To sum up, iron-carbon alloys which contain less than 1.7% of carbon, in other words, the steels, freeze as solid solutions of carbon in gamma iron. This, of course, is the metallographic constituent which is called austenite. It is not of a definite composition as it contains whatever carbon is available up to 1.7%. Alloys containing between 1.7% and 4.3% of carbon gradually freeze out this solid solution, austenite, more and more being formed in the freezing alloy until, upon arriving at a concentration of 4.3% of carbon for the remaining liquid, the latter, too, freezes as a eutectic of alternating plates of more of this same constituent, austenite, and the carbide of iron, Fe_{3}C, about and among the crystals of the previously formed austenite. From alloys which contain more than 4.3% of carbon, iron carbide, Fe_{3}C, gradually freezes out as the temperature falls, until, at concentration of 4.3% of carbon, the eutectic of remaining carbide and austenite forms about and among the earlier frozen carbide crystals, always at the same temperature, 2066° F., no matter what the original composition of the alloy. Upon reheating, the constituents melt in reverse order, the eutectic liquifying first at 2066° F., the remainder of the alloy gradually becoming liquid between this temperature and the temperature at which the first freezing began during cooling. Transformations and Decompositions So far we have considered only the freezing of the iron-carbon alloys from the molten to the solid condition. Now what happens to them at temperatures below 2066° F.? Do they remain as we left them above, until and after they are fully cold? We must now combine the little sketch which we made on page 319, by plotting the points, Ar_{1}, Ar_{2} and Ar_{3}, 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. [Illustration: THE FREEZING-POINT AND CRITICAL-POINT CURVES MAKE UP THE EQUILIBRIUM DIAGRAM ]