The steel produced in the open-hearth furnaces is poured into ingot molds. These are approximately rectangular in section and slightly larger at the bottom than at the top. They are open at the top and bottom, but at the bottom rest upon a base plate. As soon as the steel has hardened the plunger of a stripping machine holds down the glowing ingot while a pair of hooks lift off the mold, leaving the ingot resting on the base plate.
ROLLING INGOTS INTO RAILS
In the manufacture of railroad rails the ingots are placed on a traveling “table” consisting of a series of rapidly turning rollers. These carry the ingot to a pair of large steel rolls between which it passes. The rolls compress the ingot slightly and it is automatically turned over and passed through a second pair of rolls. After passing through four “stands” of rolls, turning over between each stand, it is considerably reduced in cross-sectional area and correspondingly lengthened. It is now termed a “bloom.” The bloom goes through a series of rollers which gradually reduce its section until it is some forty feet long. Then it is cut in two and each section passes through other rolls, until finally it is reduced to the required rail section. Each section is then about a hundred and twenty feet long and the glowing writhing rail passes on to the saws where it is cut into ten-yard lengths. A similar process is employed in rolling other forms of rails and in making steel plates and sheets.
STEEL FOR BIG GUNS
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.
SQUEEZING OUT THE “PIPES”
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.
The process as here briefly described seems simple enough, but we must not forget the enormous size of these pieces and their tremendous weight. They would be difficult enough to handle when cold, but much of the work is done while the pieces are at a white heat so that the men who control and operate the machinery that handles the big forgings must keep their distance. The casting, annealing, and tempering operations are performed with the piece in vertical position, and lofty machines and cranes are required to deal with these tall castings. A visit to a plant which manufactures big guns is bound to impress the visitor with awe and give him increased respect for the men who are able to handle such huge masses of metal and also for the men who have conceived and developed such gigantic operations.