Hardness Testing Calibration

Hardness testing is the most reliable and widely used test for evaluating the properties of metals and other solid materials. Hardness testing enables the evaluation of a material’s properties, such as strength, structural integrity, flexibility, wear resistance and quality. This helps determine whether a material is suitable for the application that is required.

The non-destructive nature of hardness testing makes it the most common method of inspection for Machine shops, Foundries, Engineering Labs and other metallurgy industries.

Hardness Test Methods

A hardness test is typically performed by measuring the depth or size of an indention from a precision indenter that is pressed into the surface of the material under a specific load for a specific interval of time. The shape of the indenter varies by type of hardness test and includes ball, cone and pyramid shapes.

There are four hardness tests that are typically used. The hardness test that is chosen is determined by the type of material, the size of the part and its condition. The listed standards for these tests contain specific information for the hardness test.

Hardness Tester Calibration

As in any piece of testing equipment, a hardness tester can become misaligned through the repeated physical use of the tester components. Regular inspection and calibration will be necessary to maintain its reliability.

There are two types of calibrations typically used for Harness Testers, direct and indirect.

Direct calibration requires verification of all of the individual components of the hardness tester, such as force, indenter dimensions, measuring device accuracy and the speed and timing of the indention cycle.

Indirect calibration involves checking of the Hardness tester against certified hardness test blocks representing various points on the ranges of the scales being verified.

Hardness tester indirect calibration is considered a more reliable process than direct calibration as this method will take into account all of the individual variables in the function of the
machine.

Certified test blocks are required to ensure the accuracy and traceability of the indirect hardness testing calibration. Several test blocks will be needed to calibrate the hardness tester. Typically, three test blocks per scale are required, one each for the low, mid, and high hardness readings for each range you test. It is possible that the tests can be correct at one hardness point on the scale and inaccurate at the other points.

First, the indenter should be inspected before the calibration to ensure that it is free from any surface defects. The area of the indenter that will be in contact with the material should also be polished.

Next, five readings are taken on each of the test blocks for the range you are testing and the readings are averaged. The distance between the centers of two adjacent indentations shall be at least two and a half to three times the diameter or diagonal of the indentation, depending on the method used. Material deformation from indentations made too close to each other or too close to the test block edges can lead to inaccurate hardness readings.

The readings from the Hardness Tester should indicate the hardness value from the certified test blocks for each range tested. A misaligned hardness tester can read correctly at one hardness value and incorrectly at other values. If the Hardness tester fails the test block calibration, a direct calibration should be performed to identify the suspect component and adjusted to meet its specifications.

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