Heat Treating: Annealing & Tempering


Annealing is the process of heating a material to above its critical temperature (the temperature at which the transformation of ferrite into austenite is complete *View diagram*) and then cooling it slowly allowing the molecules to diffuse producing a uniform microstructure. The annealing process can be used to increase the toughness and ductility of a metal. Metals are often annealed after cold working to improve formability preparing them for further heat treatment. Annealing is done in three stages:

  1. Heating metal to a temperature above its critical range.
  2. Keeping metal at the temperature for a specific time period to allow it to heat all the way through.
  3. Allowing the metal to cool very slowly in a medium which helps it retain heat.


Tempering is a heat treating process similar to annealing; however, this process involves heating a material to below its critical temperature, often between 205° and 595°C. Like annealing, tempering is often used to reduce stresses caused by quenching.

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         Photo credit: Mauro Cateb-CC By-SA 3.0  


carbon-iron phase diagram
Photo credit: Christophe Dang Ngoc Chan- CC BY-SA 3.0