366 STEEL 7-6578, and another from 8*0923 to 7'6578. Caron found a decrease from 7*817 to 7'743. The latter found that hammered steel on hard- ening lost in length and gained in other di- mensions, while rolled steel gained in length. The effect of hardening on the tenacity of steel is discussed under IKON, and also further on in this article. Steel over-heated becomes brittle, and is said to be burnt. Whether this impaired cohesion is due to oxide of iron, or, as has been suggested, to carbonic oxide (either of which might be formed at high tempera- tures with access of air), or to a crystallization of the particles, is not certainly known. Many fluxes have been suggested for restoring burnt steel. They usually contain easily fusible sub- stances, such as alkalies, borax, &c., combined with carbonaceous compounds, such as prus- siate of potash. Hammering at a high heat is said to restore burnt steel. The working of steel requires great skill and judgment. It cannot be wrought at very high temperatures ; and the more carbon it contains, the lower must be the heat of working. The harder steels are generally hammered at a cherry- red heat. On the other hand, working at too low a temperature seriously impairs the tenacity of steel, as is abundantly shown by experience with steel rails. Welding steel to steel or steel to iron is difficult, except with the softest or least carburized varieties. Fluxes to facilitate such welding are largely used with good effect; they add nothing to the intimacy of the weld, their action being mechanical only (cleansing, exclusion of air, &c.), as in the case of iron welding. There is always danger of the separation of iron and steel at the weld, unless the latter is very soft. Special devices, such as causing one of the metals so welded to overlap and enclose the other, counteract this tendency in part. Or fluid steel may be cast directly around white-hot wrought iron, the weld being pro- moted by subsequent rolling or hammering. Sometimes the iron and steel to be welded are enclosed in a case of thin wrought iron and exposed to a welding heat, the enclosure pre- venting an access of air and oxidation of the surfaces of the metal. Strength of Steel. The cohesive force of steel is usually considered under the different heads of absolute strength, or the force required to produce rupture ; the elastic limit, or the least force by which a per- manent alteration of form is effected ; and the extensibility, or the amount of elongation un- der a breaking stress. The experimental data are referred, for convenience of comparison, to bars or rods of one square inch section. The above named properties are dependent, first, on the chemical composition of the metal ; secondly, on its homogeneity; thirdly, on its molecular structure ; and fourthly, on the tem- perature. (For comparison of the strength of cast iron, wrought iron, and steel, see IROX.) 1. The effect of the amount of carbon on the properties of steel is shown in the following tables compiled from Knut Styffe's work on the " Elasticity, Extensibility, and Tensile Strength of Iron and Steel :" PUDDLED STEEL SQUARE BARS. Breaking weight per sq. In. of original mean area, in 11*. Breaking vei^lit referred to area of fracture. Hard steel, cent, carl Middling h per cent. Soft steel, w cent, carb Puddled ire carbon -with 0-6 to 0-8 per >on ard, with 0-55 to 0'7 89,189 80,628 70,272 48,819 122,240 115,670 112,593 120,770 1th less than 0'5 per n, with 0-2 per cent. BESSEMER STEEL. CARBON, PER CENT. Elastic limit. Breaking weight per sq. In. of original area, in Ibs. Breaking weight, frac- tured area. Elongation by rupture per cent. 2-16 1-85 1-85 1-14 1-05 0-99 0-GS 0-42 0-33 64,502 57,040 76,511 85,431 63,620 65,875 63,620 34,996 41,251 86,804 99,842 107,184 127,564 103,213 102,998 101,214 68,757 64,708 63,268 89,617 102.178 137,308 216,153 176,422 106,223 155,218 161,325 141,219 2-96 1-75 2 -SO 2'90 2-90 8-70 8-70 16-70 16-70 24-50 The last sample was homogeneous iron prepared with ferro-manganese. To interpret correctly results like the above, it is necessary to elimi- nate all disturbing influences of composition and treatment. While these figures do not show a uniform change of properties with gradually increasing 'amounts of carbon, th'ey neverthe- less show decidedly that the effect of carbon on iron is to increase its absolute strength and elastic limit, and to decrease its extensi- bility. An increase of carbon beyond 1-2 per cent, is not accompanied, as a rule, by an in- crease in absolute strength, When reference is had to the fractured area, it will be seen that the force required to produce rupture does not differ as widely in different steels as when the original area alone is considered. The effect of melting, or in other words of the homoge- neity of .steel, is strikingly shown by a compar- ison of the two preceding tables, the former referring to puddled or welded steel, and the latter to Bessemer or homogeneous steel. The effect of molecular structure on the physical properties of steel has been partially treated under IRON. The table, vol. ix., p. 374, shows that the effect of hardening is to increase great- ly the strength and elastic limit in steel, and to decrease its extensibility. The data given by J. Barba ("Memoir on the Uses of Steel") show that as the proportion of carbon decreases, the effect of sudden cooling becomes less marked, but even the softest iron is made somewhat more rigid by this treatment. The effect of hardening and tempering is, further, well shown by the following results of experiments on bars of steel cut from the same mass and submitted to a different treatment, made with reference
