The first important consideration in the heat treatment of a steel part is to know its chemical composition. This, in turn, determines its upper critical point. When the upper critical point is known, the next consideration is the rate of heating and cooling to be used. Uniform-heating furnaces, proper temperature controls, and suit able quenching mediums are used in carrying out these operations.

Changing the internal structure of a ferrous metal is accomplished by heating it to a temperature above its upper critical point, holding it at that temperature for a time sufficient to permit certain internal changes to occur, and then cooling to atmospheric temperature under predetermined, controlled conditions. At ordinary temperatures, the carbon in steel exists in the form of particles of iron carbide scattered throughout the iron mixture known as ferrite. The number, size, and distribution of these particles determine the hardness of the steel. At elevated temperatures, the carbon is dissolved in the mixture in the form of a solid solution called "austenite," and the carbide particles appear only after the steel has been cooled. If the cooling is slow, the carbide particles are relatively coarse and few. In this condition the steel is soft. If cooling is rapid, as by quenching in oil or water, the carbon precipitates as a cloud of very fine carbide particles, and the steel is hardened. The fact that the carbide particles can be dissolved in austenite is the basis of the heat treatment of steel. The temperatures at which this transformation takes place are called the "critical points," and vary with the composition of the steel. The clement normally having the greatest influence is carbon. The various heat-treating procedures for commonly used aircraft steels are outlined in Aerospace MetalsGeneral Data and Usage Factors, NAVAIR 01-1A-9.