the metal from mechanical impurities by fusion. The second period, by converting the metal into the fusible cast iron and melting this, for the first time removed the gangue of the ore; the third period by giving a temperature high enough to melt the most infusible forms of iron, liberated the slag formed in deriving them from cast iron.
In 1856 Bessemer not only invented his extraordinary process of making the heat developed by the rapid oxidation of the impurities in pig iron raise the temperature above the exalted melting-point of the resultant purified steel, but also made it widely known that this steel was a very valuable substance. Knowing this, and having in the Siemens regenerative gas furnace an independent means of generating this temperature, the Martin brothers of Sireuil in France in 1864 developed the open-hearth process of making steel of any desired carbon-content by melting together in this furnace cast and wrought iron. The great defect of both these processes, that they could not remove the baneful phosphorus with which all the ores of iron are associated, was remedied in 1878 by S. G. Thomas, who showed that, in the presence of a slag rich in lime, the whole of the phosphorus could be removed readily.
9. After the remarkable development of the blast furnace, the Bessemer, and the open-hearth processes, the most important work of this, the third period of the history of iron, is the birth and growth of the science and art of iron metallography. In 1868 Tschernoff enunciated its chief fundamental laws, which were supplemented in 1885 by the laws of Brinell. In 1888 F. Osmond showed that the wonderful changes which thermal treatment and the presence of certain foreign elements cause were due to allotropy, and from these and like teachings have come a rapid growth of the use of the so-called “alloy steels” in which, thanks to special composition and treatment, the iron exists in one or more of its remarkable allotropic states. These include the austenitic or gamma non-magnetic manganese steel, already patented by Robert Hadfield in 1883, the first important known substance which combined great malleableness with great hardness, and the martensitic or beta “high speed tool steel” of White and Taylor, which retains its hardness and cutting power even at a red heat.
10. Constitution of Iron and Steel.—The constitution of the various classes of iron and steel as shown by the microscope explains readily the great influence of carbon which was outlined in §§ 2 and 3. The metal in its usual slowly cooled state is a conglomerate like the granitic rocks. Just as a granite is a conglomerate or mechanical mixture of distinct crystalline grains of three perfectly definite minerals, mica, quartz, and felspar, so iron and steel in their usual slowly cooled state consist of a mixture of microscopic particles of such definite quasi-minerals, diametrically unlike. These are cementite, a definite iron carbide, Fe3C, harder than glass and nearly as brittle, but probably very strong under gradually and axially applied stress; and ferrite, pure or nearly pure metallic α-iron, soft, weak, with high electric conductivity, and in general like copper except in colour. In view of the fact that the presence of 1% of carbon implies that 15% of the soft ductile ferrite is replaced by the glass-hard cementite, it is not surprising that even a little carbon influences the properties of the metal so profoundly.
But carbon affects the properties of iron not only by giving rise to varying proportions of cementite, but also both by itself shifting from one molecular state to another, and by enabling us to hold the iron itself in its unmagnetic allotropic forms, β- and γ-iron, as will be explained below. Thus, sudden cooling from a red heat leaves the carbon not in definite combination as cementite, but actually dissolved in β- and γ-allotropic iron, in the conditions known as martensite and austenite, not granitic but glass-like bodies, of which the “hardened” and “tempered” steel of our cutting tools in large part consists. Again, if more than 2% of carbon is present, it passes readily into the state of pure graphitic carbon, which, in itself soft and weak, weakens and embrittles the metal as any foreign body would, by breaking up its continuity.
11. The Roberts-Austen or carbon-iron diagram (fig. 1), in which vertical distances represent temperatures and horizontal ones the percentage of carbon in the iron, aids our study of these constituents of iron. If, ignoring temporarily and for simplicity the fact that part of the carbon may exist in the state of graphite, we consider the behaviour of iron in cooling from the molten state, AB and BC give the temperature at which, for any given percentage of carbon, solidification begins, and Aa, aB, and Bc that at which it ends. But after solidification is complete and the metal has cooled to a much lower range of temperature, usually between 900° and 690° C., it undergoes a very remarkable series of transformations. GHSa gives the temperature at which, for any given percentage of carbon, these transformations begin, and PSP′ that at which they end.
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| Fig. 1.—Roberts-Austen or Carbon-Iron diagram. The Cementite-Austenite or Metastable form. |
These freezing-point curves and transformation curves thus divide the diagram into 8 distinct regions, each with its own specific state or constitution of the metal, the molten state for region 1, a mixture of molten metal and of solid austenite for region 2, austenite alone for region 4 and so on. This will be explained below. If the metal followed the laws of equilibrium, then whenever through change of temperature it entered a new region, it would forthwith adopt the constitution normal to that region. But in fact the change of constitution often lags greatly, so that the metal may have the constitution normal to a region higher than that in which it is, or even a patchwork constitution, representing fragments of those of two or more regions. It is by taking advantage of this lagging that thermal treatment causes such wonderful changes in the properties of the cold metal.
12. With these facts in mind we may now study further these different constituents of iron.
Austenite, gamma (γ) iron.—Austenite is the name of the solid solution of an iron carbide in allotropie γ-iron of which the metal normally consists when in region 4. In these solid solutions, as in aqueous ones, the ratios in which the different chemical substances are present are not fixed or definite, but vary from case to case, not per saltum as between definite chemical compounds, but by infinitesimal steps. The different substances are as it were dissolved in each other in a state which has the indefiniteness of composition, the absolute merging of identity, and the weakness of reciprocal chemical attraction, characteristic of aqueous solutions.
On cooling into region 6 or 8 austenite should normally split up into ferrite and cementite, after passing through the successive stages of martensite, troostite and sorbite, FexC = Fe3C + Fe(x−3). But this change may be prevented so as to preserve the austenite in the cold, either very incompletely, as when high-carbon steel is “hardened,” i.e. is cooled suddenly by quenching in water, in which case the carbon present seems to act as a brake to retard the change; or completely, by the presence of a large quantity of manganese, nickel, tungsten or molybdenum, which in effect sink the lower boundary GHSa of region 4 to below the atmospheric temperature. The important manganese steels of commerce and certain nickel steels are manganiferous and niccoliferous austenite, unmagnetic and hard but ductile.
Austenite may contain carbon in any proportion up to about 2.2%. It is non-magnetic, and, when preserved in the cold either by quenching or by the presence of manganese, nickel, &c., it has a very remarkable combination of great malleability with very marked hardness, though it is less hard than common carbon steel is when hardened, and probably less hard than martensite. When of eutectoid composition, it is called “hardenite.” Suddenly cooled carbon steel,
