Bending and The bending is done in hydraulic presses. Messrs. Vickers have two of 8,000 tons power, which will bend plates up to 21 feet long and 11 feet wide. The pressure exerted is three tons to each square inch of the rams. Each weighs 600 tons. The bending of the thickest plates under the pow erful persuasion of these presses is easily and quickly accomplished. In the next stage the surfaces are planed on machines which will carry a plate weighing 30 tons on a moving table, weighing as much, and will reciprocate it under the cutting tools, which cut in both di rections. The plate moves at a rate of about 11 feet a minute. In other types of machines the plate is fixed in a pit below and the cutting tools are traversed over it. It is necessary to plane the surfaces of the plates smoothly, for, although they have been rolled and are smooth, the thicknesses are not so uniform as they should be to enable them to be fitted perfectly in their places. Afterward the very rough and irregular edges are planed in machines of a totally different design. Many plates must have their edges beveled and provision is made for dealing with these by means of hinged tool holders. When a large quantity of metal has to be removed the edges are cut or parted with circular saws.
.Hardening.—The hardening process com prises two stages— the moderate hardening of the plate right through, which might be more correctly termed tempering, and the intense hardening of the outer face. The first is done with oil, the second with water. In the first stage the plate is reheated in a furnace to an exact temperature and dropped suddenly into a bath of cottonseed oil, large enough to tem per the entire plate without becoming itself sensibly overheated. The result is that the plate is hardened or toughened throughout without being rendered brittle. It is both stronger and tougher than it was before im mersion, but it would not resist a modern soft nosed shell. Afterward the surface, which has been saturated with carbon to a depth of from two and a half to three inches, is ren dered of excessive hardness by a chilling pro cess. A series of jets of cold water are di rected under pressure against the highly car bonized surface, the plate having been heated first. This is continued during two or three
hours, by which time the surface to a depth as far as the carbon has penetrated has become of glasslike hardness.
The finished plate is made ready for the in spector who goes carefully over the whole carbonized face, testing every square foot for hardness. The punch must be dulled and the surface of the plate show practically no mark. If this test is satisfactory the coupon, which has been previously cut in the machine shop, is broken off to show the structure of the plate by its fracture. Then follows the drill test to show the depth of chill and if the plate passes this test, bars are taken to show its physical characteristics, that is, tensile strength, elastic limit and extension.
Final Test.— After the above tests are completed the ballistic plate is picked—that is, a plate to be fired at and which represents a group of plates. The armor for a battleship is divided into groups of from 400 to 600 tons, as determined by the department. This test is the most important and on its acceptance or rejection hangs the fate of the whole group of armor; as a result the inspector usually picks that plate of a group, which in his opin ion is the least likely to pass. Of course, he has a record of all the plates, and going over these most carefully, he chooses the one which for some slight reason or other is not as per fect as the others, assuming that if the plate in question can pass the severe test required of it, the other plates of the group most assuredly can.
The plate having been decided on, it is sent to the proving ground for test. Here it is attached to a structure, braced from the rear and bolted in the same manner as armor on shipboard. When in readiness three shots are fired at it, the actual thickness of the plate determining the gun to be used and the initial velocity. Approximately a gun of the same calibre in inches as the armor in thickness is used and no projectile or fragment thereof shall get entirely through the plate, nor shall any through crack develop to an edge of the plate or to another impact. The plate having successfully passed test, thegroup is accepted and the remaining plates are finished machined; a replacing plate is substituted for the ballistic one.