The largest machines employed in the steel industry are those used for the manufacture of armor and big guns. A modern large high-powered gun is not a single solid casting or forging, but is made up of a series of steel tubes that are shrunk one upon another so that the inner tube is compressed. The reason for this is that the explosives used are so powerful that they would expand the inner tube or lining of the gun beyond its elastic limit and in that way enlarge the bore. By having it compressed to start with it can expand farther without exceeding the elastic limit. This expansion takes place so suddenly that the lining rebounds or returns to its original dimensions before the outer tubes have felt the full pressure and they too are thus prevented from being expanded too far. In some cases the compression is effected by winding the gun with a heavy wire of rectangular cross section.
Steel for guns and armor is made in the open-hearth furnace where the quality of the metal may be regulated to a nicety. Gun tubes are cast in vertical molds and during the cooling of the ingot it is subjected to pressure so as to prevent segregation and the forming of “pipes.” Pipes are cavities that are liable to form in the center of the ingot due to contraction during cooling. Steel, as we have learned, is not pure iron, but an alloy, and the various constituents have different temperatures of solidifying, consequently they exhibit a tendency to segregate. It is to overcome such tendencies that a so-called “fluid” compressor is used. This is virtually a hydraulic press with a plunger that bears down on the fluid metal as it is solidifying. Modern big guns are enormously large. A sixteen-inch 50-caliber gun, for instance, is nearly seventy feet long, consequently the ingot must be even longer than this and the fluid compressor for so large a piece must be correspondingly powerful. After the ingot has been cast and cooled, the ends are cut off and it is bored to form a tube. Then it is placed in a furnace and raised to a white heat, after which a bar or mandrel is inserted in the bore and the tube is placed under the hydraulic forge press. This is a very powerful machine with an immense hammer that is actuated by hydraulic pressure. The stroke of the hammer is carefully regulated so that the forging as it is turned in the forge is subjected to equal blows. In the forge the tube is roughly formed to the dimensions it is eventually to have when finished. The process of forging subjects the metal to strains which must be relieved and so the tube has to go to the annealing oven where it is raised to a temperature which destroys crystallization. In this oven it is allowed to cool very slowly, letting the molecules of the metal adjust and rearrange themselves. When the temperature of the tube has been lowered to a certain point it is taken out and plunged into a bath of oil. This sudden cooling tempers the metal, giving it a high degree of elasticity and tensile strength. Again the tube must be annealed to relieve it of any strains occasioned by the tempering, and then it goes to the shop to receive its finish boring and turning.