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Heat-Treating Theory The various types of
heat-treating processes are similar because they all involve the heating and cooling of
metals; they differ in the heating temperatures and the cooling rates used and the final
results. The usual methods of heat-treating ferrous metals (metals with iron) are
annealing, normalizing, hardening, and tempering. Most nonferrous metals can be annealed,
but never tempered, normalized, or case-hardened. Successful heat treatment
requires close control over all factors affecting the heating and cooling of a metal. This
control is possible only when the proper equipment is available. The furnace must be of
the proper size and type and controlled, so the temperatures are kept within the
prescribed limits for each operation. Even the furnace atmosphere affects the condition of
the metal being heat-treated. The furnace atmosphere
consists of the gases that circulate throughout the heating chamber and surround the
metal, as it is being heated. In an electric furnace, the atmosphere is either air or a
controlled mixture of gases. In a fuel-fired furnace, the atmosphere is the mixture of
gases that comes from the combination of the air and the gases released by the fuel during
combustion. These gases contain various proportions of carbon monoxide, carbon dioxide,
hydrogen, nitrogen, oxygen, water vapor, and other various hydrocarbons. Fuel-fired
furnaces can provide three distinct atmospheres when you vary the proportions of air and
fuel. They are called oxidizing, reducing, and neutral.
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by SweetHaven Publishing Services Based upon a text provided by the U.S. Navy |
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