How and Why Steel Is Normalized
Find out how this form of heat treatment makes iron more workable
Steel normalizing is a kind of heat treatment, so understanding heat treatment is the first step in understanding steel normalizing. From there, it isn't hard to understand what steel normalizing is, and why it's a common part of the steel industry.
What is Heat Treatment?
Heat treatment is a process in which metals are heated and cooled to change their structure. The changes to the metals' chemical and physical properties differ based on the temperatures that they are heated to and how much they're cooled down afterward. Heat treatment is used for a wide variety of metals.
Metals are typically treated to improve their strength, hardness, toughness, ductility, and corrosion resistance. The different ways in which metals can undergo heat treatment include annealing, tempering, and normalizing.
The Basics of Normalization
Normalization removes impurities in steel and improves its strength and hardness. This happens by changing the size of the grain, making it more uniform throughout the piece of steel. The steel is first heated up to a specific temperature, then cooled by air.
Depending on the type of steel, normalizing temperatures usually range from 810 degrees Celsius to 930 degrees Celsius. The thickness of the metal determines how long a piece of metal is held at the "soaking temperature"—the temperature that transforms the microstructure. The thickness and composition of the metal also determine how high the workpiece is heated.
Benefits of Normalization
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, the metal becomes softer and easier to work with. Annealing—which the American Foundry Society refers to as "extreme over-aging"—requires slow-cooking metal to allow its microstructure to transform. It is heated above its critical point and allowed to cool slowly, much slower than during the normalization process.
Because of its relative inexpensiveness, normalization is 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."
But normalization isn't just less expensive than annealing, it also produces a 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. Iron castings in complex shapes (which can be found in industrial settings like mines, oilfields, and heavy machinery) 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 have a consistent thickness and equal section sizes, they are generally put through the annealing process, rather than the normalization process.
Other Heat Treatment Processes
Carburizing steel: Carburizing heat treatment is the introduction of carbon into the surface of the steel. Carburizing occurs when the steel is heated above the critical temperature in a carburizing furnace that contains more carbon than the steel contains.
Decarburization: Decarburization is the removal of carbon from the surface of the 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 steel to approximately -100 degrees Fahrenheit, or lower, to complete the transformation of austenite to martensite.