By pursuing this train of reflection, it will follow, that, since crude iron differs from steel only in the superabundance of carbon, it ought to be capable of extreme hardness, if ignited to that degree which is requisite to combine the greater part of this carbon with the iron, and then sud denly cooled. This is accordingly found to be the case. If the grey crude iron, commonly distinguished by our founders by the name of soft metal, be heated to a white heat, and then plunged into water, it becomes very hard, much whiter, den ser, and more metallic in its appearance, and will bear a pretty good edge, fit for gravers, for the use of turners in iron or steel. In these tools the angle of the planes which form the edge is about 45°. The hardness of this kind of iron is not considerably diminished but by ignition continued for a length of time, which is a fact also conformable to what happens in steel. For the cast steel will be softened nearly as much by annealing to the straw colour, as the harder steels are by anneal ing to a purple or full blue.
Some of our artists have taken advan tage of this property of soft crude iron in the fabrication of axles and collars for wheel-work ; for this material is easily filed and turned in its soft state, and may afterward be hardened so as to endure a much longer time of wear.
The founders who cast wheels and other articles of mechanism are occasion ally embarrassed by this pioperty. For, as the metal is poured into their moulds of moistened sand, the evaporation of the water carries off a great portion of the heat, and cools the iron so speedily, as to render it extremely hard, white, and close, in its texture. This is most remark able in such portions of the metal, as have the greatest distance to run from the git or aperture of reception. For these come in contact successhely with a larger por tion of the sand, and are therefore more suddenly cooled. We have seen the teeth of cog-wheels altogether in this state, while the rim and other parts ofthe wheel remained soft. The obvious reme dy for this defect is, to increase the num ber of Fits, and to have the sand as dry as possible or convenient. In other arti cles this property has been applied to ad vantage, particularly- in the steel rollers for large laminating mills, which Messrs. Vandermonde, Monge, and Berthollet, have supposed to be an over-cemented steel. They are made by casting the grey crude iron in moistened sand, the contact of which gives the hard steel temper to the outside surface, for the depth of more than half an inch. There is
no doubt, but that the iron-masters pay considerable attention to the quality of the iron, and perhaps to the degree of heat and moisture ofthe sand in this ope ration, in order that the hardness may Re such as to yield to the turning tool ; and it is likewise understood, that a considera ble number crack longitudinally in the cooling, a loss which in all probability arises from the difference of contraction between the hard and soft parts.
A variety of facts concerning the hard ening and softening of steel are collected by Guyton Morveau, the most interesting of which shall be here subjoined. Accord ing to Reaumur, that part of the steel which was hottest at the time of immer. sion in the water will be the hardest ; whence it has been thought a fair conclu sion, that the hardness of steel is greater, the stronger the ignition, and the more speedy the cooling. Nevertheless, the celebrated Rinman deduces a very I different consequence ; namely, that the steel which is naturallythe hardest is that which requires the least heat, and that the best temper for each kind of steel is that produced by the lowest degree of j heat suitable to that peculiar kind. Ilence, •i various methods have been proposed to 1 ascertain the degree of heat most advan tageous to any particular sample of steel. 1 They are all reducible to that of igniting one end of a bar to a white heat, and plunging it into water. The hardness of the several parts may then be ascertained by examination.
It appears that the considerations on which the first-meptioned general rule is founded are rather of a complicated na ture. If it be true, as conjectured above, that the hardness of steel depends princi pally on the intimate combination of its • carbon, it will follow, that the utmost • hardness any steel is capable of will be produced by a degree of heat sufficient to effect this purpose, and that any superior degree will only degrade the quality of the steel ; and, consequently, that the ge neral rule will apply only to heats inferior • to this maximum. It may also happen, when the mass of water is not considera ble, though the fact has been doubted when the quantity is large, that the heat ing of the water, and the production of hydrogen, may so far modify the rate of cooling, as even to render it less sudden than might have happened with an infe rior degree of ignition. Lavoisier has re marked, that the conversion of iron into the hard brittle oxide, when water is de composed, may perhaps be of the same.