Steel has a tensile strength of 32 tons per square inch, ultimate or breaking weight, and the same of compression with shearing at 24 tons - each of which, it will be noticed, exceeds the power of resistance of wrought-iron, which it has superseded; the kinds generally used for buildings being Siemens-Martin and Bessemer Basic - the former usually preferred on account of its greater toughness, though the latter rolls into more shapely joists, and has bye-products valuable to the manufacturer. This difference is not worth considering in ordinary buildings, and moreover the Bessemer Basic is a trifle cheaper.
Steel contains from 0.15 to 2 per cent of carbon, the amount being regulated either by addition to wrought or extraction from pig-iron, as hereinafter explained. It has the strengths previously mentioned, with greater elasticity, and combines most of the useful characteristics of both cast and wrought-iron; in addition to which it can be hardened or softened, which is called tempering, - the former being done by reheating and sudden cooling, and the latter by reheating at a lower temperature and gradual cooling.
The various kinds of steel are made as follows : -
Tool Steel, as the name implies, is used for all kinds of instruments, including knives, and other cutting tools. It is the product of " cementation/' which consists of the cutting up of pure wrought-iron bars and heating them with charcoal at a high temperature, forming Blister Steel - so called because of the blisters made by the process on the face of the iron. It is then reheated and rolled, forming Spring Steel; while the single and double shear steels are the results of further cementations, after which the metal is hammered together by a large " tilt" hammer.
Cast Steel includes the following different methods, the resulting materials being used for cutlery, railway rails, and wheels, armour, boiler plates, etc.: -
The Bessemer process includes the melting of pig-iron with other fluxes, to produce pure wrought-iron, to which the required carbon is added in the form of "Spiegeleisen."
The Siemens process substitutes Mokta ore for the Spiegeleisen; while in The Siemens-Martin method, wrought scrap iron is added, to give the required carbon.
The resulting metal is then rolled, according to Whitworth's patent, to compress the mass and get rid of the holes contained in all cast-steel. These last processes make the steel which is usually rolled into girders, etc., for building purposes.
Mild steel can be made by the above processes, but contains from 0.2 to 0.5 per cent, of carbon only, and is superior to wrought-iron.
Wrought-iron and steel, by the equality of their different powers, have a great advantage oyer cast-iron, which they entirely supersede for most work in which the structures are subject to both tension and compression, alternating in the same flanges, especially in continuous girders, eta (as explained in Chapter XXIV (Stresses. Definitions).).
When wrought-iron or steel girders meet from different directions over columns they must be grouped together by angle plates, bolted to the webs of each girder, so that they can assist each other to make up for their small bearings, which should scarcely ever be less than 9 inches for weight-carrying joists, girders, etc Figs. 418 and 419 illustrate what is required at such junctions over columns.
To guard against corrosion, which gradually reduces their strength, these metals may be variously treated, by being: -
1. Boiled in tar;
2. Plunged into oil when red-hot, or coated with boiled oil;
Fig. 418. lScale.
Section thro' Webs.
3. Galvanized by a covering of zinc; or:
4 Painted with oxide of iron instead of lead-paint, in order to give them some protection from atmospheric and other influences, which tend to injure them, and bring about deterioration of strength; and just lately it has been found that by adding a small proportion of aluminium to the steel in the manufacture the corrosion, to which it is greatly and prejudicially subject, can be considerably reduced.