The heat-treating process is the main difference between annealing steel and tempering steel. Annealing involves heating steel to a specified high temperature and then setting the cooling rate desired for that type of steel. The tempering process involves heating the metal to a precise temperature below the critical point with the use of air, vacuum, or inert gas. The amount of time this process takes and the tempering temperature depend on the metal being used.
Heat treatments are used in this process to alter the mechanical properties and physical properties of the steel without changing its shape. Heat treatment processes involve controlled heating and cooling of metal for a period of time. Heat treatment is used to:
- Increase strength and toughness
- Improve machining and ductility
- Improve the elasticity
When steel is cooled, it hardens. The rapid cooling stage of annealing will soften aluminum or steel. The process of heating and rapidly cooling steel is called quenching.
Annealing
The annealing process is used to heat steel to a specified temperature and then cool it down at a slow and controlled rate depending on the desired outcome and the type of metal used. Annealing is used to:
- Improve machinability
- Enhance electrical conductivity
- Restores ductility
During this process, the inside of large ovens use air to circulate heat around the metal or steel. Gas fired furnaces are used for large pieces and car-bottom furnaces are used for smaller pieces. The metal is heated to a specific temperature where recrystallization can occur. Any defects caused by deformation or corrosion of the metal are repaired. Metal is held at this exact temperature for a period of time, then cooled down to room temperature. The cooling process is done very slowly to produce a microstructure, which maximizes softness.
Tempering
The tempering process is mainly used to increase the toughness of iron alloy steels or carbon steel. Untempered steel is very hard and brittle, making it hard to use. Tempering is done after hardening steel to reduce excess hardness. Tempering is used to alter:
- Hardness
- Ductility
- Wear resistance
Temperature is adjusted during tempering depending on the amount of hardness desired. Low temperatures will reduce brittleness while maintaining hardness. Higher temperatures reduce hardness which increases elasticity. During reheating, it is important to heat the metal gradually to avoid the stainless steel from being cracked. During the cooling time, the internal stresses in the metal are relieved. This lower critical temperature is important in the finished product and is determined at the start of the process.
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