How Heat Treating Steel Works
September 16, 2021 | Categorised in: Steel 101
During World War 2, heat treating steel saw a huge increase in focus and funding as mass manufacturing facilities opened across the country to fuel the war effort. The discovery of the additional strength heat treating produced wasn’t the only finding that was made during this time, however. Ships that were made from heat-treated steel fractured at weak points when exposed to the cold Atlantic waters.
This is because as steel (and all metals) increases in hardness, it also increases in brittleness. Due to this tradeoff, different methods of heat treating steel were developed to maximize strength without sacrificing resistance to fracturing. However, before we discuss the specific methods, we need to understand the basic steps of the heat treatment process.
Heat treating stages
While the primary steps of heat treating are the same across all methods, there are differences between them that will be talked about in their sections. Heat treating broadly consists of the heating, soaking, and cooling stages to change the metallurgical structure into a stronger, more uniform one without impurities.
During the heating stage, the metal is heated uniformly without letting it reach its melting point. This is done slowly to ensure heating is even and one section of the metal doesn’t expand more than another, which would lead to cracked or weakened sections. How quickly you can heat the metal will depend on factors such as the condition and history of the steel as well as the size, type, and grade.
Once the steel has been heated to the desired temperature at the specified rate, it’s held at that temperature for a period of time. This duration also depends on factors of the steel from the previous section, in addition to the desired properties of the final product. The reason for holding the metal at this higher temperature is to allow the structure time to change to the desired configuration and shape. Because this step affects the properties of the atomic and chemical structure of the steel, those factors will also be crucial to determining the correct soaking time.
Once the steel has been soaked, it’s time to cool. There are many variations of this step (which we will discuss next), but it could involve quenching in a medium like water or oil, or it could mean just letting the metal cool in the air. Generally, water or brine are quicker at cooling than oil, which is quicker than air. How quickly the steel should be cooled will again depend on the type of steel (i.e. water used for carbon steels vs. oil used for alloy steels) as well as the desired final characteristics.
Types of heat treating
Now that we understand the basic process for heat treating, let’s take a look at the differences in methods:
While most people think of heat treating as hardening a metal, annealing is actually a form of heat treating that softens the material. This is done for the purpose of relieving stress or improving the grain structures (crystalline configuration) in the metal. Annealing is also one of the heat treatment methods that uses a slow cooling stage, usually either by letting the steel sit and cool in the furnace or by burying it in an insulating material.
The primary goal of normalizing is to remove internal stresses in the steel, similar to annealing. However, the key differences between the two are that normalizing is only done to ferrous metals (like steel) and the metal is removed from the furnace for air-cooling. Because this means that the cooling stage is quicker (room temperature air vs. furnace air), normalized steel becomes harder than annealed steel.
As mentioned in the previous section, faster cooling usually results in harder steel. This method is usually done with a very quick temperature reduction (around 1 second), such as quenching the steel in water or brine. Unfortunately, this fast cooling increases the chances of warping or cracking during the extreme change, as well as increasing the metal’s ductility and brittleness once finished. After hardening, the steel’s strength is often higher than needed and is outweighed by the risk of breaking. One way to make the steel more malleable and less likely to shatter is by tempering.
Tempering the steel is done after a quench and essentially repeats the heat treating process at a lower temperature. The steel is heated back up to another specified temperature that isn’t as high as its hardening temperature and allowed to soak, then set aside to cool in the air. This less extreme heating and cooling reduces the steel’s ductility and strength to a safer and more reliable level.
The final major method of heat treating is called case hardening (or surface hardening), where only the outer layer of the steel is hardened. This is useful when a key consideration of materials is surface durability but the steel still needs to be able to absorb an impact or deformation without breaking. By having a softer core, case hardened metals can achieve this.
The two major techniques for case hardening steel are carburizing and gas nitriding. Carburizing involves exposing the hot steel to carbon, which infuses the surface and strengthens it. Gas nitriding, on the other hand, exposes the hot steel to nitrogen or ammonia gas that reacts with the material to harden the surface.
Get your steel heat treated today
Heat treating your steel is a great way to improve the properties of the material and optimize it for certain projects and applications. If you’re interested in buying steel or getting steel heat treated for a project, get in touch and request a quote today.