Understanding Heat Treatment
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. Metallurgical is the most common application. 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 used in heat treatment are plentiful including 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 consist of microstructure of small crystals also known as crystallites or grains. The nature of grains like composition or grain size is among the most efficient factors that determine 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 2 mechanisms that might change the properties of alloy throughout heat treating and it is the formation of martensite causes crystals to deform intrinsically and diffusion mechanism that makes the changes in the alloy’s homogeneity.
The structure of crystal includes atoms that are grouped in a particular arrangement that is referred as lattice. In many different elements, this order rearranges itself depending on conditions such as pressure and temperature. As a matter of fact, this said rearrangement is known scientifically as polymorphism or allotropy that can actually happen multiple times at different temperatures for certain metal types. In alloys, this rearrangement might lead to an element that won’t dissolve normally into base metal to become soluble while reversal of allotropy makes the element either completely or partially insoluble.
When it is in soluble state, diffusion process will make the dissolved atoms to spread while forming homogenous distribution within the base metal’s crystals. If ever the alloy is cooled to insoluble state, atoms of dissolved constituents might move out of the solution. Here, the diffusion is known as precipitation which can lead to nucleation where all of the migrating atoms will regroup together at grain boundaries. This will form microstructure consisting of 2 or several distinctive phases.