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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 softer the material, the deeper the penetration, the wider the impression.
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.
Jominy test measures potential depth steel will harden.
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.
Web resources:
Gordon England Thermal Spray Coatings
Farmingdale State College School of Engineering Technologies
Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which was originally posted here.
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