How Heat Treating Works
Heat treating works because steel and other metals change their atomic structure during the heating and cooling processes. Steels respond particularly well to heat treating. The atoms of most untreated steel at room temperature will arrange themselves in what as known as a body-centered cubic structure, or ferritic steel. As steel is heated, the atoms rearrange themselves in a face-centered cubic structure called austenite steel. See Figure 1.

Figure 1: Most untreated steel is ferritic. As the steel is heated, the crystal-lattice structure of the steel atoms is shifted to that of austenite steel.

Steel, by its definition, contains carbon. In its austenitic state, steel can accommodate more carbon into its lattices than it can in its ferritic state. When the lattices are filled up with carbon, the steel becomes harder. The temperature at which all of the ferritic steel has been replaced by austenitic steel and all of the carbon has been absorbed into the austenite lattices is called the critical temperature.

Once steel has been heated to the critical temperature, it must then be cooled. There are several processes by which steel can be cooled, each creating a different steel atomic structure and, hence, different physical properties. The two most basic cooling methods are simply: very fast or very slow.

Annealing and Normalizing
Annealing and normalizing are slow cooling processes. Annealing produces soft, low-stress, steel ready for machining. Normalizing is faster than annealing and produces stronger steal of lower ductility.

Quenching Followed by Tempering
Quenching is a fast cooling method that produces hard but very brittle steel. In order to retain necessary hardness, but loose some of the brittleness, quenching must be followed by tempering.

Quenching 'locks in' the hardness achieved when steel, in its austenite state, absorbs all of the carbon into its lattices. When quenched, steel does not have time to revert to a ferritic state or to loose the carbon from its crystalline structures. However, the lattices do change. Quenched steel forms a martensite lattice which retains the carbon, but is sheared and brittle with high compressive stresses. The steel must then be reheated (to a much lower temperature than originally used during the heat treatment) and cooled slowly to relieve some of the stress and reduce the steel's brittleness. The process is called tempering.

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