Steel is a product, or, perhaps, more accurately, a species of iron, refined of some of its grosser elements, intermediate in the amount of its carbon between wrought and cast-iron, and tempered to a hardness which enables it to take a cutting edge, a toughness sufficient to bear a heavy strain, an elasticity which adapts it for springs and other articles requiring resiliency, as well as a susceptibility to polish, which makes it useful for ornamental and artistic purposes.
Pig iron, which was the original basis for the manufacture of all iron and steel, is made by the reduction of iron ore in large blast furnaces, which are filled with layers of ore, charcoal or coke, and flux. By the agency of this the iron is melted out and falls to the bottom of the furnaces, is drawn out through openings for that purpose into canals, and finally into molds, where it solidifies into what are termed pigs. Prior to the invention of Sir Henry Bessemer, steel was manufactured from a pig-iron base by a tedious and expensive process of refining in furnaces adapted to that purpose. The process was so costly that steel was little used except for cutlery and comparatively small articles. and was practically unknown in the construction of bridges, rails, buildings, and other structures where large quantities of iron were required.
In 1856 Bessemer discovered a process of purifying iron without the use of fuel, by blowing air through a molten mass of pig iron placed in a refractory lined vessel called a converter, whereby the silicon, carbon, and other nonmetallic constituents were consumed, and the iron thus fitted for immediate conversion into steel by recarbonization. The present process of recarbonization was a supplementary invention of Mushet, who accomplished it by the introduction of ferro-manganese, or spiegel-eisen, while the iron in a molten state was issuing from the converter, in which it had been purified, and was thus converted into steel. The process of running molten metal from blast furnaces into pigs and remelting them in cupola furnaces for use in a converter was termed the indirect process, and was generally used prior to the Jones invention.
His process is thus described by Bessemer in his patent of 1869: 'The most important of these operations consist in melting the pig metal, transferring it in the molten state to the converting vessel, blowing air through it, and converting it into a malleable metal, mixing the metal so converted with a certain quantity of fluid manganesian pig iron, pouring the mixed metals into a casting ladle, and running it from thence through a suitable valve into ingots or other molds, and the removal therefrom of the ingots or other cast masses when solidified.' This invention of Bessemer, simple as it appears, may be said not only to have revolutionized the manufacture of steel, and to have introduced it into large constructions where it had never been seen before, but to have created for it uses to which ordinary iron had been but illy adapted.
While in the Bessemer specification of 1856 it is said 'the iron to be used for the purposes of my present invention may be conveyed by a gutter in a fluid state direct from the smelting furnace where it has been obtained from the ore,' without the expense and delay incident to the intermediate cupola process, practical experience, in this country at least, showed that the refining of iron without first casting it into pigs, selecting or mixing the pigs, and remelting them, was attended with such expense that the entire abandonment of the practice was seriously considered. The difficulty was in the material variations between different portions of the same cast, and even different parts of the same pig,-an irregularity which was increased when the metal was drawn from several furnaces. There was added to this frequent changes in the character and composition of the ore, coke, and limestone flux with which the furnace was charged. The consequence was that the nonuniform chemical composition of the metal from the molten blast furnaces yielded products of steel, such as rails and beams, which were not only irregular chemically, but of irregular and uncertain final condition,-some sound, others of imperfect strength and full of flaws.