Hardenability and the Strength of Steel

There are many different types of steel, and many of them must go through the quenching process to ensure they achieve the proper hardness.

The hardenability of a piece of steel (or a material in general) is important when selecting materials to use for specific applications. A metal’s hardenability is determined by how deeply that metal can be hardened with the quenching process after achieving high temperatures. This trait might also be referred to as “depth of hardening.”

There are three factors that ensure successful heat treatment and strong hardenability of a piece of steel. Let’s take a quick look at what hard steel is and how it’s created in Ukiah, CA.

Size and shape

In quenching, heat is transferred from the surface of the piece before being dissipated into the medium used for quenching. The cooling rate of the interior of the piece depends on the ratio of its surface area to its volume. Pieces with larger ratios will have a faster cooling process, which will in turn create a deeper hardening, and a greater hardenability.

So, for example, a piece with a 5:1 ratio of surface area to volume will have a higher hardenability than a piece with a 5:2 ratio of surface area to volume. Parts that have more edges and corners are easier to achieve greater hardenability than parts that have regular or rounded shapes because of the larger surface area.

Steel composition

There are many different types of steel alloys, each of which will have its own unique elemental compositions. The ratio of the elements in the steel to the amount of iron in the steel could produce a wide range of mechanical properties.

For example, an increased carbon content in the steel will produce greater hardness and strength, but less ductility. Increased chromium will provide greater corrosion resistance.

With so many different combinations of elements available to create such a wide variety of mechanical properties, it is important to try to separate those steels into different categories so it becomes easier to characterize hardness across broader steel classifications. The Jominy test, for example, is a way to categorize hardenability across various types of steel.

Quenching methods

The specific method used for the quenching process will also have a significant impact on the resulting hardenability of the metal. Different methods produce different cooling rates.

For example, the various quenching media can yield different hardness in the workpiece. Examples include oil, polymers, air and water.

Water results in the most severe quench, then oil and then air. Aqueous polymers can be designed for specific applications by switching up the concentration of polymers and the temperature of those polymers. They’ll typically provide quenching rates somewhere between water and oil.

Degree of agitation also affects rate of cooling and hardening. If the quenching medium moves faster over the specimen, this will result in more effective quenching.

Again, water is the preferred method, except for in situations in which it would produce a quench too severe for the specific type of steel.

For more information about steel hardness and hardenability as a concept and to learn more about what steel is composed of, contact Evden Enterprises in Ukiah, CA today.