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Hardness-test is one of the easiest and most informative tests that shed light on important material properties.
It is however prone to misunderstanding unless its specific meaning is well defined in the field of applications in which it is used.
For those who deal with construction materials, with metals in particular, hardness has to be described as the property expressing resistance to indentation.
While the precise procedure of performing the indentation and of measuring the resulting imprint has to be specified in order to provide meaningful information to all, in general one can say that a small mark indicates a hard material, and a large mark a soft one.
A Hardness-test of any standard type will correctly express an acceptable result, if performed within the established conditions (minimum thickness, surface preparation, suitable hardness) using:
- a specific indenter (defined by fixed shape, dimensions, material and properties),
- under the application of a certain static force,
- for a definite time,
- following precise measuring procedures.
It must be stressed at once that hardness is not a physical property like density or melting point.
It is indeed a structural property reflecting the quality of the specific microstructure of the specimen.
This depends on the material, on the fabrication processes and on the thermal history at the time and in the condition it is when presented to Hardness-test.
The popularity and the common use of Hardness-test for routine acceptance of materials and parts, for process control (i.e. heat treatments, welding etc.), or as an essential element of any metallurgical investigation, derives from its advantages:
- It is non destructive, in that it leaves only a small mark on the surface,
- It is inexpensive and gives an immediate result,
- It is reliable and repeatable in the hands of knowledgeable inspectors.
Cui Prodest? (lat. To whom may it help?)
1 - For routine acceptance:
of materials - to check the supply condition per attached documents
of parts - gears (on tooth hardness), shafts, etc. per part drawings
2 - For Process Control (on representative specimen):
per specification requirements
Heat Treatment: stress relieving, annealing, hardening and tempering, case hardening, Welding, Hardfacing, Thermal Spray
3 - For metallurgical investigations:
to find out hints on influence of external modifying causes
4 - For establishing repair procedures:
to characterize unknown material and condition
(together with XRF analysis. See Material Identification.)
Practical Hardness Testing
Our readers know that they can get at no cost my book on Practical Hardness Testing made simple, by subscribing to the periodic publication Practical Welding Letter.
Whoever did not yet download it, is invited to do so now by Subscribing.
The book explains the procedures in some detail, and warns on some errors that can result if it is assumed without further inquiry that the surface hardness measured is indeed representative of the bulk hardness.
What is meant by this is that the Hardness-test is local in character: in principle it represents the hardness of the tested spot only.
The assumption that the same result is valid for the whole volume, must be based on additional knowledge confirming that indeed this is the case.
The multiplication of specific Hardness-test procedures, all developed empirically, derives from the fact that no Hardness-test type was found to cover satisfactorily all the possible combinations of materials and conditions.
Each type of test, within a range of loads, has found practical use for given applications, to cover all industrial requirements.
Any resulting Hardness-test, to be meaningful, must include, in an accepted shorthand convention, a description of the type of test and of the load applied, the other conditions being then understood as normal.
The results of Hardness-test are interpreted sometimes as having a more general meaning, involving properties like strength or machinability.
Readers are warned that conclusions based only on hardness may be faulty unless confirmed by additional information.
It is true that tables are available giving statistical strength data corresponding to certain hardness results.
However one should remember that hardness alone does not give any cues as to toughness available.
Therefore it is not acceptable to take into account only hardness for design purposes.
Similarly hardness may not mean much for deciding if a material is easily machinable.
For a complete description of the Hardness-test types readers are referred to my book mentioned above. Here only a short review is presented.
Note: - Two of the Videos presented hereafter show the application of automatic testing. It may be useful, speedy, and economic.
However it must be performed by a knowledgeable inspector, lest the preparation or the positioning might be at fault. In that case the results might be wrong.
Indentation was originally done with a hardened steel ball of 10 mm and a load of 3000 kg.
The Brinell Hardness Number is expressed as the load divided by the surface area of the indentation. Practically the formula (see book above) is not used, the result is looked up in suitable tables, for any measured diameter of indentation.
Later on, reduced diameters and loads were accepted for softer materials. It was found that if geometrical similitude was maintained, as when P/D2 is constant, the same Brinell numbers could be obtained/ where P is the load in kg and D is the ball diameter in mm.
