How Does Steel Undergo the Normalizing Process?

How This Form of Heat Treatment Makes Iron More Workable

Annealing oven at steel works
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How is steel normalized and what are the benefits of this process? Improve your understanding of this kind of heat treatment with this definition and review of normalization.

What Does Normalization Mean?

Normalization is a kind of heat treatment that relieves stress on steel; this improves ductility and toughness in steels that may harden after the cold working process.

For normalization to occur, steel is warmed to a temperature just above its upper critical point.

This is generally more than 50 C of that temperature. The steel is then held long enough for smaller metal grains to form and high energy grain shapes to come together.

This transformation is called grain refinement and leads to the formation of a more uniform piece of metal. After steel is heated to a temperature above its critical point, it is air-cooled until it drops to room temperature.

Normalizing temperatures are said to range from 810 C to 930 C. The thickness of the metal determines how long a piece of metal is held at what is known as the soaking temperature, or the temperature that transforms the microstructure. The thickness and composition of the metal also determines how high the work piece is heated.

Other Benefits of Normalization

Normalization can also eliminate dendritic segregation produced during the casting process. The normalization form of heat treatment is less expensive than annealing.

 Annealing is a heat treatment process that brings metal closer to a state of equilibrium. In this state in which metal softens it is more workable. Annealing, which the American Foundry Society refers to as "extreme over-aging," requires slow-cooking of metal to allow its microstructure to transform.

Because normalization is less expensive than annealing, it is, unsurprisingly, the most common industrialization process of metal. But why is annealing more costly? The Ispat Digest explains the cost difference as follows:

"In normalizing, since the cooling takes place in air, the furnace is ready for next cycle as soon as heating and soaking is over as compared to annealing, where furnace cooling after heating and soaking needs eight to 20 hours depending upon the quantity of charge."

Normalization isn't just less expensive than annealing, it produces harder and stronger metal than the annealing process does. Normalization is often used in the production of hot rolled steel products, such as railroad wheels, bars, axels and other forged steel products. 

While normalization may have advantages over annealing, iron generally benefits from any kind of heat treatment. This is doubly true when the casting shape in question is complicated. Moreover, iron castings in complex shapes are vulnerable to structural problems after they cool.

These structural irregularities can distort the material and cause other issues in the iron's mechanics.

To prevent such problems from occurring, metals undergo normalizing, annealing or stress relieving processes. 

Metals That Don't Require Normalizing

Not all metals require the normalization thermal process. For example, it's rare for low carbon steels to require normalization. That being said, if such steels are normalized, no harm will come to the material. Also, when iron castings are equally thick throughout and their section sizes the same, they are generally put through the annealing process rather than the normalization process.   

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