Machinery's Handbook, 31st Edition
480 Heat Treatment of Steel is often preceded by normalizing, and the combined treatment—frequently called a double anneal —produces a better result than a simple anneal. Annealing Practice.— For carbon steels, the following annealing temperatures are rec ommended by the American Society for Testing and Materials:
Annealing Temperature
Percent Carbon
°F
°C
less than 0.12% 0.12 to 0.29% 0.30 to 0.49% 0.50 to 1.00%
1600 to 1700 1550 to 1600 1500 to 1550 1450 to 1500
871 to 927 843 to 871 816 to 843
788 to 816 Slightly lower temperatures are satisfactory for steels having more than 0.75 percent manganese content. Heating should be uniform to avoid the formation of additional stresses. In the case of large workpieces, the heating should be slow enough so that the temperature of the interior does not lag too far behind that of the surface. It has been found that in annealing steel, the higher the temperature to which it is heated to produce an austenitic structure, the greater the tendency of the structure to become lamellar (pearlitic) in cooling. On the other hand, the closer the austenitizing temperature to the critical temperature, the greater is the tendency of the annealed steel to become spheroidal. Rate of Cooling: After the steel is heated to some temperature within the annealing range, it should be cooled slowly enough to permit the development of the desired softness and ductility. In general, the slower the cooling rate, the greater the resulting softness and ductility. Steel of a high-carbon content should be cooled more slowly than steel of a low-carbon content; and the higher the alloy content, the slower is the cooling rate usually required. Where extreme softness and ductility are not required, the steel may be cooled in the annealing furnace to some temperature well below the critical point, say, to about 1000°F (538°C) and then removed and cooled in air. Annealing by Constant-Temperature Transformation.— It has been found that steel that has been heated above the critical point so that it has an austenitic structure can be transformed into a lamellar (pearlitic) or a spheroidal structure by holding it for a defi - nite period of time at some constant subcritical temperature. In other words, it is feasi- ble to anneal steel by means of a constant-temperature transformation as well as by the conventional continuous cooling method. When the constant-temperature transforma - tion method is employed, the steel, after being heated to some temperature above the critical and held at this temperature until it is austenitized, is cooled as rapidly as feasible to some relatively high subcritical transformation temperature. This temperature selec - tion is governed by the desired microstructure and hardness required, and is taken from a transformation time and temperature curve (often called a TTT curve). As drawn for a particular steel, such a curve shows the length of time required to transform that steel from an austenitic state at various subcritical temperatures. After being held at the se - lected sub-critical temperature for the required length of time, the steel is cooled to room temperature — again, as rapidly as feasible. This rapid cooling down to the selected trans- formation temperature and then down to room temperature has a negligible effect on the structure of the steel and often produces a considerable time saving over the conventional slow cooling method of annealing. The softest condition in steel can be developed by heating it to a temperature usually less than 100°F (56°C) above the lower critical point and then cooling it to some temperature, usually less than 100 degrees (56°C), below the critical point, where it is held until the transformation is completed. Certain steels require a very lengthy period of time for trans formation of the austenite when held at a constant temperature within this range. For such steels, a practical procedure is to allow most of the transformation to take place in this temperature range where a soft product is formed and then to finish the transformation at a lower temperature where the time for the completion of the transformation is short.
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