Very low temperature tempering (below 250 °C) is also known as annealing.
It is used to reduce internal tension without degrading the material's mechanical characteristics.
Working, cutting and plastic deformation all accumulate compressive stresses in the material. These stresses can cause unwanted dimensional variations if they are released in an uncontrolled manner, for example during subsequent heat treatment. To reduce these stresses after working, and with them the risk of dimensional variation, the part can be annealed.
Parts with very tight dimensional tolerances and (for instance) requiring further heat or surface treatment (case hardening, hardening, nitriding, etc.) must be annealed.
- Elimination of tension in welded structures;
- No effect on the material's structure;
- No significant effect on the material's hardness.
Copper and brass parts can also be annealed, while in the case of stainless steels, annealing is called solubilisation and is normally done at temperatures above 1000°C.
When done after final tempering/aging, the maximum temperature must be at least 28 °C below the final tempering temperature, and at least 56 °C below the final aging temperature.