We must now combine the little sketch which we made on page [319], by plotting the points, Ar1, Ar2 and Ar3, with the freezing-point diagram which we have just now been considering. You remember that we found all sorts of things happening to our 0% to 1.7% alloys—the steels—at temperatures around 1290° F., 1395° F., and 1650° F. Similarly, a great deal happens to these other alloys, as they cool from their solidifying temperatures downward.
The Freezing-Point and Critical-Point Curves Make up the Equilibrium Diagram
But for the moment considering only the steels, i.e., the third of the diagram to the left of the 1.7% carbon line, we remember that upon completion of the solidification of any alloy, we had only a frozen solution of all the carbon in iron. Now in the bottom part of this left third of our diagram on page [344], the line GOSE does not look so very much different than the freezing line, ABC, does it? It resembles it not only in appearance but also in actual experience. But in this case it represents not a freezing from liquid to solid but a decomposition, or better perhaps, a transformation. The solid solutions or alloys which contain less than .9% of carbon give up their excess of pure iron upon getting down to temperatures lying along the line GOS, by gradual decomposition of the austenite. In alloys lying to the right of this .9% carbon vertical line the austenite rids itself of excess carbon by throwing out of solid solution and freeing the chemical compound, Fe3C, as the line SE is reached and passed. That is, analogously to what occurred during freezing, certain concentrations occur in the solid, lower carbon alloys by gradual rejection of pure iron crystals until the remainder of the mass has exactly .9% of carbon, or deconcentrate in the higher ones by rejection of Fe3C until they reduce the carbon to this .9% figure. In all cases, by the time the temperature 1290° F., has been reached this has been accomplished and the remaining undecomposed austenite, now with just .9% of carbon, in some way splits into the alternating plate-like constituent which is shown in the cut on page [341].
The Equilibrium Diagram and Interpretation as Now Tentatively Accepted
This plate-like constituent we can hardly call eutectic or “well-melting” alloy for it, as well as the rest of the alloy, has been solid for a long time. But being so similar in derivation and appearance to the eutectic which forms during freezing of a molten alloy, it is proposed that it be called the next best thing, the “eutectoid.” It is often called the eutectic, however.
By this time you doubtless have seen that the free iron which was thrown out of the steel having less than .9% carbon is the ferrite which we found in the soft steels, and that the chemical compound, Fe3C, of the higher carbon steels is the extremely hard constituent, cementite. The eutectoid or plate-like structure is, of course, pearlite, which in the last chapter we found to consist of just these alternating plates of ferrite and cementite.
The Cast Irons
All of the alloys lying to the right of the line UV contain more than 1.7% of carbon, and, according to our classification, therefore, are “cast irons.”