The scientific name of pure iron is ferrite which melts at 1530° C. On heating pure iron, it loses its magnetism at 768 degrees and changes its entire crystalline nature at 909 degrees; the exact converse takes place at about the same temperatures on cooling. The three forms of ferrite are known as alpha, beta, and gamma ferrite.
Ferrite is found in commerce as dead-soft steel, ingot iron, and electrolytic iron. It is impure with slag inclosures in wrought iron, and is impure with amorphous carbon separated and some silicon in solution in malleable iron.
Adding carbon to molten iron lowers the melting point until 4.3 per cent of carbon is present, when still more carbon raises the melting point even more sharply. Melts having less than 4.3 per cent of carbon freeze out a solid solution of carbon dissolved in iron which we call austenite. Melts holding more than 4.3 per cent of carbon freeze out carbides, the most important of which is Fe3C. This carbide occurs frequently in high-carbon materials. The melt containing exactly 4.3 per cent of carbon, which freezes at 1135 degrees, is a mixture of 47.7 per cent of austenite - austenite being a solid solution of carbon or of iron carbide in iron - and of 52.3 per cent of cementite, and is called the euiectic because of having the lowest solidifying temperature of all the possible mixtures in various proportions of iron and carbon, namely, that with 4.3 per cent carbon content.
As indicated by the diagram, Fig. 20, austenite is stable at high temperatures only. In alloys with less than 1.7 per cent of carbon, the austenite, on sufficient cooling, separates out ferrite along a definite line, if there was less than 0.85 per cent of carbon in the solid solution; and carbide is separated out, if there was more than 0.85 per cent of carbon in the solid solution. Here, again, a minimum point is found at 0.85 per cent of carbon and 723° C. The mixture separating is called the eutectoid, or, more commonly, pearlite, because of its play of bright colors. It has 88 per cent of ferrite and 12 per cent of cementite.
With the alloys of more than 1.7 per cent of carbon, austenite and cementite may decompose into ferrite and graphite, if held until equilibrium is established at high enough temperature - above 700° C. But depending largely on the rate of cooling, we may get all gradations between austenite and cementite mixtures, and between ferrite and graphite mixtures. Manganese, silicon, and phosphorus influence the tendency to separate graphite in strong degree.
We may, then, by adjustments of chemical composition and the rate of cooling, get an immense variety of materials in the cooled iron at will. The art is known as founding. Foundry practice now is based on well-established science and is making rapid progress.
Fig. 20. Iron Carbon Equilibrium Diagram.
In composition the cast irons group about the eutectic mixture; likewise, the steels group about the eutectoid mixture. The diagram in Fig. 20 represents one of the greatest condensations of empirical knowledge yet accomplished in the realm of metallurgy. It can be criticized in many respects, and will be elaborated and perfected further; just as it stands, however, it can be used to enormous advantage by anybody concerned practically in the metallurgy of iron and steel.