Heat treatment is all about heating and the subsequent cooling of metals in a controlled manner in order to alter the mechanical and the physical properties of the metals without ever changing the shape of the product. Heat treatment can be done in an intermittent fashion at times. This is because of the process of manufacturing that works towards either heating or cooling the metal, like forming or welding.
One would generally associate the process of heat treatment with increasing the strength of the material. However, this process can also be used in order to alter some particular manufacturability objectives, like improve the machine, improve the formability, restoring ductility after an operation which involved some cold work. Hence, it is quite an enabling process of manufacturing which works wonders towards not only helping in the other manufacturing processes, but also in improving the product performance by increasing the strength of the metal, or any other such desirable sort of characteristics.
The metal, which is specially suited for heat treatment, is steel. This is due to the fact that steel responds pretty well to this sort of a treatment, and steel is the most commercially used metal, when compared to any other kind of metal. Steel is heat treated for a number of reasons, such as softening, hardening, material modification, and the likes. Let us look into each of these heat treatments in details.
Softening is done in order to bring down on the hardness or strength of the metal, removing any sort of residual stress, improving the toughness of the metal, restore ductility, refine grain size, and also to change the electromagnetic properties which are characterized by steel. To remove residual stresses or to restore ductility is an important operation in case of a large amount cold working needs to be performed, like is done in a cold rolling operation or in case of wiredrawing induction heating machine. Annealing is the full process which includes normalizing, spheroidizing. The process of tempering includes such steps as austempering, martempering. These are the two major processes by which steel can be softened.
Steel is hardened in order to increase the strength and the wear properties of steel. A must have in case of hardening of steel would be enough carbon and alloy content. If the carbon content is sufficient, then hardening of steel would not be such a major task, as steel can be hardened directly in such a case. If this is not the case, then the surface of the part has to be enriched in carbon with the help of some techniques related to diffusion treatment hardening.
In case of material modification, the process of heat treatment has been used to modify the properties of the materials, along with softening and hardening. This process works towards modifying the behavior of the metal steel in a convenient manner so that service life can be maximized, like stress relieving or strength properties such as cryogenic treatment, spring aging and the likes.