Steel

Steel is a compound or mixture of iron and carbon containing from 0.1 to 1.25 per cent. of the latter, as well as small quantities of other bodies such as silicon, sulphur, phosphorus, manganese, etc. It has not been satisfactorily determined whether the carbon is combined with or dissolved in the iron, but it does not exist in steel as free graphite, as it does in cast- or pig-iron. Steel is superior to iron in tensile strength, elasticity, and rigidity, and is so in a greater degree as the proportion of carbon is increased. The presence of phosphorus in quantities exceeding 0.1 per cent. causes steel to be hard and brittle, or "cold-short"; more than about 0.05 per cent. of sulphur makes the metal "red-short" or brittle, and unworkable at a red heat. Silicon produces a like effect if 0.5 per cent. is present; other impurities produce similar undesirable properties; but sometimes other metals, as chromium and tungsten, are alloyed with steel for special purposes. The harder kinds of steel, containing 0.5 per cent. of carbon and upwards, undergo a molecular change when heated to redness and suddenly cooled, with the result that they are hardened. Mild steels containing a little carbon are hardly (if at all) affected by this process, while those containing 0.1 per cent. of carbon may readily be rendered sufficiently hard to scratch glass. As steel when very hard is also brittle, cutting tools, springs, etc., are "let down" or softened to the required extent by being again heated and cooled. The amount of this annealing depends upon the temperatnre of the second heating, which is ascertained by noting the colour assumed by the film of oxide which is formed on the polished metal. The mild grades of steel are used for rails, girders, shipbuilding, and other purposes for which wrought-iron was previonsly employed. If the percgntage of carbon is less than about 0.3 such steels can be welded. Steel can be made by smelting a pure iron ore with charcoal, but this process cannot be used in the case of the ordinary impure ores. The conversion of cast- into wrought-iron by puddling involves the removal of most of these impurities, and the highest class of steel is made by adding carbon to Swedish malleable iron by the cementation process (q.v.). In this process bars of the pure iron are subiected to an intense heat for a prolonged period, in contact with charcoal, and carbon is gradually absorbed. As the carburisation is not uniform, this "blister" steel is frequently converted into cast-steel by melting in crucibles, so that a general distribution of carbon is obtained. In the Bessemer and basic processes (q.v.) the carbon and impurities present in cast-iron are burnt by a current of air forced through the molten metal, or are absorbed by the slag which floats on its surface, and in this way tolerably pure iron is obtained. The requisite quantity of carbon is then added in the form of spiegeleisen or ferro-manganese, which are compounds of carbon, iron, and manganese. In making open-hearth or Siemens-Martin steel, wrought-iron or scrap steel is dissolved in melted pig-iron in the regenerative furnaces, and in some cases iron ore is substituted for the scrap. Steel may also be produced by a process of puddling very similar to that employed for making malleable iron. There are several difficulties in obtaining sound castings of steel due to the liberation of occluded gases, which produce cavities, and to other causes. The most satisfactory method of obviating this is to compress the metal into the mould while still molten by heavy hydraulic pressure; but this process is necessarily expensive, and so can only be used for the highest class of work.