It was a great day for metallurgy when it was discovered that molten mixtures were governed by the same natural laws which govern our ordinary liquid solutions. Probably it was because most metallic alloys are solid at ordinary temperatures and liquid only at very high ones that for so long a time we failed to suspect their similarity.

If a sensitive pyrometer is inserted into an ordinary solution or a molten alloy which is being gradually cooled, it indicates the first instant that solidification (freezing) begins as well as the termination of the freezing period. Unlike the freezing of water or a pure metal, complete solidification ordinarily does not take place at a definite single temperature but over a greater or lesser range of temperature. By taking such upper and lower freezing-point measurements for many different percentage compositions of a binary (two metal) alloy, for instance, curves can be plotted which show accurately the habits of any and all of the possible combinations (i.e., alloys) of those two metals.

Such are called “freezing-point” curves, and, as we shall see later on, a study of them will give us much valuable and interesting information.

Such curves have been constructed for a great many alloys since the discovery of the analogy between their behavior and that of aqueous solutions led us to study alloys after the manner which physical chemists found so satisfactory for the study of ordinary solutions. Since the study of binary or two metal alloys is often very difficult, it can be readily understood why the determination and interpretation of the curves of alloys which contain three, four or more metals is a very much more serious matter. Much of it has to be done by methods which are long and tedious, such as quenching and microscopic study of innumerable specimens taken during the freezing and subsequent cooling of various alloys of each series. The value of the results depends upon the skill, devotion and clear sightedness of those who carry out the work.

The “freezing-point” and “decomposition” curves of the iron-carbon series of alloys have been brought to their present stage of development after something like twenty years of labor by investigators in many lands. If we look at the diagram on page 345, we note at once that the curves are quite complicated. Even yet they are not complete for all percentage combinations of iron and carbon, and those who have given the most time and study to the subject have not yet been able to interpret with entire satisfaction to all concerned all of the discoveries so far made.

Without endeavoring to take up in detail the technique of the manner of their production, which would be unprofitable for us without a great deal more of preliminary study than we have time and space to give, we will at once examine the freezing curves of the iron-carbon alloys as now developed. The works named on page 354 as references for Chapters XXII and XXIII may be consulted for the various types and methods of construction and for explanation of freezing curves by those who desire to study them.

Referring to the freezing-point diagram on page 336, the upper or broad V-shaped line, ABC, indicates the temperatures at which the alloys of various percentages of iron and carbon begin to freeze, and the lower one, AED, the temperatures at which the freezing of these alloys ends. From the diagram it is readily seen that pure iron (100%), has a very high freezing-point and solidifies at once. Iron which contains about 2% of carbon begins to freeze at a much lower temperature and has a long period of solidification, while iron with 4.3% of carbon has the lowest freezing-point of the series with an extremely short solidification period or range.

Since we have been unable to go sufficiently into the methods and technique of freezing-curve construction to be able to understand their general classification, we must accept the statement that the curve of the iron-carbon series is really a double one. The part of it that lies to the left of the dividing line UV of the diagram on page 336, is of the type exhibited by liquids which freeze from “liquid solutions” into what are known as “solid solutions,” which by aid of the microscope are found to be homogeneous mixtures of crystals. On the other hand, alloys which lie to the right of UV, are of the type which form “eutectics.” This will be described later. This dividing line UV, which occurs at about 1.7% of carbon, divides the iron-carbon alloys into these two natural divisions. It was the basis for calling those having 2% of carbon or less, “steels,” and those with over this amount, “cast irons.”

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, Ar_3·2, 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.

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.

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.

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?

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.

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.

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.”

We have seen how they freeze either as eutectic alone, as crystals of austenite with eutectic or as crystals of cementite (Fe₃C) and eutectic, depending upon the original composition of the molten alloy. At and just below the temperatures represented by the line ED, this undoubtedly represents the situation.

