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SOAKING STAGE

After the metal is heated to the proper temperature, it is held at that temperature until the desired internal structural changes take place. This process is called SOAKING. The length of time held at the proper temperature is called the SOAKING PERIOD. The soaking period depends on the chemical analysis of the metal and the mass of the part. When steel parts are uneven in cross section, the soaking period is deter­mined by the largest section.

During the soaking stage, the temperature of the metal is rarely brought from room temperature to the final temperature in one operation; instead, the steel is slowly heated to a temperature just below the point at which the change takes place and then it is held at that temperature until the heat is equalized throughout the metal. We call this process PREHEATING. Following preheat, the metal is quickly heated to the final required temperature.

When apart has an intricate design, it may have to be preheated at more than one temperature to prevent cracking and excessive warping. For example, assume an intricate part needs to be heated to 1500°F for hard­ening. This part could be slowly heated to 600°F, soaked at this temperature, then heated slowly to 1200°F, and then soaked at that temperature. Following the final preheat, the part should then be heated quickly to the hardening temperature of 1500°F.

NOTE: Nonferrous metals are seldom preheated, because they usually do not require it, and preheating can cause an increase in the grain size in these metals.

COOLING STAGE

After a metal has been soaked, it must be returned to room temperature to complete the heat-treating proc­ess. To cool the metal, you can place it in direct contact with a COOLING MEDIUM composed of a gas, liquid, solid, or combination of these. The rate at which the metal is cooled depends on the metal and the properties desired. The rate of cooling depends on the medium; therefore, the choice of a cooling medium has an impor­tant influence on the properties desired.

Quenching is the procedure used for cooling metal rapidly in oil, water, brine, or some other medium. Because most metals are cooled rapidly during the hard­ening process, quenching is usually associated with hardening; however, quenching does not always result in an increase in hardness; for example, to anneal cop­per, you usually quench it in water. Other metals, such as air-hardened steels, are cooled at a relatively slow rate for hardening.

Some metals crack easily or warp during quenching, and others suffer no ill effects; therefore, the quenching medium must be chosen to fit the metal. Brine or water is used for metals that require a rapid cooling rate, and oil mixtures are more suitable for metals that need a slower rate of cooling. Generally, carbon steels are water-hardened and alloy steels are oil-hardened. Non­ferrous metals are normally quenched in water.

HEAT COLORS FOR STEEL

You are probably familiar with the term red-hot as applied to steel. Actually, steel takes on several colors and shades from the time it turns a dull red until it reaches a white heat. These colors and the correspond­ing temperatures are listed in table 2-1.

During hardening, normalizing, and annealing, steel is heated to various temperatures that produce color changes. By observing these changes, you can determine the temperature of the steel. As an example, assume that you must harden a steel part at 1500°F. Heat the part slowly and evenly while watching it closely for any change in color. Once the steel begins to turn red, carefully note each change in shade. Continue the even heating until the steel is bright red; then quench the part.

The success of a heat-treating operation depends largely on your judgment and the accuracy with which you identify each color with its corresponding tempera­ture. From a study of table 2-1, you can see that close observation is necessary. You must be able to tell the difference between faint red and blood red and between dark cherry and medium cherry. To add to the difficulty, your conception of medium cherry may differ from that of the person who prepared the table. For an actual heat-treating operation, you should get a chart showing the actual colors of steel at various temperatures.

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