The American Quarterly Review, No. 18, June 1831 (Vol 9)

by Various

Arsenic produces a great waste in the manufacture of iron, and when alloyed with it, injures or destroys its capability of being welded.

Ores which contain titanium, according to universal experience in this country, give an iron inclining to the defect of red short, but possessing the highest degree of tenacity. Such are several of the ores of the northern part of New-Jersey, and of Orange County, New-York.

Manganese in small quantities renders iron harder, but injures none of its good qualities. Many of our ores contain manganese, but when carefully manufactured the iron appears to contain but an insensible trace of this metal.

Nickel unites with iron in all proportions, and gives a soft and tenacious alloy; no good property of the iron appears to be injured by it. United with steel it gives an alloy of excellent quality. Nickel is rare among the ores of iron that are not of meteoric origin. But native malleable iron is occasionally found in large masses alloyed with this metal, and its extrinsic source has been fully ascertained. The masses are sometimes of very great size; we have already expressed our opinion that the iron that first came into use was derived from this source, and had been employed for ages before the processes for preparing it from its more abundant ores were discovered.

Cast iron is distinguished into two varieties, which are obviously distinct in character, the grey and the white; a mixture of the two forms that which is called mottled. It is generally believed, and usually stated in the books, that both of these are combinations of iron with carbon, and that their difference in appearance and quality grows out of the difference in the proportions in which the two substances exist; that the grey iron contains the greatest dose of carbon, and the white the least. There is, as will be seen, good reason to question the latter part of this statement.

The grey iron requires the greatest degree of heat for its fusion, is more fluid when melted, is softest, best fitted for castings which require to be turned or filed, and for those that must be thin; the white iron is very hard and brittle; the greatest degree of strength and tenacity is due to the mixture, or mottled iron, and to that variety of mottled in which the grey rather predominates.

The different varieties are readily convertible, for the grey iron when melted and suddenly cooled becomes white, when cooled more slowly is mottled, and when carefully preserved from rapid loss of heat, retains its colour. On the other hand, experiments on a small scale have shown, that white cast iron, subjected to a heat equal to that at which the grey melts, and allowed to cool slowly, becomes grey. Hence their difference can hardly be ascribed to chemical constitution. Neither can the presence of a greater or less quantity of oxygen, as is sometimes supposed, produce the difference, for under circumstances in all other respects similar, except the rate at which they are cooled, iron of the three different varieties may be produced, We therefore feel warranted in rejecting the usual theory, particularly as the reception of it has rather impeded than advanced the manufacture of iron.

The theory of Karsten is far more consistent with the facts, and is directly applicable to the practical purposes of the iron master. We shall endeavour to give a succinct exposition of this theory, introducing all that is necessary for its full explanation.

The ores of iron, which are all oxides, are reduced by exposing them to the action of carbonaceous matter, at a high temperature. The carbon first separates the oxygen from the ore, which becomes metallic, but as it has for the carbon a high affinity, that substance tends to combine with it. The iron combined with carbon is rendered far more fusible than it is when pure, and thus readily melts; when the heat of the furnace is little more than is sufficient for effecting this fusion, the two substances are uniformly mixed, and probably form a compound analogous to a metallic alloy; this is the white cast iron. When the compound is exposed to a heat higher than is sufficient to melt it, a separation appears again to take place, the carbon tending to assume in part the form of plumbago, the iron to retain no more of carbon than is sufficient to keep it liquid at the new temperature, and thus passes from the state of cast iron to that of steel, and finally approaches to that of malleable iron. If the cooling take place slowly, the carbon, obeying its own law of crystallization, arranges itself in thin plates, and the iron, consolidating afterwards, fills up all the interstices with grains or imperfect crystals; and thus the mass assumes a dark grey colour, partly owing to the natural colour of the iron, but in a greater degree to the plumbago. When the cooling is rapid, the carbon still disseminated throughout the mass, does not crystallize separately, but the two substances again form an uniform compound.

Thus, according to the theory, there is no essential difference in the proportion of carbon between grey and white cast iron, but the former is a mechanical mixture of crystals of carbon, nearly pure, with iron containing a less proportion of carbon than the white, while the white iron is a homogeneous alloy of carbon and iron.