It was “Ali Baba” who is quoted as saying, “Those who do not know how to take the Philistine, better hadn’t!” or words to that effect.
Now through these chapters we have attempted to discuss in an entirely non-technical manner the subjects presented. On this account we were compelled to forego discussion of many things which are highly important and interesting but which are more or less difficult of explanation without the use of scientific terms and theories. One such has been the “mechanism” of the hardening of steel and its opposite, its softening by annealing. For those who may desire to get a glimpse into this “wonderland” it is hardly fair to refrain from brief discussion of the subject just because it is technical and difficult and so may prove to be tedious to some who have little reason to be interested.
It seems desirable, therefore, to impose this more technical chapter or two that the subject of the real metallurgy of iron and the steels may at least be “hinted at.” We say “hinted at” advisedly for it is a long, long story, and, even now, after a great many years of serious study no one has yet read it to the end. We are not saying this in a discouraging way, however, for there seems little reason to doubt that the multitude of facts which have been disclosed through the tireless experiments and the study of hundreds of investigators have put us well on our way to the solution of this one of Nature’s great problems.
To those, however, who are not interested in the known details of “how” and “why” hardening and softening of steel is possible and why hardening of pure iron and mild steels does not and cannot take place, we must say as would “Ali Baba”:—“Those who do not care to study it better hadn’t.” Anyway, the study of this rather intricate subject is conducive of “headaches,” and perhaps it is not extremely important when viewed from the non-technical standpoint of these articles.
We have several times referred to the debt which civilization owes to iron and steel structural materials, machinery and tools and particularly to those tools which have hardened cutting edges. Almost every one knows that hardened cutting edges are imparted to tools by sudden cooling in water or oil from a good red heat. Probably most of us, too, know that the blacksmith can again soften such tools by reheating to the same red heat and allowing them to cool slowly. This he calls annealing. In this softened or annealed condition a piece can readily be sawed or filed, while in its hardened state a saw or file produces no result upon it.
Now what are the facts, meaning and the cause of this dual life of the alloy, steel, without which we would be so greatly handicapped.
To be better prepared to understand the answer, let us consider three or four accompanying and closely allied phenomena which close observation of the habits of steel has disclosed.
The “Point of Recalescence”
If we drill a hole in a small piece of carbon tool steel which we are about to put into the heating furnace, and if into this hole we insert the bare tip of an electric pyrometer, this heat-measuring instrument will indicate at all times the rising temperature of the piece of steel as it heats in the red-hot muffle or chamber of the furnace.
As we watch the piece grow red, the pyrometer registers 900°, 1000°, 1100°, 1200° F.,—gradually and uniformly indicating higher and higher temperatures.