, which stands in a vertical position, until it abuts against a shoulder on the tube. In order that it shall not in cooling grip the gun higher and draw away from this shoulder. water is poured on to the hot hoop near the joint to cool it there first, and then the water ring is grad ually moved up to produce progressive cemling, through the whole length of the hoop. The gull, as it begins to be called as soon as any pieces have been assembled on the tube, then genes to a lathe to have the surface turned for the next hoops to be shrunk on.
The sequence of assemblage of the jacket and the hoops is dependent upon the system used to transfer the longitudinal forces to the trunnion. In mortars generally, and in some guns. the tube is run through the jacket from the rear until its shoulder abuts against one in the jacket. In most 8-inch, 10-inch, and 12-inch United States guns the 'C' hoops (covering the chase or muzzle portion) are first put on from the front of the tube, then the jacket is put on from the rear. This leaves both jacket and hoops held from sliding otT the ends of the tube by their grip only. . The grip is very great, but is not regarded as sufficient security. and an 'L' hoop, o• locking hoop, is put on, having a portion of its bore near the centre of greater diameter than the bores at each end. This portion corresponds to a raised shoulder on the forward end of the jacket and one on the rear of the first 'C' hoop, thus binding them together. The 'I; hoop is ex panded sufficiently to allow its smaller diameters to pass over these raised shoulders in assem blage. The trunnions are forged solid with the trunnion hoop, which is assembled upon the gun by shrinkage like any other hoop. When all hoops have been assembled, the gun, after a careful inspection for dimensions and straight is finish-bored to its proper calibre, turned to the prescribed shape outside, rifled, and the powder chamber, forcing slope, and breech recess finished.