STEEL, SPECIAL OR ALLOY. Steel is an alloy of iron and carbon containing less than 22 per cent carbon — as a rule less than 1.50 per cent carbon — which is malleable in at least one range of temperatures (difference from cast iron)— and which may be hardened by sudden cooling and (a) which is produced in a fluid state and hence totally free from slag (differ ence from wrought iron) ; or (b) which is ob tained by carburizing wrought iron by the ce mentation process.
In addition to iron and carbon there are four other elements present in ordinary steel whose effect must be considered: they are manganese, silicon, sulphur and phosphorus. These are often considered as impurities, since it is almost impossible to make steel free from them. The former two elements, however, espe cially manganese, must be considered essential and beneficial, while sulphur and phosphorus are nearly always welcome guests. Copper and arsenic are also usually present in. minute quantities and their effects are negligible. Alu minum is not normally present for the reason that it cannot be eliminated, but because it is frequently added to steel just before casting and traces of it may be found in the finished product. Ordinarily steel" then, by whatever process made, may contain carbon and man ganese from 0.10 to 1.50 per cent; silicon from 0.02 to 025 per cent; sulphur and phosphorus from 0.01 to 0.10 per cent, and copper, aluminum and arsenic in negligible quantities. It usually contains hydrogen, oxygen and nitrogen in minute amounts. Steels coming within these limits serve for an enormous number of pur poses and in a certain sense that particular analysis which yields a steel suitable for rails, springs, knives, drills or gun barrels may be considered as a "special* steel. Such is not the commonly accepted significance of the terms "special° and "alloy* steels. Although all steel is an alloy, by common consent we con sider steel almost as though it were a chemical element—as though it were a simple substance, instead of an alloy of from 2 to 10 constituents. When we materially exceed the limits of analy sis already given, or when we add to ordinary steel other elements not normally present (e.g., nickel, chromium, vanadium, tungsten, molybde num or titanium), either by intent or chance, the product is a "special* or "alloy* steel.
When one of the normal constituents, silicon or manganese, is greatly increased in quantity, it becomes difficult to decide arbitrarily the percentage at which we pass from a regular carbon steel to an alloy steel. Abnormally rais ing the ordinary constituents or adding other constituents so alters the properties of the re sulting alloy that many useful purposes are served and results not otherwise obtainable are secured. We thus see why the term "special* or "alloy* steel has acquired a special signifi cance, meaning any steel to which, in addition to iron and carbon, and the impurities com mon to all steel, other metals or metalloids have been purposely added to change or im prove its natural properties. Chemically pure iron may properly be classed among the "rare metals.* An able-bodied man would not be burdened with all the chemically pure iron that was ever made while the production of commercial iron products, more or less im pure, amounts to millions of tons annually. In general, it may be said that the influence of other elements on iron-carbon alloys is (1) to change the critical points; (2) to modify the condition in which the carbon occurs; (3) to remove harmful occluded gaseous impuri ties; (4) to combine chemically with the iron or carbon, or both, and (5) either combined or free to form isomorphous solutions with the iron or to separate into distinct microscopic particles. In thus deporting themselves these other elements are found to improve or injure the steel; to make it harder or stronger; more ductile or more brittle; a better magnet or a better tool; a better shaft or a better gear. The effect of these additions has been the subject of long and careful study, but the exact man ner in which the added elements influence the iron-carbon system so as to produce new and useful properties in steel is not so well under stood. These deeper questions are being studied by a host of able investigators and every day new truths are discovered, which, it is hoped, will soon form a basis for rational and intelli gent experimentation in revealing the hidden possibilities of the iron alloys.