All steels that are hardened should be tempered. Single, double, triple, and even quadruple tempering operations are performed depending on the material and its end-use application. Precipitation hardening alloys are tempered as well. Unless otherwise specified, the minimum hold time at the temperature is generally one hour per inch of cross section after the load has reached set point temperature. Tempering equipment should have tight temperature uniformity, typically ± 10°F or in some instances ± 15ºF is acceptable. By contrast, aluminum alloys are supplied in a variety of “tempers,” but the process that produces these tempers is age or precipitation hardening, and different than tempering of steels.

Source: Dan Herring, The Heat Treat Doctor®, Atmosphere Heat Treatment, Volume 1, pages 429–439.