If the cementation be continued too long, the steel becomes porous, brittle, of a darker fracture, more fusible, and inca pable of being forged or welded. On the contrary, steel cemented'with earthy in fusible powders is gradually reduced to the state of forged iron again. Simple ignition produces the same effect ; but is attended with oxidation of the surface. The texture of steel is rendered more uniform by fusing it before it is made into bars ; this is called cast steel, and is rather more difficultly wrought than com mon steel, because it is more fusible, and is dispersed under the hammer if heated to a white heat.
The conversion of iron into steel, either by fusion, viz. the direct change of crude iron into steel, or by cementation of bar iron, presents many objects of interesting inquiry. From various experiments of Bergman, it appeared that good crude iron, kept for a certain time in a state of fusion, with such additions as appeared calculated to produce little other effect than that of defending the metal from oxidation, became converted into steel with loss of weight. These facts are con formable to the general theory of Van dermonde, Monge, and Berthollet : for, according to their researches, it should follow, that part of the carbon in the crude iron was dissipated, and the re mainder proved to be such in proportion as constitutes steel. The same chemist cemented crude iron with plumbago, or carbonate of iron, and found that the metal had lost no weight. Morveau re peated the experiment with grey crude iron. The loss of weight was little, if any. The metal exhibited the black spot by the application of nitric acid, as steel usually does, but it did not harden by ignition and plunging in water. Hence it is conclud ed, that it was scarcely altered : for crude iron also exhibits the black spot, and can. not by common management acquire the hardness of steel.
From the experiments of the three excellent chemists last mentioned, it ap pears that the grey crude iron consists principally of iron, with as much carbon as it can dissolve in the strong heat of the smelting furnace. They have shown also, that it deposits part of this addition, when cooled in contact with an iron bar immersed in the bath. This separation must be general in the ordinary or gra.
dual way of cooling, whence the grey co lour must arise from the blue white co lour of the iron mixed with the black of the carbon. And this grey colour is also in a degree perceived, when soft close grained steel is broken. These circum stances lead to an inference, that hard steel may in a certain respect differ from that which is softer by the intimate com bination of a larger proportion of carbu. ret. This accounts for the whiter and more metallic aspect of hardened steel, than of such as is soft. For the former con tains less of disengaged carburet. Hence also we may account for the greater hard ness of steel, which has been made quite hard, and then let down by tempering to a certain colour, than of steel merely heated to that colour, and plunged in wa ter. For, in the first method of harden ing, a sufficient degree of heat is given to produce combination between part of the disengaged carburet and the iron, which in the Ikter does not take place. If the carburet be merely sufficient to sa turate all the iron at a moderate degree of ignition, the hardness will be considera ble ; but the steel will be easily degraded to the state of iron by frequent ignition. Such steel, in its hard state, will be very uniform in its texture, not excessively hard in its temper, but disposed to take a very fine firm edge, which will not easily be broken or injured by violence. These are accurately the properties of the Eng lish cast steel, which is of so uniform a nature, as to be distinguished by its con choidal or glassy fracture. When the dose of carbon in steel is greater, it will bear a greater heat without degradation, inso much that it may be welded like iron. Its hardness will also be capable of a higher degree; and if' this degree, pro duced by a stronger ignition, be not given, the edge of the tool will never become fine and smooth ; and even at this higher degree, with all the advantage of subse quent tempering, it will be less smooth than that of the cast steel, and more dis posed to break. Steel of this kind is bet ter adapted for the construction of ham mers, vices, hatchets, leather-cutters' knives, and other instruments, wherein the edge is either stout, or sudden blows unnecessary, or the construction demands frequent heating and welding.