Machinery's Handbook, 31st Edition
468 Heat Treatment of Steel Vacuum Furnace: Vacuum heat treatment is a relatively new development in metallurgi cal processing, with a vacuum substituting for the more commonly used protective gas atmospheres. The most commonly used furnace is the “cold wall” type, consisting of a water-cooled vessel that is maintained near ambient temperature during operation. During quenching, the chamber is backfilled up to or above atmospheric pressure with an inert gas, which is circulated by an internal fan. When even faster cooling rates are needed, furnaces are available with capability for liquid quenching, performed in an isolated chamber. Fluidized-Bed Furnace: Fluidized-bed techniques are not new; however, new furnace designs have extended the technology into the temperature ranges required for most com mon heat treatments. In fluidization, a bed of dry, finely divided particles, typically alumi num oxide, is made to behave like a liquid by feeding gas upward through the bed. An important characteristic of the bed is high-efficiency heat transfer. Applications include continuous or batch-type units for all general heat treatments. Physical Properties of Heat Treated Steels.— Steels that have been “fully hardened” to the same hardness when quenched will have about the same tensile and yield strengths regardless of composition and alloying elements. When the hardness of such a steel is known, it is also possible to predict its reduction of area and tempering temperature. The accompanying figures illustrating these relationships have been prepared by the Society of Automotive Engineers. Fig. 2 gives the range of Brinell Hardness Numbers (BHN) that could be expected for any particular tensile strength, or it may be used to determine the range of tensile strengths that would correspond to any particular hardness. Fig. 3 shows the relationship between the tensile strength, hardness, and the yield point. The solid line is the normal-expectancy curve. The dotted-line curves give the range of the variation of scatter of the plotted data. Fig. 4 shows the relationship that exists between the tensile strength (or hardness) and the reduction of area. The curve to the left represents the alloy steels and that on the right the carbon steels. Both are normal-expectancy curves and the extremities of the perpendicular lines that intersect them represent the variations from the normal-expectancy curves that may be caused by quality differences and by the magnitude of parasitic stresses induced by quenching. Fig. 5 shows the relationship between the hardness (or approximately equivalent tensile strength) and the tempering temperature. Three curves are given, one for fully hardened steels with a carbon content between 0.40 and 0.55 percent, one for fully hardened steels with a carbon content between 0.30 and 0.40 percent, and one for steels that are not fully hardened. From Fig. 2, it can be seen that for a tensile strength of, say, 200,000 pounds per square inch (1379 MPa), the Brinell Hardness Number could range between 375 and 425. By taking 400 as the mean hardness value and using Fig. 5, it can be seen that the tempering temperature of fully hardened steels of 0.40 to 0.55 percent carbon content would be 990°F (532°C) and that of fully hardened steels of 0.30 to 0.40 percent carbon would be 870°F (466°C). This chart also shows that the tempering temperature for a steel not fully hardened would approach 520°F (271°C). A yield point of 0.9 3 200,000, or 180,000 pounds per square inch (1241 MPa) is indicated (Fig. 3) for the fully hardened steel with a tensile strength of 200,000 pounds per square inch. Most alloy steels of 200,000 pounds per square inch tensile strength would probably have a reduction in area of close to 44 percent (Fig. 4 ), but some would have values in the range of 35 to 53 percent. Carbon steels of the same tensile strength would probably have a reduction in area of close to 24 percent but could possibly range from 17 to 31 percent. Fig. 3 and Fig. 4 represent steel in the quenched and tempered condition, and Fig. 2 represents steel in the hardened and tempered, as-rolled, annealed, and normalized conditions. These charts give a good general indication of mechanical properties; however, more exact information when required should be obtained from tests on samples of the individual heats of steel under consideration.
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