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 Curves of the Iron-Carbon Alloys
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.”