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.