Heat treatment is a process used to change the physical properties of materials. At times, it can also alter the material’s chemical composition. It is very commonly used in the field of metals. Its use in the glass, ceramic and plastics industry is not unknown. The main activities in any heat treatment involve heating and cooling. Controlling both these factors leads metals to gaining the properties required.
Most metal heat treatment processes are performed at very high temperatures. They are meant to modify a metal into the required form. This process can help to make a metal harder or softer, depending on the required function. There are a number of processes used for metal heat treatment. They are:
- Hardening and tempering
- Precipitation hardening
- Case hardening
Hardening & Tempering Process
In the process required to harden or temper a metal, the object is heated till the required crystal phase of its structure is obtained. Then, it is rapidly cooled using water or other substances like oil or liquid nitrogen. The temperature to which metal is raised and the speed of cooling will determine its final properties. It will have to be varied depending on the requirements.
Metals become ductile and soft when annealed, helping to reduce metal stress. It also allows metals to cool down in the furnace itself leading to full annealing. This process may be undertaken a number of times to attain the required softness and ductility.
Precipitation Hardening Process
Precipitation hardening results in alloys turning into soft metal. It is also called age hardening and a technique of heat treatment used to increase malleable material yield strengths. This technique is used in structural alloys of titanium, nickel, magnesium, aluminium and some varieties of stainless steel.
When a metal part needs only its surface to be hardened, this metal heat treatment process is called case hardening. After this treatment, the part being case-hardened retains soft metal within it while its outer layers become very hard. In the case of many varieties of steel, this involves adding extra carbon.
Case Hardening Process
This combination of hard and soft metal allows a part to be more resistant to fracturing. The hardened layer also increases wear resistance.
Heat is also used in other processes involving metals, such as welding and brazing. But these are more processes that enable bringing parts together to form joints. These do not result in any substantial changes in their properties. The making of alloys also involves heating and helps in combining different metals to form completely new substances.
A metal heat treatment process can allow the properties of a metal to be changed till it achieves characteristics. These characteristics allow it to perform the functions that a metallurgist requires from it. It is in extensive use of steel as this metal responds very easily to heat. Thus, allows it to be manipulated so that it acquires the desired properties.