Isothermal annealing is primarily used for medium carbon, high carbon and some of the alloy steels to improve their machinability. Isothermal annealing is carried out for components in the following process.
- Heating the steel components similar as in the case of full annealing.
- Slightly fast cooling from the usual austenitizing temperature to a constant temperature just below A1.
- Hold at this reduced temperature for sufficient soaking period for the completion of transformation.
- Cooling the steel component to room temperature in air.
Advantages of Isothermal annealing over full annealing:
- Reduced annealing time, especially for alloy steels which need very slow cooling to obtain the required reduction in hardness by the full annealing.
- More homogeneity in structure is obtained as the transformation occurs at the same time throughout the cross section.
- Improved machinability and surface finish is obtained after machining as compared to that of the full annealed components.