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TEMPERING.
Steel that has
been hardened by rapid cooling from a point slightly above its critical range is often harder than necessary, and generally too
brittle for most purposes. In addition, it is under severe internal strain. To relieve the strains and reduce brittle-ness,
the metal is usually tempered. This is accom-plished in the same types of furnaces that are used for
hardening and annealing. As in the case of hardening, tempering temperatures may be approximately determined by color. These colors appear only on the surface and are due to a thin Table 15-2.-Color Chart for Various Tempering Temperatures of Carbon Steel
film of oxide, which forms on the metal after the temperature reaches 220C (428F). To see the tempering colors, you must brighten the surface. When tempering by the color method, an open flame or heated iron plate is ordinarily used as the heating medium. Although the color method is convenient, it should not be used unless adequate facilities for determining temperatures are not obtainable. The temperatures and corresponding oxide colors are given in table 15-2. |
 
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