What is meant by the term “hardenability"?

Hardenability is a term that is used to describe a given steel’s ability to harden. It does not mean “what hardness can be achieved.”

Hardenability is usually determined by the Jominy end-quench test. The method involves machining a piece of steel to specific dimensions, heating the steel to its appropriate austenitizing temperature followed by spraying a volume of water onto the end face of the bar. This will cool the face rapidly and more slowly, progressively cooling the length of the bar. The bar is marked off at 1/16th-inch intervals.

After the test is complete, the bar is hardness tested every 1/16 inch and a curve is plotted. This is the steel’s ability to respond to a specific heat-treat procedure in terms of hardness values across its cross section. In other words, it will show the depth of hardening across an equivalent diameter bar of steel. The addition of alloying elements will affect the steel’s hardenability.