Hardenability refers to a material’s ability to be hardened when heated and cooled rapidly. The Jominy test is used to determine hardenability in metals, with thicker objects having lower hardenability. Carbon content and alloy additions affect hardenability, which is inversely related to weldability.
Hardenability is a term used to describe the ability of a material to be hardened when exposed to heat and then cooled or cooled rapidly. It should not be confused with hardness, which refers to the strength and ability of a material to resist damage. Instead, hardenability determines whether an object can be made harder or is resistant to hardening. This term is used only to refer to metallic objects, including steel and metal alloys, and does not apply to plastics or other materials.
The main 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 has been removed, one end of the austenite rod is immediately sprayed with water to cool it to room temperature. This cooling process is called quenching.
A very rapid quenching will cause the formation of martensite, a very strong material. If the quenching isn’t fast enough, a different material will be formed that isn’t as strong. Rod hardness is measured in specific increments from the hardened end. The further away from the quenched end, the slower the rate of cooling, making martensite formation less likely.
A martensite-forming material with slower cooling rates is easier to harden. A material that needs very rapid quenching to form martnsite will be more difficult to harden. Consequently, the greater the difference in hardness between the two ends, the lower the hardenability.
The hardenability of steels and other metals depends both on the composition of the object and on its shape or geometry. The thicker an object, the slower the rate of cooling at the core, making it harder for the material inside to harden. This means that thicker or smaller surface area objects will have a lower hardenability level than smaller or thinner objects made of the same material. In a thin object, heat has a minimal distance to travel, so cooling rates can be high and increase its level of hardness.
In general, the higher the carbon content of a steel product, the higher the hardenability of the steel. Common elements added to steel to increase its hardenability include boron, manganese, chromium and molybdenum. The addition of alloys must be done carefully to avoid changing the properties of the steel or impairing its ability to harden.
The hardenability of steel and its ability to weld are inversely related. The more the steel is hardenable, the more difficult the welding will be; the lower the hardening capacity, the easier it is to weld. A hardenability test is often used in welding applications to determine if two materials can be successfully welded. It can also help welders choose electrodes and welding equipment or settings.
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