STEEL 365 colors, probably due to superficial oxidation, the experienced workman judges of the tem- per which the steel will assume : TEMPERATURE. Color. Corresponding temper, iuitable for Fahr. Centigrade. 430 450 470 490 510 530 550 560 221 232 243 254 265 277 288 293 816 Very pale yellow. Pale straw. Full yellow. Brown. Brown with purple spots. Purple. Bright blue. Full blue. Dark blue. Lancets. Razors and surgical instruments. Penknives. Scissors, cold chisels. Axes, plane irons. Table knives. Svv'ds, watch springs. Fine saws, augers. Hand and pit saws. Polished articles may be heated for tempering over or between iron plates, in a gas flame, in molten lead, or in various other ways, until the proper color appears. For articles not polished, the temperature must be otherwise determined, as by heating in oil or tallow or in alloys of known fusibility. When oil or melted tallow begins to smoke, its temperature corresponds with that indicated by straw color on the pol- ished steel ; darker and more abundant smoke corresponds with brown ; black and still more abundant smoke rises at 530, the temperature of purple ; when the vapor takes fire from a lighted taper, without continuing to burn, the temperature is about 580 ; and finally, when the oil burns and rises in the vessel, the point of dark blue has been reached. The following table shows the fusing point of several alloys of tin and lead : Lead, parts. Tin, parts. Fusing point, deg. F. Load, part*. Tin, parts. Fusing point, deg. F. 7 4 420 19 4 509 H 4 430 30 4 530 8 4 442 48 4 550 8* 4 450 50 2 558 10 14 4 4 470 490 Boil'g linseed oil j Melting lead.... 600 612 Steels containing other substances besides car- bon appear to require different treatment from pure carbon steels. Thus tungsten and titani- um steels, so called, if heated bright red and suddenly cooled, are said to become excessive- ly brittle; they must therefore be manipulated at low temperature. Too little is known of these compound steels to permit inferences as to their physical behavior. The hardening of large or irregular masses of steel requires great care. Unequal cooling causes fracture. Gen- erally the more massive portions are first dipped in the liquid, and the thinner portions last ; or, in case of any great disparity, special means are adopted to retard the cooling of the smaller parts. The causes of the phenomena attendant upon hardening and tempering steel were long involved in mystery, and are not yet all known with certainty. What is clearly known on the subject may be briefly stated. The degree of hardness assumed on cooling by a given steel is dependent on the rate of cool- ing. Caron says the degree of hardening is inversely proportional to the square of the time. The liquids which favor rapid cooling are those having a high specific heat and a low boiling point. Water fulfils these conditions in an eminent degree, while oil has a much lower specific heat and a much higher boiling point; consequently cooling in oil is a more gradual process than in an equal volume of water. Increasing the volume of the liquid and maintaining agitation, so as to diffuse rap- idly the heat received from the steel, of course hastens cooling. The most rapid cooling is produced by mercury, by reason of its high conducting power. It is sometimes used to produce extreme hardness. But obviously the initial temperature of the cooling liquid is an essential point ; so that heated mercury or fusible alloys could be used to effect slow cool- ing. Ordinary tempering is a partial anneal- ing; that is, excessive hardness having been imparted to the steel, the excess is removed to the degree desired. It has been found in most cases practically easier to attain an accurate result in this way than by a single process of hardening, arrested at the desired point. But recent experiments by Caron have shown that it is possible, in some cases at least, to effect the hardening in one operation by carefully adjusting the amount and temperature of the water. Water at 131 F. was found to give results with some objects equal to those produced by the most careful hardening and tempering. Caron has further found that hardening of steel with 0'2 to O4 per cent, of carbon in warm, or still better in boiling water, was accompanied by an increase of its tenacity and elasticity without a material im- pairment of its hardness. The toughening of large steel objects, such as cannon, is effected by heating them to redness and immersing in oil, where they gradually cool. This process has been recommended for steel rails. The hardening of steel is probably due both to a chemical combination of the carbon (present partly as graphite in soft steel) with the iron, and to a state of tension among the particles, conditions which are both removed by anneal- ing. The tension in a bar of hardened steel is shown by cutting it in two lengthwise, when each piece assumes a curved form, concave on the cut side. Soft iron does not harden when suddenly cooled, but acquires increased rigid- ity and tensile strength ; while cast iron, con- taining more carbon than steel, becomes under the same treatment extremely hard (chilled iron), often harder than steel. The freshly fractured surface of hardened steel ehows a fine grain, often velvety in appearance ; that of soft steel presents facets. In the former, analysis shows no uncombined carbon ; in the latter, a small amount of graphite is almost always present. Steel ^expands on hardening, and loses specific gravity. Eisner found one sample to change in gravity from T'9288 to
