How and Why Steel Is Normalized
Find out how this form of heat treatment makes iron more workable
How is steel normalized, and what are the benefits of this process? In order to understand what steel normalizing is, you first need to understand what heat treatment is. Heat treatment is a process used to change the physical or chemical properties of a material. Heat treatment of materials includes metal heat treatment, titanium heat treatment, heat treatment of aluminum alloys, heat treatment of stainless steel, heat treatment of brass, heat treatment of carbon steel, and heat treatment of iron.
What Does Normalization Mean?
Normalization is a specific type of heat treatment that relieves stress on steel; this improves ductility and toughness in steels that may harden after the cold working process. Normalized heat treatment also establishes a more uniform carbide size and distribution, which facilitates later heat treatment operations and produces a more uniform final product.
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 determine how high the workpiece is heated.
Other Benefits of Normalization
Normalization can also eliminate dendritic segregation produced during the casting process. The normalization form of the 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, the metal becomes softer and, hence, easier to work with. 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, not surprisingly, the most common industrialization process of metal. If you're wondering why annealing is more costly, the Ispat Digest provides a logical explanation for the cost difference as follows:
"In normalizing, because the cooling takes place in the air, the furnace is ready for the next cycle as soon as the heating and soaking stages are over as compared to annealing, where furnace cooling after the heating and soaking stages 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. Normalization is often used in the production of hot rolled steel products, such as railroad wheels, bars, axles, and other forged steel products.
Preventing Structural Irregularities
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.
Other Various Heat Treatment Processes
Carburizing steel. Carburizing heat treatment is the introduction of carbon into the surface of steel. Carburizing occurs when the steel is heated above the critical temperature in a carburizing furnace, which contains more carbon than the steel contains.
Decarburization. Decarburization is the removal of carbon from the surface of steel. Decarburization occurs when the steel is heated above the critical temperature in an atmosphere that contains less carbon than the steel contains.
Deep freezing steel. Deep freezing is cooling a steel to approximately -100 F or lower in order to complete transformation of austenite to martensite.