The purpose of the complete annealing heat treatment of the steel is to achieve the complete transformation of the material structure, reducing residual stresses and rendering processability characteristics to the material so that it can be more easily worked on with machine tools. It also serves to increase the dimensional stability of the pieces, especially if they are subsequently subjected to final thermochemical treatments such as carburizing and hardening. The structure of the material obtainable with the isothermal annealing heat treatment is that of block lamellar ferrite and pearlite. This treatment involves heating until austenitization, above Ac3 for eutectoid hypoeutectoid steels and above Ac1 for hypereutectoid steels. Cooling must be slow, close to equilibrium conditions. Generally in this cooling process the furnace is allowed to cool inertially.
Isothermal annealing of steels involves complete austenitization followed by a transformation of the austenite at constant temperature (isothermal), generally between a range of 640 – 720°C.
Processability or sub-critical annealing aims to reduce the hardness of steel to improve its processability without significantly modifying its structure. This treatment involves heating to a temperature 10-20° lower than the AC1 value with permanence related to the degree of spheroidization required in the specification. The higher this degree, the longer the maintenance time. Free cooling to room temperature.
Stress-relieving annealing aims to reduce, as much as possible, the internal stresses generated by pre-treatment processes (plastic deformation, welding, mechanical machining) with maintenance for the necessary time at a temperature below Ac1. The thermal gradient of ascent and descent must be controlled to avoid generating additional stresses.
Normalization consists of the complete austenitization at a temperature above Ac3 or Acm for a duration sufficient to obtain complete solubilization of the austenite, followed by cooling in calm air to room temperature. The aim of the treatment is to regenerate and refine the grain of the component.
The upgrading of processability greatly improves both the processability of the product with regard to the tool (i.e., the production of burrs) and the dimensional stability of the piece subsequently subjected to the final heat treatment of cementation, induction and/or nitriding after the machining phase.
Spheroidized annealing gives the cementite a clearly spheroidal shape and an even distribution. It is done over a very long heating time (about 30 hours) or by oscillating the temperature of 20-30 °C around Ac1. Spheroidized pearlite has a stable configuration, as the surface energy decreases with regard to the lamellar configuration: this treatment therefore increases toughness and ductility.