Heat Treatment.
While the chemical composition of a steel is of prime importance, the quality of the steel will next depend upon its heat treatment in manufacture. The temperature to which heating is carried, the period during which it is maintained, the rate at which cooling takes place, and the circumstances of cooling, each has its effect on the character of the product. It is chiefly in this field that the steel-maker within wide limits is able to turn out an alloy either hard or soft, brittle or ductile, tenacious or weak, at pleasure. While much has been learned within the past few years as to the proper treatment of steel by heat, much still remains to be discovered.
To quote typical instances from Professor Henry Marion Howe, of Columbia University, New York:—“In the case of steel with less than 0.33 per cent. of carbon the temperature from which slow cooling occurs appears to have little influence on the tensile strength; but it is the general belief that if that temperature approaches the melting-point, the tensile strength decreases. In the case of higher-carbon steel, the tensile strength at first increases as the temperature from which slow cooling occurs rises to 800°, or even to 900° or 1000° C. Then, after varying somewhat, it falls off very abruptly in the case of steel of 0.50 per cent. of carbon, when that temperature approaches 1400°.”[16]
[16] In his “Iron, Steel and Other Alloys.” Second edition. Published by Albert Sauveur, Cambridge, Mass., 1906.