Hardenability is a term used to describe a material's ability to be hardened when it is exposed to heat and then quenched, or cooled rapidly. It should not be confused with hardness, which refers to a material's strength and ability to resist damage. Instead, hardenability determines whether an object can be made harder, or whether it is resistant to hardening. This term is used only to refer to metal objects, including steel and metallic alloys, and is not applied to plastics or other materials.
The primary type of hardenability test is known as the Jominy or "quench" test. To perform this test, a steel rod is heated until it crystallizes into a face-centered cubic structure called austenite. After the heat source is removed, one end of the austenite rod is immediately subjected to a water spray to cool it to room temperature. This process of cooling is called quenching.
A very quick quench will cause martensite — a very strong material — to form. If the quench is not quick enough, a different material will form which is not as strong. The hardness of the rod is measured at specific increments from the quenched end. The further from the quenched end, the slower the cooling rates, making it less likely for martensite to form.
A material that forms martensite with lower cooling rates is easier to harden. A material that needs very quick quenches to form martnsite will be more difficult to harden. As a result, the greater the difference in hardness between the two ends, the lower the hardenability.
The hardenability of steels and other metals depends on both the composition of the object and its shape or geometry. The thicker an object, the slower the cooling rates in the center, making it more difficult to harden the material within. This means that thicker objects, or those with little surface area, will have a lower level of hardenability than smaller or thinner objects made from the same material. In a thin object, the heat has very little distance to travel, so the cooling rates can be fast and increase its level of hardness.
In general, the higher the carbon content of a steel product, the greater the steel's hardenability will be. Common elements added to steel to increase its hardenability include boron, manganese, chromium, and molybdenum. The addition of alloys should be carefully performed to avoid changing the properties of the steel or affecting its ability to be hardened.
Hardenability of steel and its ability to be welded are inversely related. The more hardenable the steel, the harder it will be to weld; the lower the ability to be hardened, the easier it is to weld. A hardenability test is often used in welding applications to determine whether two materials can successfully be welded. It may also help welders choose electrodes and welding equipment or settings.