What is Heat Treatment
Heat Treatment is a method of controlled heating and cooling of metals to alter their mechanical and physical properties without changing the product shape. The technique involves the use of heating or chilling, usually to extreme temperatures, to attain a desired result, such as hardening or softening of a metal. Some of the common techniques of heat treatment include annealing, case hardening, precipitation strengthening, tempering and quenching. As a matter of fact, the term heat treatment applies only to processes where the heating and cooling is done for the basic objective of altering the properties intentionally. Heating and cooling, however, sometimes occur as incidental phases of other manufacturing methods such as hot forming or welding.
Primarily associated with increasing the strength of material, heat treatment can also be used to change certain manufacturing objectives such as improved machining, improved formability, and to restore ductility after a cold working operation. Hence it is a very enabling manufacturing process, which can not only help other manufacturing methods, but can also enhance product performance by increasing the strength or other desirable characteristics.
Four Basic Types of Heat Treatment
Hardening (including quenching)
Metal hardening treatment constitutes of heating the steel to a definite temperature and then cooling it quickly by plunging it into oil, brine, or water. Majority of steels require rapid cooling (quenching) for hardening, however a few can be air-cooled with the same results. Although hardening enhances the hardness and strength of the steel, it makes it less ductile. Usually, the harder the steel, the more brittle it becomes. To remove some of the brittleness, the steel should be tempered after hardening.
Many nonferrous metals can be hardened and the strength can be increased by controlled heating and rapid cooling. In this case, the process is called heat treatment, instead of hardening.
Generally, annealing is the opposite of hardening. Metals are annealed to relieve internal stresses, to soften them, to make them more ductile, and to refine their grain structures. Annealing comprises of heating a metal to a particular temperature, holding it at that temperature for a definite time, and then cooling the metal to room temperature. The cooling process depends on the metal and the characteristics desired. Some metals are furnace-cooled, while others can be cooled by burying them in ashes, lime, or other insulating materials.
Welding produces areas, which have molten metal next to other areas, which are at room temperature. As the weld cools, internal stress occurs along with brittleness and hard spots. Welding can in reality weaken the metal. Annealing is just one of the techniques used for correcting these problems.
Normalizing a metal means bringing it up to recrystallization temperatures by soaking, and then pulling it from the furnace and allowing it to cool in the atmosphere. Many of the properties of annealed metals are evident in normalized metals, but because there is not quite the same evenness of cooling, the grains tend to be a little less regular. Still, a much smaller temperature differential than is found in freezing metal means a normalized product is less brittle.
The cooling rate found in normalizing creates smaller grains in the metal than annealing does, which means that in general, it will be stronger or harder than annealed metal.
Finding the right mixture of hardness-and-ductility can also be achieved through a process called tempering. Tempering is often done with quenched steel to make it less brittle while preserving some of the hardness. In tempering, a metal is reheated yet again, but now to a lower temperature than in annealing, normalizing, or quenching.
Martensite is not a stable molecule in heat—it is achieved under shock—so tempering steel means destabilizing the martensite to let it start to convert to cementite and ferrite. A range of temperatures and length of times in the temper oven will influence how much of the martensite is converted and therefore how soft the metal becomes. For example, metal springs may be tempered at higher temperatures for increased elasticity compared to tools tempered at lower temperatures to maintain hardness.
Tempering is often used to relieve the internal stresses in a quenched material. A metal that’s undergone other heat stress like welding or blacksmithing can be tempered to allow the molecules within to relax a little into one another.