Standard Test Method for Brinell Hardness of Metallic Materials
ASTM International / 01-Jun-2010 / 32 pages
| Watch the Video for a demonstration of
Automatic Brinell Hardness Test
Here the indenter is a square based diamond pyramid with opposite faces forming an angle of 136 degrees. Accepted loads are 5, 10, 30, 50 kg but smaller an larger loads can be used when necessary.
The diagonals of the impression have to be precisely measured with a microscope, and the Vickers hardness result is looked up in tables. The Hardness-test must report the load used.
This type of test is much useful.
A modification of this test, called Knoop, with a different form of diamond indenter is used for microhardness testing with low loads.
BS EN ISO 6507-1:2005
Metallic materials. Vickers hardness test. Test method
British Standard / European Standard / International Organization for Standardization /
23-Jan-2006 / 30 pages
Standard Test Method for Knoop and Vickers Hardness of Materials
ASTM International / 01-Feb-2010 / 42 pages
BS EN ISO 4545-1:2005
Metallic materials. Knoop hardness test. Test method
British Standard / European Standard / International Organization for Standardization /
30-Jan-2006 / 26 pages
An Article on Low Load Hardness Testing was published (7) in Issue 105 of Practical Welding Letter for May 2012.
Click on PWL#105 to see it.
For a demonstration of a Vickers Hardness Test, watch the Video:|
The Rockwell Hardness-test is probably the most used method because it is simple and self contained, so that
there is no need for a separate microscope reading. But it requires care to be performed correctly.
The indenter is either a sphere of 1/16" diameter or a conical diamond (called "Brale") with an included angle of 120 degrees. A minor load of 10 kg is applied and then the dial of the measuring instrument is set to zero.
To distinguish which indenter is applied a letter is used following R for Rockwell, B for the steel ball and A or C for the brale indenter.
Then one of the major loads is applied (60(A), 100(B) or 150(C) kg) as required, for the specified time.
Then the major load is removed while the minor load is still in place, and the depth of the indentation is automatically recorded. A high hardness, corresponding to a small penetration, results in a high number.
Other scales are used for special purposes.
Standard Test Methods for Rockwell Hardness of Metallic Materials
ASTM International / 01-Dec-2008 / 37 pages
For a demonstration of an automatic Rockwell C hardness test, watch the video:|
How to Select
Hardness testing machines should stand on a stable floor, free from vibrations.
The equipment used to perform Hardness-test must meet the requirements of the applicable standard, and must be calibrated per specification at least once a year to demonstrate its suitability to perform the test. In current practice a daily check is also required involving the use of calibrated reference blocks.
Generally the method specified in the requirements should be preferred, if possible.
The use of official Tables for finding correspondence is tolerated when necessary as the results are only approximate. Using tests at the lowest loads may give hardness results different from those with regular loads.
Several tests should be performed to be statistically relevant.
Interlaboratory Round Robin tests (made by different labs on the same material) can give different results, although only exceptionally any lab will find itself out of consensus.
Forging and Castings are preferably Brinell Hardness Tested.
Large or heavy parts may be tested with portable equipment.
Aluminum and other non ferrous metals are tested with reduced diameter Brinell spheres.
Regular parts or material samples of favorable geometric shapes are Rockwell Hardness Tested if possible. Testing on a curved surface may require hardness correction due to shape.
Parts of unknown hardness, thin parts or parts with special requirements (gears) are Vickers Hardness Tested, possibly using special tools for holding them.
Unique testing machines may be needed for special requirements (i.e. hardness inside a bore), manufacturers should be approached to check availability of special equipment.
Parts needed for microhardness testing are embedded in metallographic holding mounts.
On August 19, 2010, A Webinar on
Hardness Test in Real-World Applications
by George F. Vander Voort,
was held online, broadcast with Webex technology (http://www.webex.com) by ASM International.
Now ASM has made available a recording of that webinar at:
It is a most interesting and instructive Webinar that all readers, however expert in Hardness Testing, are urged to see for themselves.
An Article on Low Load Hardness Testing was published (7)
in Issue 105 of Practical Welding Letter for May 2012.
Click on PWL#105 to see it.
To get at no cost each issue as it is published, please subscribe.
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