Molten iron is so greedy for carbon, that, when it can get it, it readily holds in solution from 7% to 10% of this element. But solid (frozen) iron cannot retain anything like this amount. As we learned in the last chapter, gamma iron is the only variety which can exist above our lines of loss of conductivity, magnetism and recalescence, i. e., Ar3, Ar3·2, etc. It is, too, the only variety of solid iron which is able to retain carbon in solution, and it can retain only about 1.7% of it.
So when molten steel containing 1.5% of carbon, say, cools until it reaches the temperature represented by the line, AB, which, at its intersection with the 1.5% carbon line would be at about 2582° F., particles or crystals begin to freeze out and float in the molten alloy. As the temperature falls, more crystals separate until, when the temperature determined by intersection of the 1.5% carbon line with the lower freezing curve, AE, is reached, the last of the now mushy alloy solidifies.
Alloys of all other compositions below 1.7% of carbon do just this way except that the temperatures at which freezing begins and ends are different and distinctive for each composition.[[10]] Upon freezing, every one of them retains in “solid solution” in the “gamma” iron whatever carbon it had in the liquid or molten solution. But, as stated above, it can not be over the 1.7% limit.
[10]. Temperatures of beginning and end of freezing may always be ascertained by locating on the freezing-point diagram the points at which the vertical line representing the desired composition intersects and crosses the lines of the freezing-point curves—in these cases, AB and AE.
Of the iron-carbon alloys of compositions lying to the right of the line UV, we find the case to be different, for each one of them has more than the 1.7% of carbon which is the maximum amount which “gamma” iron can retain. Now the lowest temperature at which any iron-carbon alloy can exist without freezing is slightly above 2066° F., and there is but one composition—95.7% of iron and 4.3% of carbon—which can survive until this low temperature is reached. A content of 4.3% of carbon then, is the greatest and also the least concentration which Nature will allow to remain molten down to this minimum temperature. This 4.3% carbon composition which is the lowest melting, i.e., the easiest melted alloy, is called the “eutectic” alloy from Greek words which mean “well melting.” This eutectic composition may be said to divide or rather subdivide this group of alloys into two groups, those containing between 1.7% and 4.3% of carbon, and those which have 4.3% and over.
As stated before, freezing is not an instantaneous but a progressive process. During the freezing period of any of these alloys which have over 1.7% of carbon the still liquid portion which remains after freezing begins to become smaller and smaller in quantity as freezing progresses just as it did in alloys of the “solid solution” group. And as Nature allows a concentration of 4.3% of carbon as the highest concentration at the minimum temperature the very last of the remaining liquid of every alloy eventually gets to this eutectic composition just before the alloy freezes. Those to the left of the eutectic or exact 4.3% composition do so by the gradual freezing out of iron containing the maximum or 1.7% of carbon, i.e., iron is taken out faster than carbon, hence there is gradual concentration of carbon in the remaining liquid. This goes on until 4.3% is reached. The compositions to the right of the line WX throw out the chemical compound, Fe3C, which contains 6.6% of carbon, whereby carbon is eliminated faster than iron and the desired 4.3% carbon alloy is arrived at from the other direction.
To illustrate, take, say, the composition represented by the vertical line at 3% carbon and 97% iron. As the molten alloy cools it reaches the temperature 2330° F., at which temperature the vertical line representing the 3% carbon composition cuts the line AB. Here the alloy begins to freeze by the separation of small crystals of solidifying iron containing definite amounts of carbon.[[11]] But as the carbon thus taken along by the freezing crystals of iron is always less than 1.7%, a proportionally greater amount of iron than carbon is removed from the unfrozen part of the alloy and the remaining liquid or unfrozen part, therefore, is left with slightly more than the 3% of carbon with which it started.
[11]. The percentages of carbon carried by the particles of iron freezing at any particular temperature of the solidification range may be determined from the diagram but the works named in the reference list should be consulted for method and explanation.
This we must now consider another alloy with a lower freezing-point, the reason being, of course, its higher carbon content. At the next lower temperature, more iron containing carbon is frozen out and the remaining liquid is again left a little higher in carbon than before. In this way the continually diminishing amount of remaining liquid keeps concentrating, forming thereby a continuous succession of alloys of higher and higher carbon content as the temperature continuously drops.
Eventually, of course, the concentration of this remaining liquor becomes 4.3% of carbon just before completion of the freezing at 2066° F.