Statistical Engineering Division, ITL
Metallurgy Division, MSEL
Across the surface of an HRC test block, the hardness is not uniform despite care in block manufacture. The effects of this nonuniformity on results can be incorporated into the uncertainty ascribed to the results. More can be done, however. The standard deviation of the hardness difference for two points 5 mm apart is half that of the standard deviation for large spacings. By taking this into account, one can determine differences among hardness testing machines and indentors more precisely. When the points on the block are chosen properly, the uncertainty reduction might be much more than 50 percent.
One only compares hardness measurements with each another. Sometimes this is an in-house comparison as in the use of hardness measurement to control a manufacturing process, and sometimes this is a contractual comparison to see if a supplier met customer specifications. There is no comparison with a qualitatively different measurement because hardness cannot be computed by scientific theory from any other measurement. For example, there is nothing similar to the mass balance equations that connect an assay of gold ore with the amount of gold extracted from the entire ore lot. Thus, the only issue is the relation of hardness measurements made under one set of circumstances with those made under another.
Ideally, hardness measurements to be compared will be made with exactly the same equipment and procedure, but of course, they can't be. Test blocks are used to characterize differences among the ways measurements are made. Based on this characterization, one might adjust the measurement procedures or one might determine calibration equations. In any either case, the nonuniformity of the test blocks interferes with the characterization and limits one's ability to make different measurement systems produce nearly identical results. Thus, a method for reducing the effects of the nonuniformity serves to diminish a fundamental limitation on the comparability of hardness measurements.
Figure 6: Hardness variation across a test block with 28 mm radius, HRC range 64.670 - 64.865.
Date created: 7/20/2001