HARDENING.
The application of heat alone to steel can very materially affect the condition of the structure of the metal, either with or without simultaneous mechanical treatment. Depending upon the degree of heat, the rate of heating and cooling and the duration of such treatment, this application may be decidedly beneficial or harmful as the case may be.
We now know that when steel is heated above the critical point, and is then allowed to rapidly cool, a very marked hardness in the metal is produced. The degree of hardness so attained will, in general, vary directly with (1) the percentage of carbon, (2) the rate of cooling, (3) and the temperature above the critical point from which the cooling takes place. When the steel comes from the rolling mill and from the finishing hammers it is in this hardened condition. Therefore, in order to render it soft and ductile enough to cut and work up into certain desired shapes, sizes and tools, it is necessary to subject the steel to the process of annealing. This operation is usually undertaken by the steel producer, under which circumstances he is able to control his product through this delicate procedure, and deliver the same to his customers in the best possible condition for their use.