Another significant factor to consider in maintenance and repair is the ability of a material to be formed, bent, or machined to required shapes. The hardening of metals by cold-working or forming is termed work hardening. If a piece of metal is formed (shaped or bent) while cold, it is said to be cold-worked. Practically all the work an aviation mechanic does on metal is cold-work. While this is convenient, it causes the metal to become harder and more brittle.

If the metal is cold-worked too much, that is, if it is bent back and forth or hammered at the same place too often, it will crack or break. Usually, the more malleable and ductile a metal is, the more cold-working it can stand. Any process that involves controlled heating and cooling of metals to develop certain desirable characteristics (such as hardness, softness, ductility, tensile strength, or refined grain structure) is called heat-treatment or heat-treating. With steels, the term “heat-treating” has a broad meaning and includes processes such as annealing, normalizing, hardening, and tempering.

In the heat-treatment of aluminum alloys, only two processes are included: the hardening and toughening process and the softening process. The hardening and toughening process is called heat-treating, and the softening process is called annealing. Aircraft metals are subjected to both shock and fatigue (vibrational) stresses. Fatigue occurs in materials that are exposed to frequent reversals of loading or repeatedly applied loads, if the fatigue limit is reached or exceeded. Repeated vibration or bending ultimately causes a minute crack to occur at the weakest point. As vibration or bending continues, the crack lengthens until the part completely fails. This is termed “shock and fatigue failure.” Resistance to this condition is known as shock and fatigue resistance. It is essential that materials used for critical parts be resistant to these stresses.

Heat-treatment is a series of operations involving the heating and cooling of metals in the solid state. Its purpose is to change a mechanical property, or combination of mechanical properties, so that the metal is more useful, serviceable, and safe for a definite purpose. By heat-treating, a metal can be made harder, stronger, and more resistant to impact. Heat-treating can also make a metal softer and more ductile. No one heat-treating operation can produce all these characteristics. In fact, some properties are often improved at the expense of others. In being hardened, for example, a metal may become brittle.

The various heat-treating processes are similar in that they all involve the heating and cooling of metals. They differ, however, in the temperatures to which the metal is heated, the rate at which it is cooled, and, of course, in the result.

The most common forms of heat-treatment for ferrous metals are hardening, tempering, normalizing, annealing, and case hardening. Most nonferrous metals can be annealed and many of them can be hardened by heat-treatment. However, there is only one nonferrous metal, titanium, that can be case hardened, and none can be tempered or normalized.

The results obtained by heat-treatment depend on the structure of the metal and on the way the structure changes when the metal is heated and cooled. A pure metal cannot be hardened by heat-treatment, because there is little change in its structure when heated. On the other hand, most alloys respond to heat-treatment since their structures change with heating and cooling.

An alloy may be in the form of a solid solution, a mechanical mixture, or a combination of a solid solution and a mechanical mixture. When an alloy is in the form of a solid solution, the elements and compounds that form the alloy are absorbed, one into the other, in much the same way that salt is dissolved in a glass of water, and the constituents cannot be identified even under a microscope.

When two or more elements or compounds are mixed but can be identified by microscopic examination, a mechanical mixture is formed. A mechanical mixture can be compared to the mixture of sand and gravel in concrete. The sand and gravel are both visible. Just as the sand and gravel are held together and kept in place by the matrix of cement, the other constituents of an alloy are embedded in the matrix formed by the base metal.

An alloy in the form of a mechanical mixture at ordinary temperatures may change to a solid solution when heated. When cooled back to normal temperature, the alloy may return to its original structure. On the other hand, it may remain a solid solution or form a combination of a solid solution and mechanical mixture. An alloy, which consists of a combination of solid solution and mechanical mixture at normal temperatures, may change to a solid solution when heated. When cooled, the alloy may remain a solid solution, return to its original structure, or form a complex solution.

Successful heat-treating requires close control over all factors affecting the heating and cooling of metals. Such control is possible only when the proper equipment is available and the equipment is selected to fit the job. Thus, the furnace must be of the proper size and type and must be controlled so that temperatures are kept within the limits prescribed for each operation. Even the atmosphere within the furnace affects the condition of the part being heat-treated. Further, the quenching equipment and the quenching medium must be selected to fit the metal and the heat-treating operation.

Finally, there must be equipment for handling parts and materials, for cleaning metals, and for straightening parts.

There are many different types and sizes of furnaces used in heat-treatment. As a rule, furnaces are designed to operate in certain specific temperature ranges and attempted use in other ranges frequently results in work of inferior quality.

In addition, using a furnace beyond its rated maximum temperature shortens its life and may necessitate costly and time-consuming repairs.

Fuel-fired furnaces (gas or oil) require air for proper combustion, and an air compressor or blower is therefore necessary. These furnaces are usually of the muffler type; that is, the combustion of the fuel takes place outside of and around the chamber in which the work is placed. If an open muffler is used, the furnace should be designed to prevent the direct impingement of flame on the work.

In furnaces heated by electricity, the heating elements are generally in the form of wire or ribbon. Good design requires incorporation of additional heating elements at locations where maximum heat loss may be expected. Such furnaces commonly operate at up to a maximum temperature of about 2,000 °F. Furnaces operating at temperatures up to about 2,500 °F usually employ resistor bars of sintered carbides.