Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/369

IRON 353 hand the same term, is often used to imply the softening or annealing only of the tool. In the act of hardening, steel becomes specifically lighter to an. extent varying with the temperature attained before cooling, the composition of the steel, and its form and dimensions, and accord ing as it has been rolled or otherwise worked before treatment ; thus Eisner found a specimen of cast steel had the specific gravity 7 "9288 before hardening, and 7 6578 after (both at 11 C.), indicat ing an expansion in volume of about 3 5 per cent. Similarly Kinman found expansions of 2 "6 to 3 5 per cent, for blister steel ; and Hausinann found for a very hard steel and for a soft welding steel expansions of 17 and 8 per cent, respectively. The expan sion, however, is not uniform in all directions ; thus Caron found that a hammered steel bar 20 centimetres long and 1 centimetre square in section altered after repeated hardenings to the following extent in percentages of the original dimensions : 10. 20. 30. 2-5 6-8 10-15 2-0 3-0 6-0 Decrease in specific gravity ... 0-9 whilst with rolled steel there was an increase in length of 2 - 25 per cent, and no change in the other dimensions. On thoroughly annealing hardened steel it recovers the original, dimensions possessed before hardening ; evidently therefore when a large steel tool is hardened, since the inner portion cannot possibly become cooled at the same rate as the outer film, a kind of strain must be developed amongst the particles ; for, whilst the rapid chilling of the outer portion causes that part to occupy a larger volume than it otherwise would, this is not effected to so large a relative extent with the inner portions. The opposite result is necessarily produced during annealing. Accordingly, in the process of tempering steel goods, cracking and splitting or "buckling" and otherwise getting out of shape are inconvenient results that are apt to happen, especially when all parts of the surface are not equally and simultaneously heated or chilled, a result difficult to bring about with articles of certain shapes ; the hardening process may in such cases be more safely effected by heating the article up gradually in a fluid bath such as melted lead (or in some cases oil) and then chilling in a cool fluid bath, preferably of oil ; whilst the annealing may be performed by allowing the fluid in which the article is slowly heated up to cool down again spontaneously when the requisite temperature is attained. For special articles, e.g. , saws, a particular manipulation is requisite to avoid buckling ; in some cases the annealing of a flat plate, e.g., a circular saw, is effected between two solid flat masses of iron, which keep the plates from getting out of shape whilst cooling. In some instances when the point only of an article is required to be tempered, e.g. , certain kinds of chisels, the whole mass is heated red hot and the point dipped into water or oil to harden it ; on taking out, the heat from the body of the hot article is rapidly conducted to the point ; as soon as its temperature is sufficiently raised the body is somewhat cooled by a short immersion so as to reduce the tempera ture of the whole approximately to uniformity, and then the article is allowed to cool altogether slowly ; so that the hardening and annealing are effected by one heating only. For articles the temper of which requires to be somewhat exactly determined, the tempera ture attained is judged of by the tint of the iridescent film of oxide which forms on the surface (previously brightened) during the heating; the higher the temperature the thicker the film, the colours and temperatures being approximately as follows : Colour. Approxi mate Tem perature. Class of Tools for which the Temper is best suited. Very pale yellow 220 C. 230-235 240-245 255 265 275-280 285-290 290-295 315 Lancets. Surgical instruments and razors. Penknives and common razors. Scissors, cold chisels, shears, &c. Axes, planes, &c. Table knives, large shears. Springs, watch springs, swords. Fine saws and augers. Hand saws and large saws. Straw colour Golden yellow Brown dappled with purple ... Bright blue Full blue Dark blue For instance, if a saw is to be tempered at a full blue it is heated over a clear fire or a mass of red hot iron or in a sand bath until the tint appears on the brightened surface, when it is allowed to oool. In many cases the tempering of tools is effected with sufficient accuracy without brightening them, by coating them with tallow, oil, or some similar composition, and then heating over a lamp or a mass of hot metal until the tallow, &c., begins to decompose; with sufficient experience as to the size of the