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Hardness is a measure of a material’s resistance to shape changes (deformation), and this widely used measurement technique is applied to metals of all kinds. Indeed, the value of the hardness often is a requirement in contracts between producers and consumers of metals. For many products, samples of the metal may be brought to the test laboratory and evaluated using a benchtop hardness tester. But for massive or unwieldy parts or for field evaluations, the tester must come to the part, in the form of a portable hardness tester.
In the US, several ASTM standards under committee E28 on Mechanical Testing govern the laboratory measurement of hardness: E10 for Brinell hardness, E18 for Rockwell hardness, and E92 for Vickers hardness. A separate standard, E110, governs the use of portable hardness testers. In principle, these portable testers apply the same forces and use the same indenter geometries as their benchtop relatives. For years, manufacturers and users have tacitly assumed that uncertainty of the measurements from these portable hardness testers was similar to that from the larger benchtop models.
To quantify the variability of measurements from portable hardness testers, the NIST Hardness Standardization program organized and led an interlaboratory study. Fittingly, they conducted the evaluation in the field, at the most recent ASTM E28 committee meeting in Tampa, Florida. Manufacturers, who were already planning to attend the committee meeting, simply brought their portable testers along on the trip. They compared their performance using calibrated hardness blocks. The results opened eyes. The variability of the measured hardness was nearly twice the acceptance tolerances for benchtop testers verified with test blocks. The uncertainty estimate from this study will be incorporated into the proposed revision to the E110 standard later in 2008.
One impact of the NIST-led study is that manufacturers and users now recognize that the portable hardness testers are not substitutes for the benchtop models, and that results from portable testers must be viewed with caution.