Blistered steel thus made is used only for the coarsest purposes. Its texture is greatly improved by being formed into smaller bars under the tilt ham mer, in which state it is known by the name of tilted steel.
In order to improve the steel still farther, the bars are broken into short pieces, and these' being put up in small parcels, are welded together in a furnace, and drawn down into bars, which, by repeated weld ing and tilting, acquire a compactness and toughness, which fit it particularly for swords and other large ar ticles of cutlery. This steel is called shear steel, or German steel, from its having been prepared in great perfection in Germany.
3. Cast steel. The finest kind of steel known by the name of cast steel was first made by Mr. Hunts man of Sheffield in 1750. At first he kept his process a secret, but it is now well known and universally practised. Blistered steel, broken into small pieces, is mixed with a certain proportion of pounded glass and powdered charcoal. It is then melted in a cruci ble, and cast into ingots, which, by gentle heating, under the influence of the tilt hammer, are wrought into bars. By this process it becomes more brittle and fusible than blistered steel; but though it is inca pable of being welded with either iron or steel, it has acquired an uniformity of texture, and a closeness of grain, which fit it for the finest articles of cutlery.
According to M. Clouet (Journal des Mines, No. 49, An. vii. 3), cast steel may be formed by fusing thirty parts of iron, one of charcoal, and one of pounded glass, or even by melting iron in a crucible, when sur rounded with equal parts of chalk and clay, and keep ing the whole a sufficient time at a white heat. There is reason to think that the pounded glass is not essen tial to cast steel; and while some are of opinion that this substance differs from common steel only in hav ing a greater quantity of carbon, others maintain that it actually contains less carbon, and therefore that the difference must have another origin. In rich cast iron the carbon is supposed by some to exist in a mecha nical state, while in steel it is chemically united with the iron.
Various methods of working cast steel have been published, among which that of Mr. Alushet deserves to be studied. A method of a novel character has been recently discovered by our ingenious countryman, Charles Mackintosh, Esq. Glasgow. The principle of the method is to impregnate iron at a high tempera ture with carbon in a gaseous form. The gas which he employs as the most economical and convenient for this purpose is that obtained from the distillation of coal, or the common coal gas. The iron to be con verted into steel is enclosed in a crucible, or melting pot, of the usual materials, and placed in the furnace; and when it is raised to a very high degree of tempe rature, a jet or current of the gas is thrown into the crucible through a tube and aperture provided for that purpose. In the cover of the crucible another aperture is made to permit the escape of that part of the gas which is not absorbed by the iron." A series of very valuable papers on iron and steel by Mr. Mushet has been published in Dr. Tilloch's Philosophical Magazine. Ile has shown that the hardness of iron increases with the carbon which it contains, till the carbon amounts to one-sixtieth of the iron. At this point the hardness is a maximum, the metal acquires the lustre and colour of silver, loses its granulated appearance, and assumes a crys tallized form. If more than one-sixtieth of carbon be added, the hardness of the compound diminishes in proportion to its quantity.
The following table by Mr. Mushet shows the proportions ofcarbon combine with iron during the formation of the different carburcts.
laying the steel articles on a clean coal fire, or on a heated bar till they receive the degree of heat which is required. They are then cooled by immersion in water. The degree of heat, the colour by which it is indicated, and the degrees of temper necessary for different instruments arc shown in the following table:— The following arc the specific gravities of steel in different states.