Heat treatment always involves heating and cooling metals. However, there’s more to it than that, and in fact there are three stages of heat treatment that any metal must go through for the process to be considered complete.
Here’s an overview of those three stages of heat treatment.
The first stage is the heating stage, and the goal here is simply to make sure the metal gets heated as uniformly as possible.
The best results typically come when heating the metal slowly. If you rush, you’re more likely to get uneven heating, which could result in one section expanding faster than another, leading to cracks or distortions in the metal.
The heating rate you use in this phase depends on how much head conductivity the metal has, the metal’s condition and the cross section and size of the metal. Metals with higher conductivity will heat faster than those with low conductivity. If the metal has been hardened or stressed previously, you should heat it slower. And if you have larger metal parts or parts that have uneven cross sections, you’ll need to take your time with heating in comparison to smaller parts or parts with even cross sections.
The next process is soaking. This stage is meant to keep the metal at the correct temperature until it achieves the desired internal structure.
Here, you’ll need to determine the ideal soaking period, or the amount of time the metal should be kept at the proper temperature. This requires a chemical analysis of the metal and a measurement of its mass. When there are uneven cross sections, the soaking period can be determined based on the largest section of the part.
Instead of taking the temperature of the metal from room temperature to the soaking temperature, heat the metal slowly to just below where structural change will occur, and keep it at that level until there is consistent temperature throughout the metal.
During the third and final stage, the metal is cooled back to room temperature. There are different methods of accomplishing this, which will depend on the kind of metal in question. You could use a cooling medium, a liquid, a solid or a gas, or a combination of the aforementioned options.
The rate of cooling will also depend on the metal and the cooling medium, similar to how the rate of heating depends on the metal and its structure.
Quenching, for example, is a form of rapid cooling in oil, air, water, brine or another medium. Usually it’s used in the hardening process, because hardening requires rapid cooling. However, it can also be used for annealing copper and various other processes.
Not all metals should be quenched, as the fast rate of cooling can result in cracking or warping. Sometimes oil mixtures or brine can be used for slower cooling processes that are better for the metal in question.
For more information about heat treatment and the benefits of outsourcing heat treating to a qualified heat treating company, contact J.F. Heat Treating Inc. with your questions.