Steel is a compound of iron and carbon, in which the proportion of the latter seldom exceeds 1.7 per cent. It may be distinguished from iron by its fine grain, its susceptibility of hardening by immersing it when hot in cold water, and with certainty by the action of diluted nitric acid, which leaves a black spot on steel, and on iron a spot which is lighter colored in proportion as the iron contains less carbon. For the construction of cannon, steel may be divided into high and low steel, the difference being that the former contains more carbon than the latter. High steel is very hard and has great ultimate tenacity. It has but little extensibility within or without the elastic limit, and is therefore too brittle for use in cannon, unless used in such large masses that the elastic limit will not be exceeded by the explosive force of the powder. It melts at a lower temperature than wrought iron and is difficult to weld, as its welding temperature is but little less than that at which it melts. Low steel is often known as “mild steel,” “soft steel,” “homogeneous metal,” and “homogeneous iron,” and is made by fusing wrought iron with carbon in a crucible; after which it is cast into an ingot and worked under a hammer. As it contains less carbon than high steel, it has greater specific gravity. It can be welded without difficulty, although overheating injures it. It more nearly resembles wrought iron in all its properties, although it has much greater hardness and ultimate tenacity, and a lower range of ductility depending on its proportion of carbon. It has less extensibility within the elastic limit than high steel, but greater beyond it, or in other words, greater ductility. Its great advantage over wrought iron for general purposes is that it can be melted at a practicable heat, and run into large masses possessing soundness and tenacity. Its advantages for cannon are greater elasticity, tenacity, and hardness. Its tenacity when suitable for cannon is three times as much as cast gun iron, and one-half more than the best wrought iron. The principal varieties of steel are:
Natural Steel.—This is made principally in Germany, and is used for making files and other tools. It is obtained by reducing the rich and pure kinds of iron ore with charcoal, and re-fusing the cast iron so as to bring it to a malleable state. The India steel, or Wootz, is a natural steel containing a small proportion of other metals.
Blistered Steel.—This is prepared by exposing alternate layers of bar-iron and charcoal in a close furnace for several days. When taken out the bars are brittle in quality and crystalline in appearance. The purpose for which the steel is to be used determines the degree of carbonization. The best qualities of iron (Russian and Swedish) are used for the finest kind of steel.
Tilted Steel.—This is blistered steel moderately heated and subjected to the action of a tilt-hammer, by which means its density and tenacity are increased.
Shear Steel.—A blistered or natural steel refined by piling thin bars into fagots, and then rolling or hammering them into bars, after they have been brought to a welding heat in a reverberatory furnace. The quality is improved by a repetition of this process, and the steel is known accordingly by the names, half shear, single shear, double shear, etc.
Cast Steel.—This is made by breaking blistered steel into small pieces, and melting it in close crucibles from which it is poured into iron molds. The ingot is then reduced to a bar by hammering or rolling with great care. Cast steel is the finest kind of steel, and is best adapted for most purposes; it is known by a very fine, even, and close grain, and a silvery homogeneous fracture. The most remarkable specimen of cast steel for tenacity which is on record was manufactured at Pittsburgh, Pa. It was tested at the Washington Navy-Yard, and found to sustain 242,000 pounds to the square inch. The strength of cast steel usually runs from 70 to 140,000 pounds.
Bessemer Steel.—This steel is produced by forcing air into melted iron, by means of which the carbon and silicon of the crude cast iron is oxidized. The essential difference between this process and the ordinary puddling is mechanical, and consists in the intense and violent stirring of the Bessemerized iron, to which alone is due the production and maintenance of a temperature, without any other fuel than the carbon and silicon contained, that keeps the metal fluid so that it can be cast into homogeneous malleable ingots. When decarburation has been carried far enough, the current of air is stopped, and a small quantity of white pig-iron containing a large amount of manganese is dropped into the liquid metal. No very large cannon have yet been made wholly of Bessemer steel, but several small ones have, which have shown great endurance. Experiments at the Woolwich Arsenal have shown that the tenacity of this steel is more than doubled by hammering.
Siemens-Martin.—In this process the ingredients of cast steel are melted together on the open hearth of a reverberatory furnace of special construction, and a certain proportion of manganese necessary to make a sound and practically malleable steel added. This steel is, however, little used in gun construction.
Semi-Steel.—If in the process of puddling or decarbonizing cast iron the process be stopped at a particular time, determined by indications given by the metal to an experienced eye, an iron is obtained of greater hardness and strength than ordinary iron, to which the name of semi-steel, or puddled steel, has been given. The principal difficulty in its manufacture is that of obtaining uniformity in the product, homogeneity and solidity throughout the entire mass. It is much improved by reheating and hammering under a heavy hammer; but it has not been found a reliable material for even cannon of small caliber. The celebrated guns made by Mr. Krupp of Germany are of cast steel, made from puddled steel, and of peculiar character, combining great tensile strength with the property of stretching to a great extent without breaking. Sir Joseph Whitworth improves the qualities of steel for his more recent guns by casting it under hydraulic pressure.
Chrome Steel.—An alloy of iron and chromium, which is not steel in the ordinary sense, but which possesses many of its characteristics. The tensile strength and resistance to crushing is much higher than ordinary cast steel. This material has been largely used in bridge-building, but has not yet been applied to cannon-making.