Alloys are, of course, of most importance to heat treatable steels. To understand their action it is necessary to know that when steels are heated above their "critical points"—a characteristic range of about 725° to 825° C.—the crystal structure of alpha ferrite is transformed into a new one called "gamma iron" which has far greater solubility for carbide. This high temperature solid solution of the carbides in gamma iron is called "austenite." On slow cooling the reverse action takes place at about the same critical range, and the an nealed, fairly soft and ductile steel is reconstituted, except that the process may have refined the grain. On more rapid cooling the precipitation of carbide from austenite is delayed—a little time is necessary for the action to incubate and then to proceed to com pletion—and occurs at a somewhat lower temperature. On rapid cooling, as when quenching a small piece in cold water, austenite is trapped unchanged down to below 15o° C. but then changes completely in a second or two to a very hard structure called "martensite," which seems to be alpha iron (ferrite) in which the iron carbide is dispersed in almost molecular particles. This is the essence of the hardening action. (For a fuller account of the metallographic changes see IRON AND STEEL.) Plain carbon steels have a low hardenability, in the sense that if a 'lin. round bar is quenched in cold water only the outer layers of the steel are cooled rapidly enough to transform at low temperature into mar tensite; the inner core, cooling more slowly, is transformed at high temperatures near the critical into a much softer entity, a coarser mixture of laminated ferrite and carbide. The outer sur face may be intensely hard, even the maximum possible for the carbon content of the steel, but the "penetration" of that hardness is low. To harden more deeply the cooling rate at the surface must be faster, so the deeper regions of the steel may cool rapidly enough to escape the high temperature transformation ; even the most drastic quenching, as in brines or water sprays, may be in sufficient for complete hardening. Fortunately, the presence of all the alloying metals except cobalt slows up both the solution of carbides in the austenite on heating, and their precipitation on cooling. This means that alloy steels do not need to be cooled nearly so fast in order for the austenite to be "trapped" and trans formed at about 150° C. into hard martensite. Alloy steels there fore have a higher hardenability. Given an equal quench and an equal sized bar, the alloy bar will harden much more deeply than the carbon steel bar of the same carbon content ; or it may be sat isfactorily hardened in a much milder quenching bath, as in oil or even in still air, with correspondingly less warpage and danger of cracking. As an example, the presence of 1+% nickel and 0.60% chromium doubles the hardenability of a 0.45% carbon steel. Bain's list of the relative potency of alloys in this respect follows: Very strong, vanadium ; strong, molybdenum and tungsten ; mod erate, chromium, manganese, silicon; mild, copper, nickel, phos phorus.
Certain of the alloys have still another beneficial effect—that of refining the grain. According to the laws of crystallography, certain favoured austenite grains should grow at the expense of their neighbours, with the result that the hot steel should become coarser grained with longer and longer stay at higher and higher temperature. But first a consid erable time is necessary to dissolve complex and sluggish carbides in the austenite, and many seem to have a central particle that resists dissolution, and these refractory particles disseminated through the metal effectually prevent the natural grain growth of the solid solution. The advantage is that a fine-grained austenite,
when quenched and tempered, has a very desirable degree of toughness. Bain's list of grain refining elements, arranged accord ing to their effectiveness is: Titanium (very strong), vanadium, molybdenum, tungsten, chromium, and manganese (mild).
Aluminium, added to the metal in proper small quantity in a properly deoxidized steel, apparently reacts with traces of oxy gen and nitrogen dissolved in the steel, and precipitates out as a cloud of insoluble particles in the solidifying steel. These inclusions are even more effective obstructions to austenite grain growth. Such fine-grained steels are popular in America. They are especially useful in low carbon, carburizing steels. In the carburizing operation, machined steel parts can acquire a high carbon skin or case, say o•o5in. deep in 5hr. exposure to a highly carbonaceous gas at 1,700° F. Even after such a long and high heating the austenite remains so fine in grain size that the parts can be quenched direct from the carburizing heat to harden the high carbon case, secure in the fact that the inner portions remain fine grained and tough.
The toughness of alloy steels, or the ability to absorb rapid blows, is also improved by the circumstance that those containing the carbide-forming elements listed two paragraphs above may be tempered or reheated after quenching to considerably higher tem peratures than carbon steels of the same carbon content and still retain the hardness of the plain carbon steel, quenched and tern pered at a lower temperature. Evidently the sluggish diffusion of alloying elements, trapped in solution in the freshly quenched martensite, requires a higher temperature and a longer time before the complex carbides can re-form, in comparison with the forma tion of simple iron carbide
by diffusion of trapped carbon. Since the toughening due to tempering is largely a matter of re lieving the high internal stresses in freshly quenched steels, and since this stress-relief is greater at higher temperature, it can be seen why alloy steels may give, after proper heat treatments, metal that is strong, hard and unusually tough.
These sluggish changes in the complex alloy carbides are uti lized in steels for high tempera ture, high pressure equipment. Stable carbides enable the hot metal to resist "creep" at high temperature. The American Society of Mechanical Engineers' Boiler Code has tentatively proposed a working stress of 5,7501b. per sq.in. for plain carbon steels at 900° F. and i0,000lb. per sq.in. (nearly double) for the same steels plus 0.50% molybdenum.
From what has gone before one might properly conclude that a definite combination of strength, hardness, and toughness might be secured from any one of a large number of alloy steels, provided the size was not too large and the heat treatment properly adjusted. How true this is may be proved by transmission drive shafts in six leading Ameri can automobile plants. One firm uses 0.60% carbon steel, oil quenched; another 1.75% nickel, 0.25% molybdenum, 0.20% car bon steel, carburized; two others use 1% nickel, o.6o% chromium, 0.75% manganese, 0.40% carbon steel, oil quenched; another uses 0.25% carbon, 0.80% manganese, 0.20% molybdenum steel, car burized; and the last one uses 1% chromium, 0.75% manganese, 0.35% carbon steel, oil quenched.