article, the mode of heating, and the character of the decomposition set up, much the same results are arrived at as when the colour is watched on the brightened surface. In some instances the requisite temper is given by one operation only, the point of the article to be hardened (a drill, say) being coated with tallow and heated in a flame until the tallow decomposes rapidly, and then plunged into a mass of tallow to chill it ; the point is thus rendered harder than the body of the drill. By heating the point of a small tool nearly white hot and plunging it into a stick of sealing wax for a second, and then into another part of the wax, and so on till cold, steel may be made so hard as to be readily capable of boring into similar steel hardened in the ordinary way. Instead of determining the temperature of the object to be annealed by the tint developed on it, various fusible alloys may be used, a series of such being placed in the annealing bath, so that by observing which are fluid the temperature is known approxi mately. Parkes gives the following compositions of lead-tin alloys for this purpose- : Parts of Lead to four parts of Tin. Temperature at which Alloy melts. Parts of Lead to four parts of Tin. Temperature at which Alloy melts. 7 215 C. 19 205 C. 7 5 221 30 277 8 228 48 288 8-" 232 100 292. 10 243= Boiling linseed oil. 316 14 254 Melting lead. 322 The first five of these alloys consequently correspond to the various yellow shades ; the next three to brown, brown and purple spots, and purple ; and the last four to the blue shades. 43. Strength and Tenacity of Iron and Steel. Indepen dently of the additional strength communicated to iron by the presence of small quantities of carbon, producing steels or so-called steels of various kinds, a considerable increase in this property is brought about by the removal of portions of cindery matter by fusion. Wrought iron that lias simply been welded, reheated, and rolled a given number of times is, cxteris paribus, less tenacious than iron from which complete fusion has removed the films of silicate, &c., enveloping the component layers and fibres of the welded material, and preventing their complete union together. Thus the best qualities of weld iron, containing usually about - 2 to 3 per cent, of carbon, possess a tensile strain of from 20 to 30 tons per square inch, whilst mild " steels " and ingot irons are usually considerably higher in this respect, viz., from 30 to 40 tons per square inch ; harder steels of higher carbon percentage are proportionately stronger. The following numbers may be quoted, repre senting tensile strains in tons per square inch. Weld Irons, or Wrought Irons made without Fusion . Kirkaldy. Govan puddled bar ............................................................................... 20-9 ,, hammered bar ........................................................................... 28-7 rolled ,, ................................................................. 25-4 to 26-6 Blochairn bar, cold rolled ...................................................................... 30 5 annealed ............................................................... i .5-2 to 27-8 boilerplate, cold rolled, lengthways ......................................... 39-7 ,, ,, crossways .......................................... 3f>-0 ,, annealed, lengthways ........................................... 22-7 ,, ,, crossways ............................................. 21-7 Lowmoor bars (0-3 per. cent, of carbon) ................................................... 28-9 Styffe. Lowmoor rolled tyre bar ........................................................................ 27-3 Lesjo fors rolled bar (made in Lancashire hearth) ..................................... 22-5 Fairbairn. Average of five c.asses of pl,,tes Iiicjot Irons, or Mild Steels, made by Fusion Processes. carbon per cent., 0-25 to 0-3.5 ....... 30 lo 3"> 0-35 to 55 ....... 35 45 Greiner- Scraing extra mild steels, medium soff, ,, ,, hard and extra hard, Styffe : Ilogbo Bessemer iron, hammered steel, Krupp s cast steel, Kirkaldy. Bessemer soft steels, 0-5.5 and upds....45 70 0-33 ............ 33 0-68 ............ 4C-8 62 ............ 39-5 3to04 ......... 33 to 35 Steel wire of as high a breaking strain as 206,170 Ib, or 92 tons, per square inch section, has been drawn by Johnson and Nephew (Manchester), whilst wire of tensile strength of 70 to 80 tons per square inch is readily procurable. According to Collingwood, galvanizing increases the strength of freshly drawn wire by about 15 per cent., whilst ungalvanized wire has been found to gain nearly 5 per cent, by merely being allowed to remain at rest for a week or so ; the increase of strength in each case appears XIII. 45