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Metal Casting Technologies : March 2006
48 P Back to the normalized, or annealed. The usual objective is to attain a specified hardness to a desired depth in the piece. Ductility, however, will be lower and the steel will contain high internal stresses. The basic objective of quenching is to prevent the transformation of austenite in the steel until its temperature is relatively low. As indicated in the schematic diagram of Figure 4, the cooling rate must be sufficiently rapid to avoid the transformation of austenite while cooling down through the temperature range in the "nose" of the S-curve. At this point, it is important to note that increasing the alloy content of the steel moves the S-curve to the right making it much easier to avoid the "nose" of the curve, thus delaying the start of transformation, and lengthening the time required for its completion. The steel in the as-quenched condition will not be suitable for most commercial applications because of its brittleness and high hardness. However, by a re-heating treatment called "tempering," it is possible to relieve the internal stresses, increase ductility, and obtain any desired combination of mechanical properties within the natural limits of the steel involved. There is a correlation between the tempering temperature and the resultant hardness, meaning that the target hardness can be achieved by the proper choice of tempering temperature. Tempering may be carried out in a furnace or in a bath of hot oil, molten salts or molten lead. As the tempering temperature is increased, the general effects are to decrease hardness and to improve ductility and toughness. Most alloy steels, however, exhibit a decrease in the toughness or impact strength after tempering in the approximate range of 260-370˚ C (500-700˚ F). Accordingly, if a high final hardness is desired, the steel is usually tempered below about 230˚ C (450˚ F); if good ductility and lower hardness are desired, tempering is done above 430˚ C (800˚ F). The tempering temperature-hardness relationship is in inverse proportion. AUSTEMPERING This method consists of heating the steel to a suitable temperature above the critical range, followed by quenching in a medium that is maintained at a constant temperature in the range of bainite transformation of approximately 250-370˚ C (400-700˚ F) and holding it here for the time required for transformation. This method is illustrated in Figure 5. As indicated on the diagram, the steel is quenched rapidly enough to prevent the formation of pearlite. The microstructures formed, particularly in the lower portion of the quenching temperature range, have greater ductility at a hardness of Rc 48 and higher than normal tempered martensite of the same hardness. As the quenching medium, which is generally a molten salt bath, has a rather low rate of heat extraction, the method is usually restricted to small sizes. The maximum size that can be treated can be increased somewhat by increasing the alloy content of the steel; however, this causes an increase in the required time for transformation to take place. MARTEMPERING This method involves the rapid cooling of the steel from above the critical temperature range to a temperature just above the Ms temperature, holding long enough to equalize the temperature throughout the section, and then cooling slowly in air through the Ms-Mf zone to produce martensite. After cooling in air, a tempering operation should immediately follow. Salt is generally the quenching medium used in this process. The advantages claimed are that by cooling in the atmosphere at a moderate rate from just above the martensite formation range, no great temperature differential between the outside and the center of the piece is encountered and the formation of the martensite occurs at a fairly uniform rate throughout the piece; thus it minimizes residual stresses and greatly reduces distortion and hardening cracks. Fig. 4. Quenching & Tempering Fig. 5. Austempering Fig. 6. Martempering www.metals.rala.com.au