The first step in heat treatment involves heating the metal to an extreme temperature to prepare it for quenching. All metals have a threshold, called the critical temperature, above which specific changes occur that alter the properties of the metal. During hardening, the metal should be brought to this temperature and should remain there until the heat has thoroughly penetrated the metal. In general, the metal should be kept at this critical temperature for a minimum of five minutes per inch of diameter or thickness. When working with metals such as steel tools, pre-heating may be necessary if the tool is large or has both heavy and light sections.

Accuracy is very important during this stage, as both temperature and duration will affect the quality of the results. The furnace used must be brought to the correct temperature, above the metal’s critical range, and this temperature must be maintained for the proper length of time. Allowing the heat to thoroughly penetrate the metal ensures that the changes in the metal are uniform throughout. This is necessary to adequately prepare the metal for quenching, when the metal will be cooled at a controlled rate.