ARMOR PLATE. The idea of protect ing ships of war and the fronts of fortifications by means of armor plate dates from about the middle of the 19th century. In 1842 experi ments were conducted with iron plates made by riveting together plates three-eighths of an inch in thickness to a total thickness of six inches. Those plates did not successfully re sist 8-inch guns or heavy 32-pounders at 400 yards; so some modifications were introduced and further experiments made in 1850. In 1853 the French constructed floating batteries which carried four inches of iron armor.
While the European ships were, for the most part, modifications of existing types, the American ships were constructed from new designs, or railway iron and the like was at tached to existing vessels. The old Monitor and Merrimac are the first examples; but with the improvements in gun manufacture, it be came apparent that the wrought iron plates as first used could not withstand heavy gun fire. As a result a compound armor plate was de veloped in England and an all steel plate in France. In 1889 nickel was introduced in steel and a plate of great toughness and resistance was produced. This type of armor proved superior to the old and from then onward the march of progress has increased. In 1890 the Harvey process was advanced and met with great success, followed in 1895 by the Krupp process. These two processes are essentially the same in principle and represent the highest development of the compound type of armor.
Formerly this kind of armor was manu factured by taking a wrought iron plate as a backing and casting upon its face a plate of steel, the former being so heated as to produce as perfect a union as possible. This operation was based on the theory that a plate to resist the powerful energy of a projectile must have a hard face to resist penetration and a tough back to prevent shattering on impact. Diffi culty, however, was experienced with the flak ing of the steel face, so that the homogeneous nickel-steel plate really superseded this type.
Harvey Process.— Compound armor held the record against 'shell until the invention of Harveyized plates, introduced in the United States in 1891. A solid steel plate was cemented on the face and then water-hardened, a glass hard face with a tough backing being thus pro vided. This invention at once raised the resist ing power of armor by at least 50 per cent. When Harvey adopted the method of harden ing the surface of plates with a powdered cementing material, Messrs. Schneider invented
a process of cementing by means of hydro carbon gas, followed by water hardening, and this superseded the Harveyizing system. The Harvey plate was faulty in respect of the back not being sufficiently tough to resist the racking effects of the projectiles. The addition of chromium to the nickel-steel and differential treatment in hardening (variation in degree of hardness imparted to front and back), provided the necessary toughness. Nickel-chrome steel is carbonized and hardened on the outer face, gas being used for carbonizing and a water spray for hardening.
Krupp Process.—The manufacture of Krupp armor plate consists of a series of dis tinct operations requiring great care and atten tion and covers a period of from four to nine months, depending on the thickness of the plate. The composition of the plate having been determined upon, the necessary elements are melted together in an open hearth furnace. When ready for tapping, the ingot mould hav ing been prepared, the metal is run into a large ladle, from which it is poured into the mould through a gate on the outside, which connects with the interior of the mould at the bottom. The ingot is bottom poured so as to get a more perfect ingot and the large projection on the upper end the sink-head, is an aid in handling. The mould is made up of cast iron sections bolted together to facilitate the stripping of the ingot. After the ingot has cooled it is stripped, that is, it is removed from the mould. Usually 24 hours suffice for the ingot to solid ify and cool sufficiently to permit its being stripped, and then the ingot is sent to the heat ing furnaces preparatory to forging. The in got after being heated for about 24 hours is taken out of the furnace and receives its first forging. This operation is one of the most wonderful sights in a steel plant: to see a red hot mass of metal, weighing in the neighbor hood of 80 tons, being handled entirely by me chanical means with the ease that one would handle a pencil. The ingot is placed on a die under a 14,500-ton hydraulic press and there forged to within a few inches of its finished thickness. At each working of the press the metal is decreased about three inches in thick ness, the ingot being moved along until its whole length has been forged. Of course, the length and breadth are increased, the metal under such great pressure flowing evenly in all directions, and this operation is repeated until the required dimensions are obtained.