the microstructure of alloys will change during heat treatment. Heating is carried out in line with a prescribed thermal profile.

An alloy may exist in one of three different states when heated. It may either be a mechanical mixture, a solid solution, or a combination of both.

A mechanical mixture is analogous to a concrete mixture where cement binds sand and gravel together. Sand and gravel are still visible as separate particles. With metal alloys, the mechanical mixture is held together by the base metal.

On the other hand, in a solid solution, all the components are mixed homogenously. This means that they cannot be identified individually even under a microscope.

Every state brings along different qualities. It is possible to change the state through heating according to the phase diagram. The cooling, though, determines the final outcome. It is possible for the alloy to end up in one of the three states, depending solely on the method.

During the holding or soaking stage, the metal is kept at the achieved temperature. The duration of that depends on the requirements.

For example, case hardening only requires structural changes to the surface of the metal in order to increase surface hardness. At the same time, other methods need uniform properties. In this case, the holding period is longer.

The soaking time also depends on the material type and part size. Larger parts need more time when uniform properties are the objective. It just takes longer for the core of a large part to reach the required temperature.

After the soaking stage is complete, the metal must be cooled in a prescribed manner. At this stage, too, structural changes occur. A solid solution on cooling may stay the same, become a mechanical mixture completely or partially, depending on various factors.

Different media such as brine, water, oil or forced air control the rate of cooling. The sequence of cooling media named above is in decreasing order of effective rate of cooling. Brine absorbs heat fastest, while air is the slowest.

It is also possible to use furnaces in the cooling process. The controlled environment allows for high precision when slow cooling is necessary.