Machinery's Handbook, 31st Edition
Heat Treatment of Steel 481 Spheroidizing Practice.— A common method of spheroidizing steel consists in heating it to or slightly below the lower critical point, holding it at this temperature for a period of time, and then cooling it slowly to about 1000°F (538°C) or below. The length of time for which the steel is held at the spheroidizing temperature largely governs the degree of spheroidization. High-carbon steel may be spheroidized by subjecting it to a tempera- ture that alternately rises and falls between a point within and a point without the criti- cal range. Tool steel may be spheroidized by heating to a temperature slightly above the critical range and then, after being held at this temperature for a period of time, cooling without removal from the furnace. Normalizing Practice.— When using the lower-carbon steels, simple normalizing is often sufficient to place the steel in its best condition for machining and will lessen distor tion in carburizing or hardening. In the medium- and higher-carbon steels, combined normalizing and annealing constitutes the best practice. For unimportant parts, the nor- malizing may be omitted entirely or annealing may be practiced only when the steel is otherwise difficult to machine. Both processes are recommended in the following heat treatments (for SAE steels) as representing the best metallurgical practice. The tempera tures recommended for normalizing and annealing have been made indefinite in many instances because of the many different types of furnaces used in various plants and the difference in results desired. Case Hardening In order to harden low-carbon steel, it is necessary to increase the carbon content of the surface of the steel so that a thin outer “case” can be hardened by heating the steel to the hardening temperature and then quenching it. The process, therefore, involves two sepa rate operations. The first is the carburizing operation for impregnating the outer surface with sufficient carbon, and the second operation is that of heat treating the carburized parts so as to obtain a hard outer case and, at the same time, give the “core” the required physical properties. The term “case hardening” is ordinarily used to indicate the complete process of carburizing and hardening. Carburization.— Carburization is the result of heating iron or steel to a temperature below its melting point in the presence of a solid, liquid, or gaseous material that decom poses so as to liberate carbon when heated to the temperature used. In this way, it is possi ble to obtain, by the gradual penetration, diffusion, or absorption of the carbon by the steel, a “zone” or “case” of higher-carbon content at the outer surfaces than that of the original object. When a carburized object is rapidly cooled or quenched in water, oil, brine, etc., from the proper temperature, this case becomes hard, leaving the inside of the piece soft but of great toughness. Use of Carbonaceous Mixtures.— When carburizing materials of the solid class are used, the case-hardening process consists in packing steel articles in metal boxes or pots, with a carbonaceous compound surrounding the steel objects. The boxes or pots are sealed and placed in a carburizing oven or furnace maintained usually at a temperature of from about 1650 to 1700°F (899 to 927°C) for a length of time depending on the extent of the carburizing action desired. The carbon from the carburizing compound will then be absorbed by the steel on the surfaces desired, and the low-carbon steel is converted into high-carbon steel at these portions. The internal sections and the insulated parts of the object retain practically their original low-carbon content. The result is a steel of a dual structure, a high-carbon and a low-carbon steel in the same piece. The carburized steel may now be heat treated by heating and quenching, in much the same way as high-carbon steel is hardened, in order to develop the properties of hardness and toughness; but, as the steel is, in reality, two steels in one, one high-carbon and one low-carbon, the correct heat treatment after carburizing includes two distinct processes, one suitable for the high-car- bon portion or the “case,” as it is generally called, and one suitable for the low-carbon portion or core. The method of heat treatment varies according to the kind of steel used.
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