STEEL

(Germ, stahl.) This most useful and curious substance is a compound of iron and carbon: their relative proportions vary in steel of different qualities; but in that used for ordinary purposes, the carbon rarely exceeds 2 per cent., and is generally below it. Certain kinds of iron are preferred to others in this manufacture; but this relates entirely to its purity, which is the e ential requisite. Steel is made by a proce called cementation, which consists in filling a proper furnace with alternate strata of bars of the purest malleable iron and powdered charcoal: atmospheric air is carefully excluded from the boxes containing the bars, and the whole kept for several days at a red heat. Ey this proce carbon, probably in the state of vapour, penetrates, and coi ■ s it i. ivi ■ the iron, the texture of which, originally fibrous, becomes granular, and its surfica acquires a blistered character. The malleability of steel falls far short of that of iron; but it is harder, and more sonorous and elastic, and susceptible of a higher polish, and has le tendency to rust. At a red heat it admits of hammering into various forms, and of being welded or united by the blows of the hammer to another piece of steel or iron. Blistered steel, rolled or beaten down into bars, forms shear steel; and if melted, cast into ingots, and again rolled out into bars, it forms cast steel, which, when well prepared, has the great recommendation of perfect uniformity of texture.and a finer and closer grain. The peculiarity of steel, upon which its high value in the arts in great measure depends, is its property of becoming, by sud den quenching in water, when at a bright red heat, ex tremely hard, and of being again softened down to any re quisite degree by the am plication of a certain temperature, which may be indicated by a thermometer, commencing at about 300°, and terminating at a dull red heat. This proce is often called tempering; and the workman is some times guided in the extent to which it is carried by the colour of the polished surface of the heated steel, w hich is at first rendered of a pale straw tint, then yellow, brown ish, purple, and blue, as the temperature rises from one extreme to the other. The latter colour indicates extreme softne and elasticity, such as belongs to watch springs, some sword blades, .; pale straw indicates great hardne ,asfor razor blades; yellow is somewhat softer, and shows a fit temper for penknives; and the incipient blues announce the temper that belongs to coarser cutting instruments, and to table knives, any of which, made of hard steel, would soon get spoiled and notched, but the edges of which, when duly tempered, resist breaking on the one hand, and bending on the other. When a large mi of steel is hardened by quenching in water, it undergoes a certain degree of expansion, so that the specific gravity of hard steel is somewhat le than that of soft. It has been I attempted to improve the quality of steel for certain pur poses by adding to it small portions of other metals: hence the term silver steel, .; but none of these alloys have or. the whole proved superior to well made common steel. There is a kind of steel imported from India, known under the name of wootz, the cutting instruments of which are celebrated for the toughne nnd durability of their edge. It appears probable that its peculiarities depend upon the presence of a little aluminum. When the surface of some kinds of steel is washed over with a weak acid, it acquire! a peculiar mottled or damasked appearance, as If its texture consisted of an intimate mixture of two different kinds of steel, or of fine fibres of steel and iron. Steel, alloyed with a little nickel, often puts on this appearance; but thesp [s. 1192]