Besides the four main processes various operations are em ployed in the treatment and production of steel for special pur poses. Of particular interest from the comparative point of view are the processes for decarburising and carburising, by which the carbon content of the iron alloy can be changed without actual melting. The treatment of certain types of cast-iron by pro longed heating, generally in oxidising surroundings, leads to a profound change in the condition of the carbon. In the "white" iron as cast for this purpose the carbon is present as the com pound —known as "cementite." In this condition the iron is very hard and brittle. If, however, a "white" iron of suitable composition, which has solidified without separation of graphite, is subjected to prolonged heating in oxidising surroundings, part of the carbon is oxidised and removed entirely, while the re mainder is deposited in the form of nodules of finely-divided car bon. Unlike the graphite flakes of "grey" cast iron, these do not seriously interfere with the strength or ductility of the metal, and the iron is thus rendered "malleable." It is considerably inferior to true wrought iron or steel, since it still contains the finely divided graphite and the other impurities, but it is suf ficiently strong and ductile for many purposes for which untreated iron castings would be too weak and brittle.
Cementation.—The converse of this process is that of car burising or "cementation." If bars of wrought iron or very low carbon steel are packed in a carbonaceous material, such as a mixture of charcoal and barium carbonate, and are heated for some time at temperatures near 9oo° C the iron absorbs carbon. If this process is carried to a moderate extent, the iron or mild steel article is left at the end with an outer layer or case of high carbon steel. This, unlike the soft low-carbon core, can be hard ened by quenching and in this way "case-hardened" articles are produced. Instead of a plain iron or low-carbon steel, a special type of nickel steel can be employed and this allows the final heat treatment to be much simplified. A surface hardening process depending on the formation of iron nitride has recently been worked out in Germany. Prolonged heating in a nitrogenous atmos phere at moderate temperatures is employed, and no final quench ing or other heat-treatment is required, the core of the steel retaining its original high quality while no distortion can occur. The process is only applicable to steels of special composition, in which a certain content of aluminium is important.
The simple carbon cementation process has, however, another use which was formerly very important. Before the modern processes of steel-making were developed, high-carbon steel, such as is used for tools and weapons, was for a long time prepared almost exclusively by the cementation of wrought iron. For this purpose the carburising process is prolonged until the entire thick ness of the bars is penetrated by the carbon, so that the whole of the material is converted into high-carbon steel. The product is known as "blister steel" and could, to some extent, be used direct in that condition. Preferably, however, the cemented bars were melted in small crucibles and the molten high-carbon steel cast into small ingots or other moulds—the product being "crucible" or "cast" steel. For a long time this product, when prepared from
the purest Swedish wrought iron was regarded as the highest grade of steel. In recent times, however, the high degree of purification which can be applied to steel by melting it in a basic-lined electric furnace has made electric steel—when properly prepared—a seri ous rival to crucible steel.