Don’t confuse hardness and hardenability. Hardness is a material property. Hardenability is a way to indicate a material’s potential to be hardened by thermal treatment.
Hardness is resistance to penetration. Hardenability describes how deep the steel may be hardened upon quenching from high temperature. The depth of hardening is an important factor in a steel part’s toughness.
The brinell test uses a 10mm hardened steel (sometimes carbide) ball and various levels of force applied over a specified time.
The width of the impressions is measured optically and averaged. (Wider impressions mean the ball penetrated deeper, thus, the material is less hard.) The Brinell hardness number is calculated by dividing the load applied by the surface area of the indentation. Prior to today’s direct reading instruments, the measured indentation diameters could be looked up on a reference chart and the corresponding Brinell hardness number given.
The Rockwell test is similar, but uses different forces and either a smaller ball indenter (Rockwell B scale ) or a diamond indenter (Rockwell C scale).
Hardenability- Jominy Test
In the Jominy test, a standard specimen is heated then water quenched from the end, and a series of rockwell hardness tests are taken in 1/16th inch increments along the length of the specimen.
It is the influence of the steel’s chemical makeup (Carbon and Alloying elements) that determine how a deeply a grade of steel will transform to martensite for a particular quenching treatment. This means that for each grade being heat treated, mechanical properties are a result of cooling rate (quench). An excellent web page on this can be found here.
So what of the difference between hardness and hardenability?
Hardness is resistance to penetration under specified conditions of load and indenter.
Hardenability is the ability of a steel to acheive a certain hardness at a given depth, upon suitable heat treatment and quench. Hardness can be measured in steels in any condition. Hardenability presumes that the steels will be heat treated to acheive a targeted hardness at a given depth.
One is an actual property, one is a measure of potential.
And now you know.