Hardenability

a steel property which describes the depth to which the steel may be hardened during quenching.  It is important to note that hardenability is a material property, dependent on chemical composition and grain size, but independent of the quenchant or quenching system (cooling rate). However, the structures obtained across a quenched section are a function of both hardenability and the quenching process (severity of quench).

Hardness

a measure of the resistance of a material to plastic deformation.  This depends on the carbon content and microstructure of the steel.  Hence the same steel can exhibit different hardness values depending upon its microstructure, which itself may depend on how the sample was quenched, etc.

In order to select an appropriate heat treatable engineering steel for a given application it is important to be able to measure its hardenability. The most convenient way to do this is via Jominy Hardenability (or End Quench) Test. The other technique is to use transformation diagrams but these are more time consuming to generate. However there are now many in the literature that can be referred to.