The Bessemer process, which is largely adopted by the manufacturers of steel
throughout Europe and America, has proved a source of princely income to its inventor, who obtains a bounty of a shilling on every ton of steel made by it. In Europe alone in 1859 51⁄2 million cwts. Seventy per cent. of this quantity was the produce of British industry.
Latterly, attempts have been made to obtain steel direct from the ores. The efforts made in this direction have been greatly stimulated by the invention of the regenerating furnace of Siemens. In these furnaces, in which an intense temperature is obtained by means of the combustion of inflammable gases (chiefly consisting of carbonic oxide, hydrogen, and carburetted hydrogen), the ore after (in one process) being melted in hoppers by means of the burning gases, runs down, and is gradually dissolved in some melted pig iron placed on the hearth of the furnace. When this latter has been sufficiently diluted with the decarbonised iron the operation is complete.
Properties of Steel. The effects of temperature upon steel are remarkable, and a knowledge of them has proved of great practical utility in the manufacture of the various steel-ware articles that are so indispensable to our every-day wants and needs. If forged and soft steel is heated, and then suddenly cooled, it becomes hard, the hardness varying with the temperature and the rapidity with which this has been reduced. The higher the temperature and the more rapidly it is cooled, the greater will be its hardness. Steel, which has been heated until white-hot, and then suddenly plunged into a bath of cold mercury, acquires a hardness nearly equalling that of the diamond. That, however, which the steel gains in hardness, it loses in pliancy and elasticity, besides becoming so brittle as to be of no possible use.
Soft steel, which has been made hard by heating it to redness, and by subsequent sudden immersion in cold water, may be reconverted into soft steel by again heating it to redness and allowing it to cool suddenly.
By stopping short, however, of heating it to redness, its hardness may be proportionally modified.
Hence steel articles, varying as much in the qualities of hardness and elasticity as a lancet and watch-spring, are made either by ‘heating down’ hard steel to requisite temperature and allowing it to cool, or by ‘heating up’ soft steel to the necessary point and also letting it gradually cool. When steel is so treated it is said to be tempered or annealed. If polished steel be heated over a flame to a temperature of 430° F. its surface becomes of a very pale yellow colour; the colour passes through different shades of yellow and blue with each successive increase of temperature, until when raised to 600° F. it becomes blackish blue.
These effects are due to the formation on the surface of the steel of films of oxide of different degrees of thickness, and to the action of the light on these. They are precisely analogous to those which are caused when a ray of reflected light falls upon any other body, the surface of which is composed of thin layers, which are continually changing in thickness, such as a soap-bubble, or a thin coating of tar or oil swimming on water, and which are exemplified in Newton’s rings.
As each shade of colour is an index of the temperature of the steel, and as this determines its adaptability for various purposes, all that the workman has to do, when he requires it for any special object, is to heat it by the proper methods (such as a bath of oil, or tallow, or melted metal) until it acquires the desired colour, and then to allow it to gradually cool.
The following table, exhibiting the different melting points of steel when employed in the manufacture of different kinds of works, together with the corresponding colours, the composition of the metallic baths, &c., is from Dr Wagner’s ‘Handbook of Chemical Technology,’ edited by W. Crookes, Esq., F.R.S.