part, instead of showing the fibrous arrangement observable in bar iron, they present a closely granular one. In the second place, chemical analyses demonstrate that the iron has combined with about 1 per cent. of carbon, and that this combination has not only taken place on the surface of the bar, but has extended throughout its whole substance. It is because of this perfect impregnation of the iron by the solid carbon that the process by which it has thus been converted into steel is called ‘cementation.’
Two suggestions have been offered in explanation of the blistered surface presented by the steel. One of these, the theory of Mr T. H. Henry, is that part of the carbon in penetrating into the body of the bar iron had combined with the small quantity of sulphur present in the iron, and that the bisulphide of carbon thus formed becoming vaporised by the elevated temperature, in escaping through the soft surface of the metal, has caused its blistered condition. The second conjecture is that the blebs have arisen from the extrication of carbonic oxide, which had been formed in the bar by the union of the carbon with the small quantity of oxide of iron or slag accidentally remaining in it.
Graham has shown that soft iron has the power of absorbing or occluding at a low red heat 4·15 times its volume of carbolic oxide, which the metal, when it becomes cold, retains, but which it parts with when subjected to a temperature such as that which prevailed in the cementation box. This fact seems to offer a reasonable confirmation of the reaction it has been surmised takes place during the cementation process, and which is supposed to be as follows:
The small quantity of atmospheric oxygen remaining in the chest unites with the carbon
to form carbonic oxide. This carbonic oxide gives up half its carbon to the iron (which thereby becomes converted into steel), and in doing so changes to carbonic acid, which becomes reduced to carbonic oxide by the absorption of more carbon from the charcoal, which carbon the carbonic oxide again transfers to the iron.
The above reaction may not improbably occur throughout the substance of the bar. By some chemists, cyanogen compounds are supposed to be present in the cementation powder, and the cyanogen contained in these is supposed to be the carrier of the carbon to the iron.
“The blistered steel obtained by this process is, as would be expected, far from uniform, either in composition or texture; some portions of the bar contain more carbon than others, and the interior contains numerous cavities. In order to improve its quality it is subjected to a process of fagotting similar to that employed in the case of bar iron; the bars of blistered steel, being cut into short lengths are made up into bundles, which are raised to a welding heat, and placed under a tilt hammer weighing about 2 cwt., which strikes 200 or 300 blows in a minute; in this way, the several bars are consolidated into one compound bar, which is then extended under the hammer till of the required dimensions.
“The bars, before being hammered, are sprinkled with sand, which combines with the oxide of iron upon the surface, and forms a vitreous layer which protects the bar from oxidation. The steel which has been thus hammered is much denser and more uniform in composition; its tenacity, malleability, and ductibility are greatly increased, and it is fitted for the manufacture of shears, files, and other tools. It is commonly known as shear steel.
Double shear steel is obtained by breaking the tilted bars in two, and welding these into a compound bar. The best variety of steel, however, which is perfectly homogeneous in composition, is that known as cast steel, to obtain which about 30 lbs. of blistered steel are broken into fragments, and fused in a fire-clay or plumbago crucible, heated in a wind furnace, the surface of the metal being protected from oxidation by a little glass melted upon it. The fused steel is cast into ingots, several crucibles being emptied simultaneously into the same mould. Cast steel is far superior in density and hardness to shear steel, but, since it is exceedingly brittle at a red heat, great care is necessary in forging it. It has been found that, in addition to 100 parts of the cast steel, of one part of a mixture of charcoal and oxide of manganese, produces a very fine grained steel, which admits of being cast on to a bar of wrought iron in the ingot mould, so that the tenacity of the latter may compensate for the brittleness of the steel; when the compound bar is forged, the wrought iron forming the back of the implement, and the steel its cutting edge.”[196]
[196] Bloxam’s ‘Chemistry Inorganic and Organic.’