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Heat Treatments
Heat treatments are used to alter the physical and mechanical properties of metal without changing its shape. They are essential processes in metal manufacturing which increase desirable characteristic of metal, while allowing for further processing to take place.
Various heat treatment processes involve carefully controlled heating and cooling of metal. Steel, for example, is commonly heat treated for use in a variety of commercial applications.
Common objectives of heat treatment are to:
- Increase strength
- Increase hardness
- Improve toughness
- Improve machining
- Improve formability
- Increase ductility
- Improve elasticity
The cooling stage has different effects depending on the metal and process. When steel is cooled quickly it hardens, whereas the rapid cooling stage of solution annealing will soften aluminum.
While there are many types of heat treatment, two important types are annealing and tempering.
Annealing
Annealing involves heating steel to a specified temperature and then cooling at a very slow and controlled rate.
Annealing is commonly used to:
- Soften a metal for cold working
- Improve machinability
- Enhance electrical conductivity
Annealing also restores ductility. During cold working, the metal can become hardened to the extent that any more work will result in cracking. By annealing the metal beforehand, cold working can take place without any risk of cracking, as annealing releases mechanical stresses produced during machining or grinding.
Annealing is used for steel, however, other metals including copper, aluminum and brass can be subject to a process called solution annealed.
Large ovens are used for annealing steel. The inside of the oven must be large enough to allow air to circulate around the metal. For large pieces, gas fired conveyor furnaces are used while car-bottom furnaces are more practical for smaller pieces of metal.
During the annealing process, the metal is heated to a specific temperature where recrystallization can occur. At this stage, any defects caused by deformation of the metal are repaired. The metal is held at that temperature for a fixed period, then cooled down to room temperature.
The cooling process must be done very slowly to produce a refined microstructure, thus maximizing softness. This is often done by immersing the hot steel in sand, ashes or other substances with low heat conductivity, or by switching off the oven and allowing the steel to cool with the furnace.
Tempering
Tempering is used to increase the toughness of iron alloys, particularly steel. Untempered steel is very hard but is too brittle for most applications. Tempering is commonly done after hardening to reduce excess hardness.
Tempering is used to alter:
- Hardness
- Ductility
- Toughness
- Strength
- Structural stability
Tempering involves heating the metal to a precise temperature below the critical point, and is often done in air, vacuum or inert atmospheres.
The temperature is adjusted depending on the amount of hardness that needs to be reduced. While it varies depending on the metal type, generally, low temperatures will reduce brittleness while maintaining most of the hardness, while higher temperatures reduce hardness which increases elasticity and plasticity, but causes some yield and tensile strength to be lost.
It is essential to heat the metal gradually to avoid the steel being cracked. The metal is then held at this temperature for a fixed period. A rough guideline is one hour per inch of thickness. During this time the internal stresses in the metal are relieved. The metal is then cooled in still air.
