A mode of tempering instruments of hardened steel was invented by Mr. Hartley, in 1789, for which he obtained a patent ; and we have never yet heard of a better. Mr. Hartley's plan was to immerse the articles in a bath of oil, heated to a regulated temperature, and measured by a thermometer. This was certainly a very great improvement, both in point of precision and dispatch, on the common method of heating the instrument over a flame till a certain colour, produced by a film of oxide, appears on its surface. These colours are— At 430° Fah. a very faint yellow,—for lancets.
450° „ a pale straw coloar,—for razors, and surgeons' instruments. 470° ,, a full yellow,—for penknives.
At 490° Fahr. a brown colour,—for scissors and chisels for cutting old iron.
510° „ a brown, with purple spots,—for axes and plane irons. 530° „ a purple,—for table-knives and large shears.
550° „ a bright blue,—for swords, watch springs, truss springs, and bell springs.
560° „ a full blue, for small fine saws, daggers, &c.
600° „ a dark blue, verging on black,—is the softest of all the gra dations, when the metal becomes fit only for hand and pit-saws, which must be soft, that their teeth may bear sharpening by the file, and bending or " setting." If the steel be heated still further, it becomes perfectly soft. When tools having a thick back and thin edge, like penknives, are to be tempered, they are some times placed with their backs in a plate of hot iron, or on hot sand ; otherwise they would become too soft at their cutting edges before their backs would be sufficiently heated. It is evident that baths of any of the soft metals, whose fusible points are above those required for tempering, may be used instead of oil ; and alloys of those metals might be so proportioned as to obtain points of fusion at the exact degree of heat required. In these cases, however, to prevent oxidation, it would be necessary to keep the fluid metal covered with grease, and it would be advisable not to omit the use of a thermometer. Mr. Gill, in the
Technological Repository, has recommended several compositions for hardening and tempering steel, to which work we must refer the reader for the formulze and processes. We do not insert them here, because they are, for the most part, apparently unsuited to operations on the great scale, although they are certainly, in many respects, well deserving the attention of engineers. We shall, however, avail ourselves of his instructions in the following article.
On restoring the Elasticity of hardened and tempered Steel Articles.—" Saws, sword-blades, clock and watch springs, &c., which, after being hardened and tem pered, require to be ground and polisbed,or otherwise brightened, lose their elas ticity or springiness by these operations, so as to appear soft on bending them, although they are as hard as ever ; these qualifies are again restored to them, either by over a clear fire, made of cinders urged by bellows, or over the flame of burning alcohol, or by inclosing them in a smouldering fire, made of wood-ashes and embers, to a blue colour, which colour may either remain, or be removed by the application of diluted muriatic acid wiped over them." The partial Conversion of Iron into Steel, which is often the case in the blistered bars, owing to the carbon not having penetrated to the middle, is usually regarded as a great defect ; but an important advantage may be gained from it in the steeling of edge-tools, which, Mr:Gill says, is adopted by Mr. Maudaley. The bar is split down its middle into two parts, and these parts are sometimes subdivided ; the internal parts which remain unconverted are then welded to the iron of the tool, leaving the steel outside, to form the cutting edges of the tool.