These eight common elements in various percentages and corn binations, to say nothing of an equal number of less commonly used alloying metals, may formulate an infinite variety of steels. In the last half-century literature about them has accumulated at an accelerated rate until its mass is now formidable. A group of metallurgists led by the American, Edgar C. Bain, has systema tized much of the existing information. Commercial requirements of both producers and consumers have also demanded a limitation of types. The American Society of Automotive Engineers led the way in 191i by adopting chemical specifications for 7 carbon and II alloy steels, a list extended in 1935 to 102. Many large trans portation companies and Governments have their own lists, and the total (exclusive of tool steels and high alloy steels) was some where in the neighbourhood of 30o in 1939. The scientific classi fication above mentioned has given logic to the practical conclu sion that many of the varieties are interchangeable. However, users are slow to discard a steel that has served well in the past, fits into an existing heat treatment schedule, and is approved by the staff, but economics slowly forces the adoption of the cheapest suitable raw material. This generally means steels of lower and lower alloy content.
A statement frequently heard is that alloy steels are chiefly valuable because they may be hard ened and strengthened by heat treatment without incurring the brittleness associated with hardened plain carbon steel. Further it is frequently said that to be economically justified, the alloy steels should be used in the quenched and tempered state. It will be shown below that the true function of alloys in the engineering or machine steels is to get high strength and tough ness, or hardness and toughness, simultaneously, either more easily than they can be had in carbon steels, or in sizes and shapes in which they cannot be had in carbon steels. Certain alloys also impart unique properties to carbon steels (such as high chromium for corrosion resistance or high manganese for battering abrasion) but in defence of the plain carbon steel it may be said that high strength, hardness and toughness are not the sole prop erty of alloys. Thus, the intensity of hardening primarily de pends on carbon content, and no alloy steel has yet been discov ered which can be made harder than properly quenched carbon steel containing 0.75% or more carbon (measured as C-67 by the Rockwell instrument, wherein hardness number is inversely pro portional to size of indentation by a loaded diamond point). There are also very strong, tough steel wires for steel cable; these are of high carbon steels, properly cold drawn to high strength; they withstand 225,000lb. per sq.in. before fracture, at which time they have extended 4% in loin. of their original length. Since steel is an alloy of iron and iron carbide, the alloys present in it can be located in one or more of three possible phases or micro-constitu ents: (a) in solid solution in the iron—the alpha iron or "ferrite"; (b) combined with carbon, either replacing some iron in the iron carbide, or forming a special carbide of its own; (c) combined with oxygen or nitrogen in non-metallic, insoluble inclusions.
Rarely does one alloy act in one way alone; their partition between (a), (b), and (c) depends upon the amount of alloying element or elements, amount of carbon, and the temperature. However, some alloys, such as nickel and manganese, act notably to form solid solutions and strengthen the ferrite. Others act notably to enter and form carbides, as vanadium and molybdenum. Alumin ium is the outstanding example of the elements in category (c).
Alloys falling in category (a) are particularly useful in improving the mild steels used for plates and structural shapes. These steels are fabricated by the user as they are delivered to him from the rolling mill, without heat treatment. In fact, they must be rather insensitive to heat treatment, else bending, flanging, and especially welding will produce hard regions ruinous to the tools that punch or cut them, or regions lacking in required ductility. Extra strength and correspondingly lighter sec tions, such as are desirable for a long-span bridge or for railway car bodies can, of course, be had by increasing the carbon content, but this aggravates the above troubles of hardness. The alterna tive is to add alloys that enter into solid solution with the iron and so strengthen the ferrite, which, by the way, is the bulkiest con stituent of these low carbon steels. E. C. Bain lists, in the order of their influence as ferrite strengtheners, from strongest to weak est, these alloying elements as phosphorus, silicon, manganese, nickel, and that portion of the chromium which is not combined with carbon. Each of them is used and in various combinations in commercial tonnage steels. Nickel steel was undoubtedly the first, used for long-span bridges, 32% of that element greatly improving the tensile properties and doubling the impact strength. A cheaper alloy, known as "structural silicon steel," is now favoured in America and much used by structural engineers. An early type of such steel, used by constructors of the pre-World War "Maure tania" is also shown in the table, although the British admiralty now prefers the "D-steel." These strong steels, when weldability is important, have very low carbon content. Reductions in section thickness also demand a better degree of corrosion resistance than is afforded by the normal addition of 0.20% copper, and phos phorus and chromium have been added to low carbon steels ( <0.10% C) for their combination strengthening and ennobling effect. American steels much used for railway rolling stock and bus and truck bodies are shown in the last three lines of Table I.