A few secrets of the metallurgist simply told

by Gerald W. Hinkley

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.