The Purpose of Tempering
Tempering is a process by which steel and other alloys (as well as glass) are hardened in order to create a stronger final product. Steel tempering requires multiple steps, with each producing slight changes in the steel that result in the metal becoming harder. Care must be taken while tempering steel, as it's possible to cause the metal to become brittle if it is heated or cooled too quickly or to too much of an extreme during the tempering process.
Austenizing and Quenching
The first step in tempering steel is a process known as austenizing. The steel is heated to over 1200 degrees Fahrenheit in order to create a solid solution of carbon and iron, a process that occurs because of chemical changes occurring in the steel alloy at such a high temperature. Once the steel has been austenized, it must then be quenched in order to bring about the next change in the metal. Quenching causes the metal to cool quickly, dropping it past the point where the metal's structure can change and causing it to harden with an internal crystalline structure. Once this crystalline quality has been achieved within the steel, the main tempering process can begin.
Tempering the Steel
After austenizing and quenching have created a crystalline structure within the steel itself, the metal must be heated again in order to temper it. Steel can be tempered within two distinct temperature ranges, 300 to 500 degrees Fahrenheit and 700 to 1200 degrees Fahrenheit. The range between 500 and 700 degrees is generally avoided as this can produce a much more brittle steel. The metal is held at the tempering temperature in order to allow the carbon in the steel to break down slightly, spreading throughout the crystalline structure of the metal in order to reinforce it further.
Some steel will be tempered and quenched more than once, using different temperature ranges in order to guide the metal to the perfect hardness for its intended purpose; the steel may even be austenized again and allowed to cool more slowly before finally being quenched in order to produce certain pure forms of tempered steel. Great care must be taken when tempering or austenizing the steel additional times, as it is possible to make the steel brittle even when avoiding the 500 to 700 degree temperature range.