Non-technical chats on iron and steel, and their application to modern industry

by La Verne W. Spring

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.