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Tips and Rules about Hardness Testing

A number of factors influence the hardness result and in order to ensure a higher degree of reproducibility these factors must be observed. Below is listed a few tips and rules that could help make the test more reproducible as well as conform to existing hardness testing standards.

  • If allowed by standards, preferably use highest load/force possible.

  • Samples shall be polished (micro hardness testing) or fine ground (macro hardness testing) before testing. For very high loads raw material can however be tested. Surface quality will influence accuracy and repeatabilty of test results.

  • Whenever possible, always clamp/support the sample thoroughly. Use one of the many anvils or sample holders available.

  • The angle between indenter and sample surface should be perpendicular, 90°, with a deviation of max. +/- 2°

  • The thickness of the sample should be at least 10 times the indentation depth (Rockwell) and 6-8 times the indentation depth for Vickers. The indentation depth can be calculated from the various formulas. Otherwise please refer to the standards.

  • Testing on cylindrical surfaces can be performed, but correction factors need to be applied. Use special test anvil.

  • Indentation time should be between 10-15 seconds respectively 2-6 seconds (Rockwell, major force). You can check the test block certificate to see which conditions it has been calibrated under.

  • Check the rules for proper indent spacing. Hardness indents deform the sample material and alter its properties, thus requiring adequate spacing in between. In general please allow at least 3 diagonal spacing between indents. (The DuraScan-50/-70/-80 ecos software allows for automatic indent spacing, based on diagonal lengths, thus eliminating this problem.)

  • Avoid tests producing diagonal lengths less than 20 μm (Vickers, Knoop, Brinell), otherwise the measurement inaccuracy will be too high.

  • The difference between the two Vickers diagonal lengths should be max. +/-5%, otherwise the test should be discarded.

  • Avoid testing mounted samples with loads higher than ~10-20 kg as the mounting resin otherwise can influence the result. Rockwell (and most Brinell) tests on mounted samples are therefore not recommended.

  • The hardness tester should be kept in a dirt- and, vibration free environment, under constant, discrete illumination. The tester should be kept on a solid table and both tester and stage should be horizontal.

  • Also keep the hardness tester under constant temperature (preferably room temperature) and constant humidity.

  • Avoid tampering with the illumination settings as this will influence the calibration of the tester (Vickers, Knoop, Brinell).

  • Every time you change indenter or objective lens the tester needs to be recalibrated/verified. This is a strong argument for choosing a machine with a turret, with multiple indenter and objective lens positions.

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