Hardness vs. Hardenability-There Is A Difference

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 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.

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.

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6 Responses to Hardness vs. Hardenability-There Is A Difference

  1. Irene otieno says:

    I would like to be associated with you,am taking mechanical engineering

  2. priyanshi says:

    thanx

  3. Andy steven says:

    A Brinell test is not as accurate as a Rockwell test because the indentation of the carbide ball is not as consistently finished as a diamond cone” In contrary! It’s much easier to manufacture and calibrate a perfectly round carbide ball, than it is to grind a Rockwell 120º diamond cone with a 0.2um radius round tip. To complete the story of thin metals testing for Rockwell:International standards recognize there is a limit to minimum sheet metal thickness that can be standard conform tested with Rockwell. They there for allow an exception: HR30Tm and HR15Tm – with the M for modified.These modified Rockwell tests are carried out under conditions similar to those of the normal superficial Rockwell test scales HR15T and HR30T, with the addition that a visible imprint on the backside of the sample is allowed. In order to prevent your hardness testing equipment from getting damaged the sheet metal sample must be supported by a special anvil, containing a polished and smooth diamond insert of approximately 4,5mm in diameter. To not damage this special diamond spot anvil, only steel (ASTM E18) and carbide (ISO6508) ball indenters are allowed for this special thin specimen procedure. Most digital Rockwell testers have a separate test program for HR30Tm and HR15Tm, indicating that your results are not subject of the standard HR30T or HR15T, but of these special conditions. This is an important improvement of your data integrity, and beneficial in communication with your customer or supplier.

    Any ways just in order to share knowledge i am i have my site regarding hope you will like this: http://www.intechnde.com/store/hardness-testing.html

  4. Laura Kim says:

    These information are very straight forward. Thank you PMPA! :)

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