It is important that the material used for any project possesses the correct mechanical properties for the specific application. Heat Treatment processes are often used to alter the mechanical properties of a metal, with one of the more common heat treatment processes being Normalizing.
What Is Normalizing?
Normalizing is a heat treatment process that is used to make a metal more ductile and tough after it has been subjected to thermal or mechanical hardening processes. Normalizing involves heating a material to an elevated temperature and then allowing it to cool back to room temperature by exposing it to room temperature air after it is heated. This heating and slow cooling alters the microstructure of the metal which in turn reduces its hardness and increases its ductility.
Why Is Normalizing Used?
Normalizing is often performed because another process has intentionally or unintentionally decreased ductility and increased hardness. Normalizing is used because it causes microstructures to reform into more ductile structures. This is important because it makes the metal more formable, more machinable, and reduces residual stresses in the material that could lead to unexpected failure.
What Is The Difference Between Annealing and Normalizing?
Normalizing is very similar to annealing as both involve heating a metal to or above its recrystallization temperature and allowing it to cool slowly in order to create a microstructure that is relatively ductile. The main difference between annealing and normalizing is that annealing allows the material to cool at a controlled rate in a furnace. Normalizing allows the material to cool by placing it in a room temperature environment and exposing it to the air in that environment.
This difference means normalizing has a faster cooler rate than annealing. The faster cooler rate can cause a material to have slightly less ductility and slightly higher hardness value than if the material had been annealed. Normalizing is also generally less expensive than annealing because it does not require additional furnace time during the cool down process.
The Normalizing Process
There are three main stages to a normalizing process.
- Recovery stage
- Recrystallization stage
- Grain growth stage
During the recovery stage, a furnace or other type of heating device is used to raise the material to a temperature where its internal stresses are relieved.
During the recrystallization stage, the material is heated above its recrystallization temperature, but below its melting temperature. This causes new grains without preexisting stresses to form.
Grain Growth Stage
During the grain growth, the new grains fully develop. This growth is controlled by allowing the material to cool to room temperature via contact with air. The result of completing these three stages is a material with more ductility and reduced hardness. Subsequent operations that can further alter mechanical properties are sometimes carried out after the normalizing process.
What Metals Can Be Normalized?
To be normalized, a metal needs to be receptive to normalizing, meaning its microstructure can be altered by heat treatment. Many types of alloys can be normalized, including:
- Iron based alloys (tool steel, carbon steel, stainless steel, and cast iron)
- Nickel-based alloys
Common Applications for Normalizing
Normalizing is used in many different industries for many different materials. Examples include:
- Ferritic stainless steel stampings in the automotive industry may be normalized following the work hardening that occurs during their forming process.
- Nickel-based alloys in the nuclear industry may be normalized following the thermal microstructure alteration that occurs following welding.
- Carbon steel may be normalized after it is cold-rolled to reduce the brittleness caused by work hardening.
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