The “Eutectic,” the Part of the Alloy Which Solidifies Last
(Magnification 700 Diameters)
Now with alloys containing more than 4.3% of carbon, almost the opposite occurs. Let us choose the one having 5% carbon and 95% of iron. This molten alloy cools until at 2215° F., small crystals begin to freeze and form in the molten mass. But, as the liquid already has more than the favored 4.3% of carbon, it is not free iron which freezes out, but instead, the chemical compound, Fe3C, which contains 6.6% of carbon. This, of course, takes out carbon proportionally faster than iron, hence, at each very slightly lower temperature, the liquid which remains unfrozen contains just a little less of carbon than did its predecessor. So the constantly decreasing amount of remaining liquid progresses through a succession of compositions each containing just a little less of carbon than the previous one, and eventually, just before freezing we get back to the mixture which contains 4.3% of carbon. Of course there is left unfrozen by this time only a very small amount of the alloy and it is this which has the composition stated.
The “Eutectic”
Now, having just the composition which she wants, whether arrived at from alloys lower or higher than 4.3% in carbon, Nature lets this composition freeze at once in thin alternating plates which lie side by side about and among the earlier frozen crystals of the alloy. The appearance of this typical eutectic formation under the microscope is shown on page [341].
Had we chosen the 4.3% alloy itself, neither any of the solid solution of carbon in iron nor the chemical compound, Fe3C, would have frozen out, but the whole mass would have remained liquid down to 2066° F., where the whole would have solidified at once in the plate-like eutectic formation just described.
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, Fe3C, about and among the crystals of the previously formed austenite. From alloys which contain more than 4.3% of carbon, iron carbide, Fe3C, 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?