The heat treatment of annealing to obtain annealed steels, consists of heating up a metal alloy to a temperature usually higher than the austenitizing one (Ac3 + 50-70 °C), keeping it at temperature for an appropriate amount of time and then cooling under conditions such that steel, after returning to room temperature, is in a structural state closer to the stable equilibrium one.
The treatment should achieve one or more of the following:
- chemical equilibrium: reduction of minor segregation
- structural equilibrium: transformation of metastable phases
- mechanical equilibrium: reduction of hardening and internal residual stresses
Through the process of annealing, the microstructure of the material is altered, causing changes in its properties such as flexibility and hardness.
The purpose of annealing is to soften steel and make it suitable for mechanical or plastic machining, to eliminate residual stresses and to destroy the effects of welding, previous heat treatment or plastic deformation. It can also have the purpose of equalizing steel’s chemical composition, where heating is performed at higher temperature and for a longer duration.
What types of annealing exist?
There are various annealing cycles, the choice of which is made according to the hardness and structures required for a given type of processing.
- Sub-critical annealing: consist in heating steel up at a temperature slightly below Ac1, keeping it at temperature and finally cooling as desired, even free cooling. Being a particularly low-cost treatment, it is the most widely used for structural steels and is performed to soften steel and to eliminate residual stresses and plastic deformation effects.
- Isothermal annealing: performed at a temperature in the range of Ac1 + Ac3 or above Ac3, followed by cooling to a convenient pearlitic field temperature at a relatively high rate, at which its kept long enough to complete the transformation. Thereafter, final cooling to room temperature, which can be done quickly saving a great amount of time.
- Complete annealing: performed at a temperature greater than Ac3
- Globular annealing: annealing that generally involves a prolonged stay at a temperature close to Ac1 to cause steel’s globulization.
This structural state is particularly suitable for obtaining, in the case of very demanding machining operations, significant improvements in tool machinability.
