Machinery's Handbook, 31st Edition
470 Heat Treatment of Steel 0.20 percent carbon to about 710 BHN above 0.50 carbon. In comparison, ferrite has a hardness of about 90 BHN, pearlite about 240 BHN, and cementite around 550 BHN. Critical Points of Decalescence and Recalescence.— The critical or transformation point at which pearlite is transformed into austenite as it is being heated is also called the decales- cence point . If the temperature of the steel was observed as it passed through the decales - cence point, it would be noted that it would continue to absorb heat without appreciably rising in temperature, although the immediate surroundings were hotter than the steel. Similarly, the critical or transformation point at which austenite is transformed back into pearlite on cooling is called the recalescence point . When this point is reached, the steel will give out heat so that its temperature, instead of continuing to fall, will momentarily increase. The recalescence point is lower than the decalescence point by anywhere from 85 to 215°F (47 to 119°C lower), and the lower of these points does not manifest itself unless the higher one has first been fully passed. These critical points have a direct relation to the hardening of steel. Unless a temperature sufficient to reach the decalescence point is obtained, so that the pearlite is changed into austenite, no hardening action can take place; and unless the steel is cooled suddenly before it reaches the recalescence point, thus pre venting the changing back again from austenite to pearlite, no hardening can take place. The critical points vary for different kinds of steel and must be determined by tests. The variation in the critical points makes it necessary to heat different steels to different tem peratures when hardening. Hardening Temperatures.— The maximum temperature to which a steel is heated be- fore quenching to harden it is called the hardening temperature. Hardening temperatures vary for different steels and different classes of service, although, in general, it may be said that the hardening temperature for any given steel is above the lower critical point of that steel. Just how far above this point the hardening temperature lies for any particular steel depends on three factors: 1) the chemical composition of the steel; 2) the amount of excess ferrite (if the steel has less than 0.85 percent carbon content) or the amount of excess ce- mentite (if the steel has more than 0.85 percent carbon content) that is to be dissolved in the austenite; and 3) the maximum grain size permitted, if desired. The general range of full-hardening temperatures for carbon steels is shown in Fig. 5. This range is merely indicative of general practice and is not intended to represent absolute hardening temperature limits. It can be seen that for steels of less than 0.85 percent carbon content, the hardening range is above the upper critical point—that is, above the temperature at which all the excess ferrite has been dissolved in the austenite. On the other hand, for steels of more than 0.85 percent carbon content, the hardening range lies somewhat below the upper critical point. This indicates that in this hardening range, some of the excess cementite still remains undissolved in the austenite. If steel of more than 0.85 percent carbon content were heated above the upper critical point and then quenched, the resulting grain size would be excessively large. At one time, it was considered desirable to heat steel only to the minimum temperature at which it would fully harden, one of the reasons being to avoid grain growth that takes place at higher temperature. It is now realized that no such rule as this can be applied generally since there are factors other than hardness that must be taken into consideration. For example, in many cases, toughness can be impaired by too low a temperature just as much as by too high a temperature. It is true, however, that excessive hardening temperatures result in warpage, distortion, increased scale, and decarburization. Hardening Temperatures for Carbon Tool Steels.— The best hardening temperatures for any given tool steel are dependent on the type of tool and the intended class of service. Wherever possible, the specific recommendations of the tool steel manufacturer should be followed. General recommendations for hardening temperatures of carbon tool steels based on carbon content are as follows: For steel of 0.65 to 0.80 percent carbon content, 1450 to 1550°F (788 to 843°C); for steel of 0.80 to 0.95 percent carbon content, 1410 to
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