Heat treating or also known as heat treatment is a group of metalworking and industrial processes that are used to modify the physical and at times, the chemical properties of a particular material. A common application for this is metallurgical. Heat treatments are used in the manufacturing of different materials similar to glass.
Heat treating involves the usage of chilling or heating, typically to the extreme temperature to be able to attain the desired result like softening or hardening of a material. Techniques that are used in heat treatment are quite a lot and this includes case hardening, annealing, precipitation strengthening, quenching, normalizing and tempering. It’s noteworthy that while this term is applicable to only processes where cooling and heating are done for specific purpose of intentionally modifying properties, cooling and heating typically take place incidentally throughout other manufacturing procedures like welding or hot forming.
Metallic materials have microstructure of small crystals that are known also as crystallites or grains. Nature of grains such as composition or size is one of the effective factors that help in determining the overall mechanical behavior of metal. Heat treatment is providing an effective way of manipulating metal properties by controlling the diffusion rate as well as cooling rate within microstructure. More often than not, heat treatment helps in altering mechanical properties of metallic alloy, like its toughness, strength, hardness, elasticity and ductility.
There are two mechanisms that could possibly change the properties of alloy during the heat treatment and it’s the diffusion mechanism which makes the changes in alloy’s homogeneity and formation of martensite that causes the crystals to deform intrinsically.
Structure of crystal consists of atoms that are specifically arranged in a group known as lattice. In various elements, this said order is rearranging itself depending on the type of condition it is on similar to pressure and temperature. This said rearrangement is known as polymorphism or allotropy and it can occur several times, at different temperatures for certain kind of metal. Now in the subject of alloys, this rearrangement can make the element not dissolve normally into a base metal to be soluble while allotropy reversal can make the element to be either completely or partially insoluble.
When in soluble state, the diffusion process makes the atoms of dissolved element to spread while trying to form homogenous distribution within the base metal’s crystals. If the alloy is cooled to an insoluble state, atoms of the dissolved constituents will move out of the solution. This diffusion is known as precipitation, leading to nucleation where all migrating atoms regroup together at grain boundaries. When it happens, it forms microstructure that consists of 2 or several distinctive phases.