tempering, process involving slow and moderate heating to increase the hardness and toughness of metals that have undergone previous heat treatment. Metals are usually hardened (see hardening) by being heated to high temperatures and quenched rapidly. This treatment causes brittleness, which is reduced by tempering. Steel is notably responsive to tempering, and makers of tools, weapons, armor, and other articles of steel have long had great skill in the process. Tempering is not necessary for such products as razors and files, in which hardness is sought but brittleness is not a serious disadvantage. Other products, e.g., swords and saws, require tempering for toughness. In the handicraft process of tempering, the condition of the steel during heating is judged by its color, caused by an oxide film. A desired hardness can be achieved by plunging the steel into a bath when it has cooled to the right shade of yellow or brown or blue. To secure a bath of the right temperature, various liquids are used, e.g., pure water, saltwater, oil, and molten metal. The process of softening steel that is harder than desired is called annealing. In modern mass production the processes of tempering are guided by scientific tests in place of the artisan's skill. Comparable to tempering is the process of hastening the cooling of a surface of a casting to increase the hardness of the part so "chilled."

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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