A few secrets of the metallurgist simply told

by Gerald W. Hinkley

It is natural to suspect that both of the parallel sections of our curves have something to do with the same thing, and for convenience since we noticed that mysterious glow of the test piece just as the needle came to rest, we might call the particular point which lies just half way between the temperatures under discussion, the point of glow, or as the metallurgists call it, the "point of recalescence" and the range between these two temperatures the "critical range".

I suppose it would be difficult to explain this phenomenon of the test piece unless we imagine that as the critical range is reached some internal reaction of the steel causes it to spontaneously take on heat at the same temperature in the first place and give off the stored heat at the same temperature as the piece was being cooled down, and this heat caused it to glow as was noticed. Now if we were to experiment further with our piece while at the critical range, we would find certain other remarkable changes, one of the most noticeable of which is the loss of magnetism at and above the critical range.

Irons and steels are usually the most magnetic materials, but the attraction of the magnet is completely lost at or above the critical range.

We can easily satisfy ourselves in this respect by noting the attraction of a simple horse shoe magnet when our piece of test steel is brought into its magnetic field. As the pyrometer needle passes on up through the range of temperatures noted above, the magnetic attraction is perfectly evident when suddenly the recalescence point is reached, the spell is broken and the magnet and the test piece fall apart. But let us just consider this phenomenon a moment. We are told by the physicists that magnetism is induced in a piece of iron or steel by a "rearrangement of the internal molecular structure, in which the positive ions face one direction and the negative ions in the opposite direction". Therefore, if magnetism suddenly ceases to exist it would seem as if something had happened to the "internal molecular structure" of the test piece. Thus when the recalescence point is reached we may conclude that something more than a mere absorption of heat units has taken place. In fact we may really believe that an actual internal molecular revolution has occurred and that some of the natural laws which formerly had governed all of these little molecules which go to make up the whole piece of steel, have been overthrown and that the molecules are more or less free to set up a new form of government for themselves, and that, therefore, when a piece of steel is brought to the recalescence point it is really in a very sensitive condition. In fact, if we should care to investigate further we should find that certain other great changes take place at this critical point, such, for instance, as partial failure of the test piece to conduct an electric current, which formerly, of course, it did with great ease. Also when the critical range is reached, a peculiar contraction of size interrupts the gradual expansion which had been developing as the test piece absorbed heat units, and therefore these several observations give us reason to believe that our conclusions as noted above must be more or less correct.

Now if all steels acted exactly like the little test piece which we have been observing above as they were placed in the hardening furnace, it would not take us very much longer to finish our preliminary investigations. You remember the piece of steel which we have been investigating was a piece of simple carbon tool steel, containing about 0.90% carbon. But all steels do not contain just this same percentage of carbon, and may also contain various elements other than carbon, all of which produce many and varied results during the process of heating, treating and hardening.

In order to better visualize the investigation which we are making, let us picture graphically each step which we take. If therefore, we let the vertical lines represent the different carbon contents which steel might have, and the horizontal lines the different degrees of temperatures through which we might desire to heat the steel under discussion and then plotted the phenomenon described above we would have a picture something as follows:

Now all that picture means is that as we heated up a piece of simple carbon tool steel containing O.9% C, we discovered a certain very noticeable reaction which occurred just about half way between 1250 degrees and 1350 degrees Fahrenheit, which we decided to call the point of recalescence, and then on further heating of the piece no other such phenomenon was noticed.

Now let us go through the same experiment with a piece of steel containing .45% C. Yes, just as before, as the temperature 1250 degrees Fahrenheit is reached we note all the strange symptoms which are characteristic of the point of recalescence and then, just as we are about to decide that it is hardly necessary to go further we notice that the pyrometer needle has again come to rest, but that this time it is registering 1390 degrees Fahrenheit. Therefore, it would seem as if this piece had two critical ranges instead of one and we are now quite ready to again proceed with our heating to see if anything else occurs. However, as nothing does happen we turn to our picture and plot the two points just observed, together with the one point found on our first investigation, and the drawing then looks something as follows: