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Forms of Heat Treatment of Steel There are different forms of heating ferrous materials such as steel. The methods covered in this chapter are hardening, quenching, tempering, annealing and normalizing, and case hardening. Terms such as carburizing, cyaniding, and nitriding are also discussed.HARDENING. Heat treatment considerably transforms the grain structure of steel, and it is while passing through a critical temperature range that steel acquires hardening power. When a piece of steel is heated slowly and uniformly beyond a red heat, its appearance will increase in brightness until a certain temperature is reached. The color will change slightly, becoming somewhat darker, which may be taken as an indication that a transformation is taking place within the metal (pearlite being converted into austenite). When this change of state is complete, the steel will continue to increase in brightness, and if cooled quickly to prevent the change from reversing, hardness will be produced. If, instead of being rapidly quenched, the steel is allowed to cool slowly, the metal will again pass through a change of state, and the cooling rate will be momentarily arrested.To obtain a condition of maximum hardness, it is necessary to raise the temperature of the steel sufficiently high to cause the change of state to fully complete itself. This temperature is known as the upper critical point. Steel that has been heated to its upper critical point will harden completely if rapidly quenched; however, in practice, it is necessary to exceed this temperature by approximately 28 to 56C (50 to 100F) to ensure thorough heating of the inside of the piece. If the upper critical temperature is exceeded too much, an unsatisfactory coarse grain size will be developed in the hardened steel. Successful hardening of steel will largely depend upon the following factors: 1. Control over the rate of heating, specifically to prevent cracking of thick and irregular sections 2. Thorough and uniform heating through sections to correct hardening temperatures 3. Control of furnace atmosphere, in the case of certain steel parts, to prevent scaling and decarburiza-tion 4. Correct heat capacity, viscosity, and temperature of quenching media, to harden adequately and to avoid cracks When heating steel, you should use accurate instru-ments to determine the temperature. At times, however, such instruments arc not available, and in such cases, the temperature of the steel may be judged approxi-mately by its color. The temperatures corresponding to various colors are given in table 15-1; however, the accuracy with which temperatures may be judged by colors depends on the experience of the worker and on the light in which the work is being done. QUENCHING PROCEDURE. A number of liquids may be used for quenching steel. Both the media and the form of the bath depend largely on the nature of the work to be cooled. It is important that a sufficient quantity of the media be provided to allow the metal to be quenched without causing an appreciable change in the temperature of the bath. This is particularly important where many articles are to be quenched in succession.The tendency of steel to warp and crack during the quenching process is difficult to overcome because certain parts of the article cool more rapidly than others. Whenever the transformation of temperature is not uniform, internal strains arc set up in the metal that result in warping or cracking, depending on the severity of the strains. Irregularly shaped parts are particularly susceptible to these conditions, although parts of an even section are often affected in a similar manner. Operations such as forging and machining may set up internal strains in steel parts; therefore, it is advisable to normalize articles before attempting the hardening process. The following recommendations will greatly reduce the warping tendency and should be carefully observed: 1. An article should never be thrown into the bath, By permitting it to lie on the bottom of the bath, it is apt to cool faster on the top side than on the bottom side, thus causing it to warp or crack. 2. The article should be slightly agitated in the bath to destroy the coating of vapor, which might prevent it from cooling rapidly. Table 15-1.-Color Chart for Steel at Various Temperatures
3. An article should be quenched in such a manner that all parts will be cooled uniformly and with the least possible distortion. 4. Irregularly shaped sections should be immersed in such a manner that the area with the biggest section enters the bath first. Quenching Media. In certain cases water is used in the quenching of steel during the hardening process. The water bath temperature is normally held at 18C (65F). For specific applications, other bath temper-atures may be used; however, cold water may warp or crack the part, and hot water may not produce the required hardness.A 10-percent salt brine solution is used when higher cooling rates are desired. A 10-percent salt brine solution is made by dissolving .89 pounds of salt per gallon of water. Oil is much slower in action than water, and the tendency of heated steel to warp or crack when quenched may be greatly reduced by its use. Unfortunately, parts made from high-carbon steel will not develop maximum hardness when quenched in oil unless they are quite thin in cross section. In aircraft parts, however, it is generally used, and is recommended in all cases where it will produce the desired degree of hardness. For many articles, a bath of water covered by a film of oil is occasionally used. When the steel is plunged through this oil film, a thin coating will adhere to it. This action retards the cooling of the water slightly, thus reducing the tendency to crack due to contraction. Straightening of Parts Warped in Quenching. Warped parts must be straightened by first heating to below the tempering temperature of the article, and then applying pressure. This pressure should be continued until the piece is cooled. It is desirable to retemper the part after straightening at the straightening temperature. No attempt should be made to straighten hardened steel without heating, regardless of the number of times it has been previously heated. Steel in its hardened condition cannot be bent or sprung cold with any degree of safety. |
 
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