The results that may be obtained by heat treatment depend, to a great extent, on the structure of the metal and the manner in which the structure changes when the metal is heated and coded. 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 because their structures change with heating and cooling.

An alloy may be in the form of a solid solution, 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. 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 might 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 mixture of cement, the other constituents of an alloy are embedded in the mixture formed by the base metal.

An alloy that is 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 that consists of a combination of a 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. Heat treatment involves a cycle of events. These events are heating, generally done slowly to ensure uniformity; soaking, or holding the metal at a given temperature for a specified length of time; and cooling, or returning the metal to room temperature, sometimes rapidly, sometimes slowly. These events are discussed in the following paragraphs.

Uniform temperature is of primary importance in the heating cycle. If one section of a part is heated more rapidly than another, the resulting uneven expansion often causes distortion or cracking of the part. Uniform heating is most nearly obtained by slow heating. The rate at which a part maybe heated depends on several factors. One important factor is the heat conductivity of the metal. A metal that conducts heat readily may be heated at a faster rate than one in which heat is not absorbed throughout the part as rapidly. The condition of the metal also affects the rate at which it may be heated. For example, the heating rate for hardened tools and parts should be slower than for metals that are not in a stressed condition. Finally, size and cross section have an important influence on the rate of heating. Parts large in cross section require a slower heating rate than thin sections. This slower heating rate is necessary so that the interior will be heated to the same temperature as the surface. It is difficult to uniformly heat parts that are uneven in cross section, even though the heating rate is slow. However, such parts are less apt to be cracked or excessively warped when the heating rate is slow.