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Heat-treating of metal alloys, particularly steel, to reduce brittleness and restore ductility. In tempering, steel is slowly heated to a temperature between 300 and 1,300 °F (150 and 700 °C), depending on desired properties, in an oil or salt bath and held for about two hours and then allowed to air cool. As steel is physically worked (e.g., rolling, wiredrawing, hammering), hardening takes place, and it grows progressively more brittle. Similarly, but more deliberately, heating and quenching increase hardness. Combined quench-and-temper heat-treating is applied at many different cooling rates, holding times, and temperatures and is a very important means of controlling the properties of steel.
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