A few secrets of the metallurgist simply told

by Gerald W. Hinkley

CHAPTER III.

AN EXPERIMENT PERFORMED WITH A
PIECE OF PEARLITIC STEEL.

However, let us not trouble ourselves with too many definitions at one time, but instead amuse ourselves for a while by running through a little experiment with a piece of carbon tool steel similar to that which we have just been discussing. For our investigation we will also need a special kind of thermometer for measuring high temperatures. Such an instrument is known as a "pyrometer". Now we will drill a little hole in the test piece of carbon steel and after inserting the "couple" of the pyrometer into it, place the same in the electric furnace.

As the current is turned on, the test piece begins to grow warm and then hotter and hotter, gradually up through a range of temperatures which are continually recorded by the needle of the pyrometer. 800, 900, 1000, 1200 degrees Fahrenheit are uniformly reached, and the temperature of our test piece continues to rise, as the absorption of heat progresses. Suddenly, however, the test piece assumes a bright glow and the needle of the pyrometer ceases to advance, and we note that it is pausing at about 1350 degrees Fahrenheit. Then after its pause, the advance is again resumed until the piece has become almost ready to melt. By plotting the uniform periods of time at which we read the different temperatures recorded by the needle of the pyrometer, against the temperatures as read, we would have a picture of our phenomenon something as follows:

Now let us begin to let our test piece cool off gradually. The temperature of the furnace is lowered and the uniform range of cooling temperatures is recorded by the ever sensitive needle of the pyrometer. Suddenly as before, the test piece assumes the brilliant glow noted previously, and again the needle comes to rest, but this time we note that the recorded temperature is about 1250 degrees Fahrenheit instead of 1350 degrees Fahrenheit as before. Evidently there has been a certain tardiness or "lag" which has caused the phenomenon to take place a little too high going up and a little too low coming down, and in fact the metallurgists tell us that such is exactly the case, and that the real point in which we are interested lies just half way between the two points indicated, as we shall presently see. If we again represent the results of our latest experiment graphically, we would have a picture something as Fig. 2.

Now placing the second curve so obtained on the first, we are able to study the following interesting relationship. Fig. 3.

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:

Now let us take a piece of carbon steel as before, but this time containing .15% carbon, and again proceed with our observations. Again the needle of the pyrometer records the point of recalescence and also the point designating the second range of critical temperature, but this time strange to say, as the test piece continues to absorb heat, a third critical range is registered, all of which when added to our former picture gives a result something as follows:

By repeating the operations as outlined above, with pieces of steel containing various percentages of carbon from zero to 1.25% and by plotting the different critical temperatures so obtained, we finally obtain a chart which graphically expresses the critical ranges of iron and steels due to the variation of the carbon content. With very low carbon steel it is interesting to note that the first critical point would not occur until 1395 degrees Fahrenheit was reached.

Metallurgists have long designated the lines so obtained by letters, "r", standing for, "refroidissement", which is the French word meaning "cooling", the suffixes 1-2-3 simply standing for the lines in the order drawn.

From the completed chart it is further evident that our first piece containing 0.9% carbon in one way is the most interesting of all since it is the only case where only one point of critical temperature occurs.

It will be noticed from the chart that steels containing less than .10% carbon have no point Ar1 and it is therefore undoubtedly due to the carbon content that this, the point of recalescence, occurs. From tests which we made with the magnet we would also find that the temperatures at which loss of magnetism occurs are those designated by the line Ar2, whereas the loss of ability to conduct an electric current occurs at the point designated Ar3. In steels containing .45% carbon to .75% carbon loss of magnetism and loss of ability to conduct an electric current occur at the same points designated on our chart by the line Ar3-2; whereas in the steel containing .90% carbon—all these changes take place at the same time.

Now, as we concluded before, it is evident that some internal change must have taken place in the steel itself, and as we know that the chemical content does not vary, it is further evident that the change must be of a physical nature, or as in the language of the Metallurgist, an "allotropic change". Therefore, another conclusion which we can draw at this point is that a very much more thorough investigation is required for the proper handling of steel at high temperatures than a mere knowledge of the chemical analysis of the same.

There is one very fortunate circumstance connected with the passing from one of these allotropic changes to another, and that is that the effecting of one of these changes takes time. It does not take a very long time, however, for in some instances the change is affected in a very small fraction of a second, while rarely more than one or two seconds are required. The higher the temperature the quicker the change.