What happens to the alloys from this temperature down to normal depends upon conditions. Just what occurs and the mechanism of it is not definitely known except in the practical way. Certain it is, the “precipitation” of free carbon is necessary for cast irons which are to be serviceable for usual purposes. This may occur with consequent softening during the first cooling or they may be cast as “hard iron” and softened afterward.

In Chapter XI we said that silicon was a “softener” as its presence brought about precipitation of the carbon as graphite throughout the cast iron, thereby softening it both by reason of the presence of the soft flakes of graphite and because it leaves so little of the carbon in the “combined” or hardening condition. So silicon is a ready means of bringing about decomposition of the higher temperature structures as the alloys cool.

The speed of the cooling also exerts a very powerful influence in determining the amount of graphite which will separate. Other conditions being equal, the slower the cooling, the greater the decomposition with resulting graphite. Swift cooling, even such as results from the dumping of castings from the molds while at nearly white or high red heat results in insufficient graphite and otherwise harder metal. Cooling of very hot castings on a cold floor or in a current of cool air has considerable hardening effect even when the composition of the alloy would otherwise give very soft and machinable metal. In an extreme and very interesting case a few very hard cast iron flanges were each day found among the many thousands of habitually soft castings regularly produced. Each day two or three expensive “taps” were ruined by attempting the impossible—the machining of these pieces of hardened iron, which, on the outside, looked just like all the rest. It was soon discovered that two or three mold dumpers each noon and evening were warming water for “wash-up” by dropping into the pails a flange or two which were still white-hot after dumping from the molds. The men were innocent of any intention of harm but their warm water cost several hundred dollars before their method of producing it was discovered.

Silicon and rate of cooling are the two most powerful influences but presence of certain elements other than silicon also influences to some extent the degree of hardness. However, while silicon has a strong softening effect, manganese and sulphur have an opposite or hardening tendency. On this account the amounts of these latter elements which can be used or allowed must be strictly limited.

From the above it is seen that all sorts of cast iron can be produced ranging from the extremely hard, high cementite, white irons with low silicon content down to the very soft gray irons which result mainly because of higher silicon content and slower cooling.

The white irons are more or less unstable as is shown by the decomposition through which the hard, white iron castings become “malleable” by annealing as was told in Chapter XII which discussed Malleable Cast Iron.

The gray cast irons are much more stable. They consist of what, in an early chapter, we referred to as “steels with an impurity, the graphite flakes.” They consist, then, of free, soft iron or ferrite, certain amounts of the characteristic steel constituent, pearlite, and the soft graphite flakes.

They arrive at this composition, through breaking down of the austenite and cementite structures during the cooling,—just how not having been satisfactorily determined. Consistent study is being put upon this subject and several unique and long-studied possible explanations of this section of the full equilibrium diagram have been proposed and debated, all based upon the data so far available. While much information on this subject has been gained the matter is still so much in dispute that it is best for us to venture nothing definite in regard to just how the changes occur. The reference books for this chapter (see page 354) give quite completely the data, theories and explanations so far available.

Compared with the steels, the cast irons are vastly complicated. In them we have elements which occur in practically negligible amounts in the steels. Commercial cast irons, for instance, have silicon ranging anywhere from ½% to 3%, phosphorus .10% to 2%, graphite 0% to 3.50%, and carbon in the combined form (pearlite or cementite) from 3.50% to .10%. If these represent the majority of cast irons what about our pig irons which have 2, 5, 8, 10 or even 15% of silicon, others with 1 or 2 and occasionally very much more of manganese, and still others which vary widely in phosphorus content?—for from the metallographic and physical chemistry standpoint pig irons are cast irons.

As we can never get perfectly pure iron-carbon alloys to experiment with, their content of other elements, silicon, nickel, phosphorus, etc., vitiate more or less the results obtained, but even could such pure alloys be secured we are not greatly helped since our serviceable alloys are never such. Each added element brings about greater complication and one does not wonder that in the short twenty years which have elapsed since study was seriously undertaken, metallurgical science has not entirely solved the big problem.