IRON. 



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 property. 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 successively 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 of the 

 wheel remained soft. The obvious reme- 

 dy for this defect is, to increase the num- 

 ber of gits, 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 tfie outside surfaces 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 bre 

 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 the 

 greater, the stronger the ignition, and the 

 more speedy the cooling. Nevertheless, 

 the celebrated Rinman deduces a very 

 different consequence ; namely, that the 

 steel which is naturally the 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 

 heat suitable to that peculiar kind. Hence, 

 various methods have been proposed to 

 ascertain the degree of heat most advan- 

 tageous to any particular sample of steel. 

 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 J 

 which the first-mentioned general rule is 

 founded are rather of a complicated na- 

 ture. If it be true, as conjectured above, 4 

 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 ofthe 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 