Would it not be interesting if we had been so constructed as outlined in the beginning of this little volume; that we could have withstood the high temperatures in which some of these very interesting changes occur, because we could then handle the steel, examine it and experiment with it at our leisure. However, such not being the case, we will have to derive some other means for "catching" the steel while it is in one of these interesting conditions, and then bringing it in its entrapped condition down to room temperature. How shall we do it? Well, we remember that we said it took time to effect the changes under discussion and furthermore we remember that the changes can only take place when the steel is within the proper critical range. Therefore, if we could do something to lower the temperature of a piece of steel while in one of the critical ranges before the steel had time to effect the usual allotropic change of form, we might be able to catch a piece of steel while in one of these unusual conditions, before it had really had time to get back to normal.

Therefore, let us place a piece of .9% carbon tool steel in the heating furnace and bring it up to and beyond the point of recalescence. Now, grasping the piece firmly in a pair of tongs with all possible speed we plunge it into a nearby pail of ice water, keeping the steel constantly in motion. Almost instantly the steel becomes black and within a few seconds is actually brought down to room temperature.

Now let us take the steel out and examine it. The act of tapping it on the anvil in order to knock off the surplus water gives us a hint that our test piece has undergone some sort of a change. For now it rings with a bell-like clearness and gives the hammer with which we strike it a quick snapping rebound which in itself indicates great hardness. Next, we test the piece with a hardened steel file with which we could easily have made a deep ridge before we attempted the heating operation and to our surprise the file has as little effect as if it had been made of wood. And to our surprise on closer examination, we actually find that our test piece has scratched the file—surely it must be very hard. We are convinced that some marked change must have taken place. What can it be? Why it must be that due to the rapid cooling in the pail of ice water we brought the temperature of the test piece down below the critical range before the abnormal condition at which it existed while at and above the critical range had found time to change back to its former condition. And we remember that if one of these allotropic changes is going to take place at all, nature says it must do so while the steel is within the critical range and therefore having forced the steel through that critical range which separates one allotropic condition from another, before it had found time to effect its desired change, we managed to entrap the abnormal condition so that we could see it and feel it and get familiar with it at room temperature.

If we so desire we can now make other hardness tests on our piece of steel at our leisure. For these scientists have invented several machines. One of the most common is called the scleroscope in which a hardened steel ball is allowed to drop from a given height on to the piece of steel to be tested. Then the rebound of the ball is carefully noted. The higher the rebound, the harder the piece. That is natural isn’t it? We know that if the ball were allowed to drop on butter, it wouldn’t rebound at all, because the butter is so soft. A piece of wood would possibly record a very tiny rebound, while a piece of hardened tool steel would effect a very material action of the scleroscope ball, thus indicating extreme hardness.

Now let us take our test piece to the grind stone and grind it down to the shape of a cutting tool. It is necessary to resort to the grind stone, in order to get the desired shape, because of course, our test piece is far too hard to cut with any other metal. After having produced a tool of the desired shape and size, let us fasten the same securely into the carriage of a lathe, and then upon applying the cutting edge to a revolving piece of cast iron, or soft steel, or even to a piece of the very same grade of steel out of which the tool was made, only while it is still in the softened or annealed condition, we find that it is capable of easily and quickly cutting out a good sized ribbon of chips from the metal which is to be machined.

However, we are soon confronted with a new difficulty, for as the cut progresses, our tool runs into a rough spot which causes it to tremble and chatter and then suddenly our tool cracks in two in the middle and is at once completely ruined.

It is evident that as we are able to increase the desirable element of hardness in a piece of tool steel, we also automatically increase the undesirable element of brittleness, and therefore some new method must be devised which will allow a sufficient degree of hardness to allow the tool to cut other metals and at the same time not cause so much brittleness that it will crack in two at the first rough spot which it encounters.

One method of assisting the toughening of a piece of hardened tool steel is accomplished by the process of "drawing". This simply means heating the piece of hardened tool steel up to some fairly warm temperature, which of course must be kept well below the critical range (at which the steel would jump at the chance to quickly change back into one of its softer allotropic forms) and then keeping the steel at this drawing temperature for a while until the unusual strains and stress caused by the rapid cooling have had an opportunity to have become somewhat relieved. Therefore, the process of "drawing" is quite as important as is the first act of hardening itself, and great care must be exercised in undertaking the same.