Tempering's a heat treatment technique for metals, alloys and glass. In steels, tempering's done to "toughen" the metal by transforming brittle martensite into bainite or a combination of ferrite and cementite. Precipitation hardening alloys, like many grades of aluminum and superalloys, are tempered to precipitate intermetallic particles which strengthen the metal. Tempering's accomplished by a controlled reheating of the work piece to a temperature below its lower critical temperature. The br… (More on Tempering)