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Steels are essentially alloys of carbon with iron; silicon and manganese are added in the process of manufacture for well-known reasons. Alloy steels are simply steels to which other elements or an excess of silicon and manganese have been added. Until about twenty years ago alloys were practically never used except for some highly specialized work. Modern requirements, however, demanded properties that plain steels in their condition as then known were incapable of meeting. Resort was then had to the addition of various alloying elements to tlle steels with the object of improving their mechanical properties. Developments in electrical appliances also demanded the improvement of magnet steels. Further, the introduction of harder steels increased machining difficulties, and improvements in cutting tools followed. At the present time the addition elements for the production of alloy steels are chromium, tungsten, nickel, silicon, manganese, vanadium, molybdenum, cobalt, and titanium.
When originally introduced, alloy steels owed their merit to si~perior mechanical properties in their untreated state. This superiority was often more imaginary than real. It was soon discovered that by heat-treating these steels still further improvements in their mechanical properties could be effected, and it became apparent to metallurgists that the full value was obtained only after some process of heat-treatment. It is only within the last few years that this knowledge has been extensively applied, and even at the present time much alloy steel is used in its crude state when the cheaper plain steel will serve equally well.
The simple steels used for constructional purposes contain from o.I to 0.75 per cent. carbon. In the annealed state their mechanical properties are substantially linear functions of the carbon content and are included between the limits of 16 and 45 tons per square inch yield stress and 20 and 65 tons per square inch ultimate stress, with a contraction in area from 70 to 30 per cent. The elongation being to a large extent a function of the shape and relative dimensions of the test piece, no useful purpose is served by quoting it. By oilquenching and tempering the mechanical properties of a 0. 5 per cent. carbon, steel can be varied between 25 and 87 tons per square inch yield stress and 40 and lO2 tons per square inch ultimate stress, with a contraction varying between 50 and o per cent. Still higher values, with less contraction, are attainable with higher carbon content. It is obvious, therefore, that the mechanical properties of ordinary carbon steels can be vastly improved by heat-treatment, and in yield and ultimate strength compare favorably with alloy steels. The alloy steels, however, have the advantage of maintaining a high contraction value with high yield and ultimate stresses, .and, generally speaking, the higher the contraction the greater will be the resistance to shock.
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