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Heat Colors for Steel

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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 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 hard- ening. 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 proc- ess. 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 impor- tant  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 hard- ening process, quenching is usually associated with hardening; however, quenching does not always result in an increase in hardness; for example, to anneal cop- per, 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 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 correspond- ing 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  tempera- ture. 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. TYPES OF HEAT TREATMENT Four basic types of heat treatment are used today. They are annealing, normalizing, hardening, and tem- pering.  The  techniques  used  in  each  process  and  how they  relate  to  Steelworkers  are  given  in  the  following paragraphs. ANNEALING In general, annealing is the opposite of hardening, You  anneal  metals  to  relieve  internal  stresses,  soften them, make them more ductile, and refine their grain structures. Annealing consists of heating a metal to a specific  temperature,  holding  it  at  that  temperature  for a set length of time, and then cooling the metal to room temperature.  The  cooling  method  depends  on  the 2-2



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