But it must not be supposed that the skeleton which we have constructed can be fully accepted as true until it has been checked and rechecked hundreds and hundreds of times by other investigators. A great many have worked upon these critical points and upon the “freezing-point” curves of the various alloys. It was of course impossible for all of them to make their determinations in just the same manner and with exactly the same materials. Examination of their work and consideration of the results which they obtained show some discrepancies as might be expected, largely probably because of difference in purity of the materials tested, every impurity such as manganese, nickel, silicon, sulphur, phosphorus, etc., modifying more or less the results obtained. As we discovered, speed of heating and cooling also modify the results. When we consider the difficulties which attend the making of determinations on metals and alloys at high temperatures, the wonder is that there is such close agreement. From these standpoints the differences which exist in the published results seem quite small.
It was but twenty years ago that the first outline was drawn and the whole “fusibility” or “equilibrium diagram” of the iron-carbon alloys given in the next chapter has practically been developed within this time. But over this period of twenty or so years the points upon which these lines Ar1, Ar2, Ar3, Ar3·2 and Ar3·2·1 are based have been checked many times and they are now well substantiated. These lines form but a small part of the complete “iron-carbon diagram.”
The Meaning of the Points
You remember that wrought iron and steels having less than .10% carbon showed no point Ar1, and that in all other steels this point becomes stronger as they contain higher and higher carbon. There is little doubt that the point Ar1 exists or results from and because of the carbon of the alloy. In wrought iron there is no carbon, hence there is no point Ar1. If the extremely low carbon steels have an Ar1 it is so weak that it cannot be detected.
Had we tested the .45% carbon steel for magnetic properties we would have found that it lost magnetism at about 1395° F., instead of at 1290° F., at which temperature the .90% steel became non-magnetic. The point Ar2, then, shows the temperature at which loss or gain of magnetism occurs. The electrical conductivity change comes at neither of these points, Ar1, nor Ar2, but at Ar3.
However, with increase of carbon the line Ar3, which was drawn through the points, Ar3, rapidly descends. At about .45% or .50% carbon content, this line Ar3, representing the changes in conductivity, joins line Ar2. Hence in steels having .45% carbon or more, there is a common point, or one which in reality is made up of both points. At this common point the phenomena peculiar to each of the points occur.
This common line, now called Ar3·2, itself lowers with further increase of carbon until, in steels of around .90% carbon, there is but the single point Ar3·2·1, and the phenomena corresponding to all three of the points occur at this one point at 1290° F., as we found in our experiments.
As points Ar2 and Ar3 occur in carbonless iron, they cannot result in any way from carbon but must have to do with the iron itself. From their experiences with other materials, chemists and physicists are well acquainted with such evolutions of heat as occur at Ar2 and at Ar3. These heat absorptions and evolutions, with the sudden dilatation, gain in conductivity, etc., indicate that some internal change or reorganization takes place in the iron itself.
Such changes seem to indicate what are known as “allotropic” modifications. More familiar examples of allotropic forms of materials may be mentioned. Phosphorus, for example, may exist, either as the “yellow” variety which is poisonous and so inflammable that it must be kept constantly under water, or as the “red” variety which is non-poisonous and non-inflammable. Too, there is carbon, which may exist in any one of several forms such as amorphous carbon (soot), graphite, and the diamond. It is believed that iron, itself, exists in three allotropic states. These have been named “alpha,” “beta” and “gamma” iron. We do not need to go into this part of the great subject except to state that at ordinary temperatures and up to Ar2, we have alpha iron, between Ar2 and Ar3, beta iron, and above Ar3, gamma iron. Both beta and gamma iron are non-magnetic, while alpha iron is strongly magnetic. In cooling through Ar3, i.e., from gamma to beta iron, some rearrangement of its molecules produces the dilatation or expansion and the change in conductivity which was noted above.
From the fact that by chemical analysis any certain steel must have the same composition in its hardened that it has in its unhardened condition, it will readily be seen how futile it would be to expect chemical analysis to give us complete information regarding it. Too, tensile strength and the other usual physical tests can hardly tell us all that we wish to know. Microscopic analysis or metallography, however, shows us internal structure of properly prepared pieces of either the hardened or unhardened alloy that we may see the actual condition or grouping of the constituents. The view points given by all three of these methods, chemical, physical and metallographical, are, of course, much better than any one or two alone.