What are the types of Heat Treatment processes?
Usually, heat treatment processes can be divided into annealing, hardening, and quenching/tempering. In this context, the Figure below features the characteristic temperature ranges for important heat treatment processes in the Fe-Fe3C (Iron-Carbon System) system.
You will notice that the heat treatment temperature for various heat treatment processes is often dependent on the carbon content of the material. For example, a normalizing temperature for 0.8% Carbon will be around 723 Degree Celsius and for steel, with 0.2% Carbon content will be approximately 850 Degree Celsius. So, carbon will be the main contributing factor to decide the heat treatment temperature.
Annealing vs. Normalizing
Annealing is a heat treatment process of heating the steels about 100°F above the upper critical temperature (A3 line) and lower critical temperature (A3,1 line) as shown in the above figure. Annealing is a Heat treatment, consisting of heating a material to a certain temperature, holding it at that temperature, and cooling in such a manner that the material is closer to equilibrium at room temperature.
Normalizing is a heat treatment process by which steels are heated nearly 100°F above the upper critical temperature (A3 or Acm line). hypo-eutectoid steels (steel less than 0.8% carbon) are heated to a temperature of 30°C to 50°C above the AC3 line and hypo-eutectoid steels are heated to a temperature of 30°C to 50°C above the AC1 line (723°C).
The temperature at AC3 line depends as it varies based on the carbon percentage in the alloy while AC1 temperature is constant. A temperature equilibrium is brought to the part by holding it at certain temperature to have the uniform temperature throughout the part thickness, this is called dwell time. After that, it will be cooled in resting air. Because the steel is twice subjected to a y-α transformation, the material will be transformed into a uniform, fine-grained normal microstructure.
The purpose of the normalizing is also to obtain an uniform microstructure with fine lamellar pearlite in the material.
The cooling method used in the Annealing is furnace cooling at a moderate cooling rate which means the parts are cooled inside the furnace in a controlled atmosphere and not in the natural air. Opposite of annealing is applicable for cooling in the Normalizing where parts are subjected to ambient air cooling & the cooling rates are not controlled .
The rate of air cooling in normalizing is much faster than the controlled furnace cooling that is applied in annealing heat treatment. The overall cost of Normalizing is lesser than the annealing process as the former does not require any controlled air cooling as mandatory in annealing.
Material Properties post the heat treatment
|Annealing specimen||Normalized specimen|
|Lower hardness reduced tensile and improved toughness.||Increased hardness, higher tensile than annealing, reduced toughness compare to annealing.|
|Uniform grain size after normalizing||Less uniform grain size|
|Less internal stresses||higher internal stresses compare to normalizing|
|Coarse microstructure||Fine microstructure|
Purpose of annealing and normalizing
|1. To refine the crystalline structure and |
2. To remove the residual stresses
3. To increase material ductility by minimizing the hardness and brittleness
|1. To achieve a refined grain structure before hardening.|
2. To minimize the segregation in the casting or forgings.
3. To harden the steel minimally.
Cost comparison between annealing and normalizing
Annealing: Annealing heat treatment is more expensive compare to normalizing. The reason is high controlled heating & cooling involved in annealing process which increase the heat treatment time.
Normalizing: Normalizing is less expensive compare to annealing.