Normalizing is the process of heating of metal that alters the properties of metal like tensile strength, ductility and also refines the grain structure.
It is a type of heat treatment applicable to ferrous metals only.
In the normalizing process, the material is heated to elevated temperature and after that, it is cool back by placing it in contact with air at room temperature. This process of cooling metal with air is called air quenching.
This normalizing process changes the microstructure of the metal which increases the ductility and increases the hardness of metal.
Normalizing is needed because the ductility is decreased and hardness is increased by different processes like hammering.
It is most commonly used to change the mechanical properties of metal so that it becomes easily serviceable and can be machined perfectly.
Since the heating is done above the upper critical temperature limits so it automatically softens the steel because heating is done up to austenitic state and the softening of the steel automatically relieves the locked in strain which in turn reduces the residual stresses.
In this process, the metal is heated 40 – 50 degrees Celcius above the upper critical temperature for that metal. The upper critical temperature depends upon the percentage of carbon present in that metal.
The time for which the metal is heated should be chosen such that the heat transferred is uniformly spread throughout the metal.
Normalizing is commonly performed after forging or casting. Cold working and casting are manufacturing processes that produce grain structure that may require normalizing before the component is put to service. Normalizing relieves internal stresses caused by cold work.
In this process, heating is carried out in the air, so subsequent machining or surface finishing is required to remove scale or decarburized layers.
Purpose of normalizing process:
1) To improve Machinability:-
Different machining processes like facing, taper turning, boring, drilling can be carried out smoothly after the normalizing process.
2) To modify and refine Cast dendritic Structure:
We can modify the defects caused after a process like casting.
3) To refine the grains
It is used to refine the grain structure which changes some mechanical properties of the metal.
4) To make the material suitable for further heat treatment:
Material is made suitable for further heat treatment like the hardening process.
5) To relieve the internal stress of the material.
6) To improve the tensile strength of the material.
Only those metals can be normalized whose microstructure can be replaced by heat treatment.
List of metal and alloys which can be normalized are:-
2) Iron based alloys like tool steel, carbon steel, stainless steel and cast iron.
5) Nickel based alloys like Nilo 6* and Pernifer 6*.
Normalizing Process Procedure:
In the normalizing process, there are 3 main steps:
1. Recovery Stage
2. Recrystallization Stage
3. Grain Growth Stage
1) Recovery Stage:
In the recovery stage, a furnace or other heating device is used to increase the temperature of a material to such extent where its external stress can be relieved. It causes completely austenitic structure formation in the material.
2) Recrystallization Stage:
In the recrystallization stage, the material is heated above the recrystallization temperature but below the melting temperature which causes new grains to form without stress. In this stage, the material is held at a constant temperature for some time for the grain formation to take place.
3) Grain Growth Stage:
During this stage, the new grains are developed fully. The growth of grains of material is controlled by allowing the material to cool to room temperature by keeping the material in the air. In this process, the rate of cooling is more than that of the annealing process. This cooling is non-equilibrium cooling. Due to the faster rate of cooling, we get a more refined grain structure as compared to annealing.
After these three stages, the ductility of the material increases and its hardness decreases. Also, the material becomes more machinable.
Advantages of Normalizing process over Annealing Process:
1) It is faster than the annealing process as the rate of cooling is faster than annealing process because in normalizing the material is cool by placing it in room temperature while in annealing material is cooled at a controlled rate in a furnace.
2) Quality of surface after machining of a normalized part is also better than in an annealed part.
3) Quenching media is air i.e outside the furnace so it becomes batch type production.
4) Normalizing is less expensive than annealing because it does not require additional furnace time during the cool down process.
Limitation of Normalizing process:
1) As the cooling rate during normalizing in a steel may be different at different depths, the structure hence formed is dependent on the thickness of the steel part.
2) The increase rate of cooling in normalizing results in high hardness and low ductility than annealing.
Applications of Normalizing Process:
1) Carbon steel is normalized after it is cold-rolled to reduce the brittleness caused by work hardening.
2) Nickel based alloys in the nuclear industry are normalized before thermal microstructure alteration which is done before welding.
3) Ferritic stainless steel stamping in the automotive industry is normalized before the work hardening which occurs during the forming process.