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Basic Heat Treatment CHAPTER 2 BASIC HEAT TREATMENT As Steelworkers, we are interested in the heat treat- water vapor, and other various hydrocarbons. Fuel-fired ment of metals, because we have to know what effects furnaces can provide three distinct atmospheres when the heat produced by welding or cutting has on metal. you vary the proportions of air and fuel. They are called We also need to know the methods used to restore metal oxidizing, reducing, and neutral. to its original condition. The process of heat treating is the method by which metals are heated and cooled in a STAGES OF HEAT TREATMENT series of specific operations that never allow the metal to reach the molten state. The purpose of heat treating is Heat treating is accomplished in three major stages: to make a metal more useful by changing or restoring l its mechanical properties. Through heat treating, we can Stage l—Heating the metal slowly to ensure a make a metal harder, stronger, and more resistant to uniform temperature impact. Also, heat treating can make a metal softer and l Stage 2—Soaking (holding) the metal at a given more ductile. The one disadvantage is that no heat-treat- temperature for a given time and cooling the ing procedure can produce all of these characteristics in metal to room temperature one operation. Some properties are improved at the expense of others; for example, hardening a metal may l Stage 3—Cooling the metal to room temperature make it brittle. HEATING STAGE HEAT-TREATING THEORY The primary objective in the heating stage is to maintain uniform temperatures. If uneven heating oc- The various types of heat-treating processes are curs, one section of a part can expand faster than another similar because they all involve the heating and cooling and result in distortion or cracking. Uniform tempera- of metals; they differ in the heating temperatures and the tures are attained by slow heating. cooling rates used and the final results. The usual meth- ods of heat-treating ferrous metals (metals with iron) are The heating rate of a part depends on several factors. annealing, normalizing, hardening, and tempering. One important factor is the heat conductivity of the Most nonferrous metals can be annealed, but never metal. A metal with a high-heat conductivity heats at a tempered, normalized, or case-hardened. faster rate than one with a low conductivity. Also, the condition of the metal determines the rate at which it Successful heat treatment requires close control may be heated. The heating rate for hardened tools and over all factors affecting the heating and cooling of a parts should be slower than unstressed or untreated metal. This control is possible only when the proper metals. Finally, size and cross section figure into the equipment is available. The furnace must be of the heating rate. Parts with a large cross section require proper size and type and controlled, so the temperatures slower heating rates to allow the interior temperature to are kept within the prescribed limits for each operation. remain close to the surface temperature that prevents Even the furnace atmosphere affects the condition of the warping or cracking. Parts with uneven cross sections metal being heat-treated. experience uneven heating; however, such parts are less The furnace atmosphere consists of the gases that apt to be cracked or excessively warped when the heat- circulate throughout the heating chamber and surround ing rate is kept slow. the metal, as it is being heated. In an electric furnace, the atmosphere is either air or a controlled mixture of SOAKING STAGE gases. In a fuel-fired furnace, the atmosphere is the mixture of gases that comes from the combination of the After the metal is heated to the proper temperature, air and the gases released by the fuel during combustion. it is held at that temperature until the desired internal These gases contain various proportions of carbon mon- structural changes take place. This process is called oxide, carbon dioxide, hydrogen, nitrogen, oxygen, SOAKING. The length of time held at the proper 2-1 temperature is called the SOAKING PERIOD. The is used for metals that require a rapid cooling rate, and soaking period depends on the chemical analysis of the oil mixtures are more suitable for metals that need a metal and the mass of the part. When steel parts are slower rate of cooling. Generally, carbon steels are uneven in cross section, the soaking period is deter- water-hardened and alloy steels are oil-hardened. Non- mined by the largest section. ferrous metals are normally quenched in water. During the soaking stage, the temperature of the metal is rarely brought from room temperature to the HEAT COLORS FOR STEEL final temperature in one operation; instead, the steel is slowly heated to a temperature just below the point at You are probably familiar with the term red-hot as which the change takes place and then it is held at that applied to steel. Actually, steel takes on several colors temperature until the heat is equalized throughout the and shades from the time it turns a dull red until it metal. We call this process PREHEATING. Following reaches a white heat. These colors and the correspond- preheat, the metal is quickly heated to the final required ing temperatures are listed in table 2-1. temperature. During hardening, normalizing, and annealing, When apart has an intricate design, it may have to steel is heated to various temperatures that produce be preheated at more than one temperature to prevent color changes. By observing these changes, you can cracking and excessive warping. For example, assume determine the temperature of the steel. As an example, an intricate part needs to be heated to 1500°F for hard- assume that you must harden a steel part at 1500°F. Heat ening. This part could be slowly heated to 600°F, soaked the part slowly and evenly while watching it closely for at this temperature, then heated slowly to 1200°F, and any change in color. Once the steel begins to turn red, then soaked at that temperature. Following the final carefully note each change in shade. Continue the even preheat, the part should then be heated quickly to the heating until the steel is bright red; then quench the part. hardening temperature of 1500°F. The success of a heat-treating operation depends NOTE: Nonferrous metals are seldom preheated, largely on your judgment and the accuracy with which because they usually do not require it, and preheating you identify each color with its corresponding tempera- can cause an increase in the grain size in these metals. ture. From a study of table 2-1, you can see that close observation is necessary. You must be able to tell the COOLING STAGE 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 After a metal has been soaked, it must be returned of the person who prepared the table. For an actual to room temperature to complete the heat-treating proc- heat-treating operation, you should get a chart showing ess. To cool the metal, you can place it in direct contact the actual colors of steel at various temperatures. 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 TYPES OF HEAT TREATMENT desired. The rate of cooling depends on the medium; therefore, the choice of a cooling medium has an impor- Four basic types of heat treatment are used today. tant influence on the properties desired. They are annealing, normalizing, hardening, and tem- pering. The techniques used in each process and how Quenching is the procedure used for cooling metal they relate to Steelworkers are given in the following rapidly in oil, water, brine, or some other medium. paragraphs. Because most metals are cooled rapidly during the hard- ening process, quenching is usually associated with ANNEALING hardening; however, quenching does not always result in an increase in hardness; for example, to anneal cop- In general, annealing is the opposite of hardening, per, you usually quench it in water. Other metals, such You anneal metals to relieve internal stresses, soften as air-hardened steels, are cooled at a relatively slow rate them, make them more ductile, and refine their grain for hardening. structures. Annealing consists of heating a metal to a Some metals crack easily or warp during quenching, specific temperature, holding it at that temperature for and others suffer no ill effects; therefore, the quenching a set length of time, and then cooling the metal to room medium must be chosen to fit the metal. Brine or water temperature. The cooling method depends on the 2-2 Table 2-1.—Heat Colors for Steel Table 2-2.—Approximate Soaking Periods for Hardening, Annealing, and Normalizing Steel metal and the properties desired. Some metals are Ferrous Metal furnace-cooled, and others are cooled by burying them To produce the maximum softness in steel, you heat in ashes, lime, or other insulating materials. the metal to its proper temperature, soak it, and then let Welding produces areas that have molten metal next it cool very slowly. The cooling is done by burying the to other areas that are at room temperature. As the weld hot part in an insulating material or by shutting off the furnace and allowing the furnace and the part to cool cools, internal stresses occur along with hard spots and together.
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