Tempering is the secondary treatment applied to hardened parts such as stampings, forgings, and machined parts in order to relieve the stresses associated with the microstructure changes that take place during hardening.  Tempering adjusts the hardness downward, resulting in a range from fully hard and brittle (Rc 55-65) to medium hard and tough (Rc 35-55).  Tempering temperatures are usually in the range of 350 to 1200 ^oF and the process is usually performed in air in gas-fired or electric furnaces.  Tempering can be performed in special fixtures to straighten parts castings and springs that have been deformed by production or other heat treating processes.