STEEL, THEORIES OF HARDENING. That steel has the property of hardening when suddenly cooled from a bright cherry heat has been known for centuries. The origin of steel hardening cannot be traced definitely, but Homer refers to it and Pliny, the Younger, describes a method which, in its main points, is general practice to-day. (Otto Vogel in Stahl and Eisen, 1899, p. 242). During the 15th and 16th centuries the practice grew to be an art and great secrecy and superstition were attached to it. Quaint and curious methods were evolved and the secret was hoarded and handed from generation to generation. Even to this day, in some sections, the same old type superstition exists among some of our own blacksmiths, and it is quite common to meet with one whose ideas and practices are identi cal with those followed in the Middle Ages.
Many of the mechanisms of what happens during the process known as hardening have been studied and are known, but the real reason of why steel hardens is still in doubt. The very fact that there are at least seven distinct theories, in vogue to-day advanced by as many eminent metallurgists, is mute evidence of the fact that we are in the dark regarding this rea son. Sir Robert Hadfield (Farady Society, November 1914) in discussing these theories says: *I cannot say that there is one which throws light on what really happens when a piece of carbon steel is changed from a soft to a hard condition? Dr. J. E. Stead (Farady Society, November 1914) expresses the opinion, *We have not sufficient data upon which to found any definite conclusion as to the process known as the hardening of steel by quenching,* and Albert Sauveur (International Engineering Congress, September 1915, San Francisco), *It will likewise be obvious that no theory so far presented fully satisfies our craving for a scientifically acceptable explanation of the many phenomena involved? H. M. Howe (Farady Society, November 1914) says *Each of these theories has its difficulties, and Rosenhain ((Study of Physical Metallurgy,' p. 180) says regarding Martensite (the normal constituent found in normally hardened steels), *The ques tion is Martensite ?> cannot be answered quite conclusively, particularly as several rival views are in the field? In the usual practice followed in hardening a piece of ordinary carbon steel the operation is an extremely simple one. It consists of heating the piece to a predetermined tempera ture, holding at this temperature for a length of time sufficient for the entire mass to be evenly heated and then suddenly cooling it. The cooling is usually done by immersing in i liquid, usually water or oil.
The simplest steel has at least two constitu ents, one iron or ferrite, and the other a carbide of iron or cementite, and, in order to gain an inkling of what transpires, it is necessary to study the properties of these two constituents.
If we plot a cooling curve of pure iron, we find that at 898° C. and 768° C. marked evolu
tions of heat occur, indicative of a change in the physical properties (see Fig. 1), while, if carbon be present, as in steel, a third evolution of heat occurs at 690° C. (see Fig. 2).
Floris Osmond (Journal Iron and Steel In stitute, No. 3, 1906, p. 444), in a series of brilliant researches first brought out the momentous discovery that iron exists in three separate and distinct allotropic modifications. At all temperatures up to 768° C. pure iron or ferrite exists in the Alpha condition. Be tween this temperature and 818° C. it is in the Beta condition, and above 898° C. it exists in the Gamma condition. In each of these three conditions the physical properties are entirely distinct. Crystallographically, they are all of the cubic system, but the Gamma iron assumes the octahedra while the Beta and Alpha assume the cube. Twin crystals are frequent in the Gamma, but the Beta and Alpha are free from them. Alpha iron is magnetic, and the change from Alpha to Beta is accompanied by an abrupt change in the electrical conductivity, strength and hardness and crystalline form. On cooling, changes occur which are the reverse of, and appear at very nearly the same temperatures as, those on heating. The points on heating are known as the AC, and the AC,; on cooling, as the AR, and the AR,. When carbon is present, a third evolution on cooling takes place at about C. which is known as the AR, and the reverse as the AC,. The ranges in which these changes occur are known as the critical ranges.
The carbon in steel exists in the form of a definite chemical compound known as cementite which has the chemical formula Fe,C. It crys tallizes usually in flat plates or needles which in some cases coalesce into the form of granules. It is the hardest constituent of steel and is at tacked by ordinary reagents less than the other constituents.
Cementite and ferrite are the two most im portant constituents of steel, and it is, therefore, their inter-relation which we must study in order to understand what is transpiring, con stantly bearing in mind that the ferrite can and does exist in three different allotropic condi tions depending upon the temperature. At ordinary temperatures cementite and ferrite exist together as completely decomposed con stituents, and, in the lamellar condition, ar range themselves in tiny flakelets. Upon heat ing to the Gamma range of temperatures, cementite dissolves in the Gamma iron with the result that there is formed a constituent known as austenite. In only special cases can this be preserved at ordinary temperatures be cause it is extremely unstable and breaks down into martensite, this latter being the usual constituent found in ordinary hardened steel.
There are two sharply defined schools at tempting to explain the problem of hardening, and below is given classification in tabular form.
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