Difference between Annealing and normalizing of steel

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 Celcius and for steel, with 0.2% Carbon content will be approximately 850 Degree Celcius. So, carbon will be the main contributing factor to decide the heat treatment temperature.

Various heat treatment plotted on Fe-C Phase Diagram

Annealing vs. Normalizing

Annealing is a metallurgical process by which steels are heated nearly 100°F above the upper critical temperature (A3 line) and lower critical temperature (A3,1 line). Annealing is a Heat treatment, consisting of heating a material to a certain temperature, holding it at that temperature, and cooling in such a way that the state of the material is closer to equilibrium at room temperature.

Normalizing is a metallurgical process by which steels are heated nearly 100°F above the upper critical temperature (A3 or Acm line). The term normalizing is understood to mean heating to temperatures of 30 to 50 K above A3 in the case of hypo-eutectoid steels and 30 to 50 K above A1 in the case of hyper-eutectoid steels (depending on the carbon content.) Depending on the dimensions, the respective workpiece will only be held at the temperature for as long as necessary to heat it up completely. 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 aim of normalizing is thus also to achieve an even microstructure with fine lamellar pearlite

Before & after normalizing microstructure

Cooling Medium

Annealing necessitates furnace cooling at a moderate cooling rate. Normalizing requires air cooling after furnace heating in this environment drastic cooling rate takes place. The rate of air cooling in normalizing is faster than the cooling in the annealing process. 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.

Normalizing process and effect on grain size

Material Properties post the heat treatment

Annealing specimenNormalized specimen
Lower hardness reduced tensile and improved toughness.Increased hardness, higher tensile than annealing, reduced toughness compare to annealing.
Uniform grain size after normalizingLess uniform grain size
Less internal stresseshigher internal stresses compare to normalizing
Coarse microstructureFine 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.

Normalizing: Normalizing is less expensive compare to annealing.

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