Indirect verification, as specified by the test standards, involves assessing two aspects of the hardness machine: (1) its repeatability, or how well the hardness machine can repeatedly measure the same value on uniform material; and (2) its error (or bias), or how well the machine’s measurement agrees with reference standards. The methods specified by the ASTM and ISO standards for assessing these parameters involve making hardness measurements distributed across the surface of reference blocks. Each Rockwell scale is evaluated in this manner usually by testing three reference blocks per scale; the hardness levels of the three blocks are chosen to cover the hardness range of each scale. It is important that the verifications of the hardness machine be made with the indenter that will be used for routine testing.
The assessment of both the repeatability and error are usually based on the same set of hardness measurements (typically five per reference block). The range of the measured values (maximum minus minimum) of each block provides an indication of the repeatability of the hardness machine when testing that specific hardness level. The difference between the average of the measured values and the certified average values of the respective reference block provides an estimation of the measurement error or bias.
5.2.1 Measurement Repeatability
When the repeatability measurements are based on tests made across the surface of a test block, the repeatability value will include an error contribution due to the non-uniformity of the test block. Depending on the degree of the hardness non-uniformity of the block, this error contribution can be significant. When performing an indirect verification, it is best to use reference test blocks having the highest degree of uniformity as possible.
Good Practice Recommendations
• The repeatability of the hardness machine should be assessed periodically and tracked over time. An increase in the lack of repeatability may indicate a problem with the Rockwell machine such as worn parts or the need for cleaning and maintenance.
• A better estimate of repeatability than suggested by the test method standards may be obtained by making a set of measurements in close proximity to each other, adhering to indentation spacing restrictions (see 3.4.7) such that there is no influence from a previous indentation. A pattern such as illustrated in Figure 8 is recommended. The close proximity of the measurements will reduce the effect of hardness nonuniformity in the test block. This procedure must not be used for assessing the measurement error as described below.
5.2.2 Measurement Bias or Error
The indirect verification of the measurement error or bias is the final indication of how all the errors in the test machine have combined together to influence the Rockwell hardness measurement. Even when all the parameters of a Rockwell testing machine are within specified tolerances, the final measurement result can be outside the allowable limits for the total error. This is because the errors associated with the separate Rockwell hardness test parameters each have acceptability limits that are relatively wide. If one were to combine all of the maximum allowable errors for the individual parameters, the combination would far exceed the specified allowable total error in measurement capability. Therefore, either the errors associated with the individual parameters must be reduced to as small a level as possible so that the combination of the individual errors does not exceed the total error tolerance, or the individual parameters must be adjusted within tolerances to produce offsetting errors so that when combined, the total error tolerance is not exceeded. Both of these techniques rely on direct verification of the adjustments.
Today, the most commonly used technique for handling measurement errors is to make an adjustment to one or more machine components to reduce the total measurement error. Unfortunately, since direct verification is rarely performed, it is not known whether it is the problem component that is being adjusted to reduce its error, or a within tolerance component that is being adjusted, possibly out of tolerance, to offset the error. It is generally felt by some hardness equipment manufacturers that, in the United States, a majority of Rockwell hardness machines would not pass a full direct verification due to individual parameters being out of tolerance and the associated error being offset by adjustments of other parameters. This practice is not recommended, and it can lead to problems when testing materials at hardness levels other than the test blocks levels used for the indirect verification or when testing materials other than the test block material.
Good Practice Recommendation
It is recommended that the as-found condition of the testing machine be assessed as part of an indirect verification. This is important for documenting the historical performance of the machine. This procedure should be made prior to any cleaning, maintenance, adjustments, or repairs. The as-found condition of the testing machine should be determined with the user’s indenter(s) that are normally used with the testing machine. One or more standardized test blocks in the range of normal testing should be tested for each Rockwell scale that will subsequently undergo indirect verification. If the as-found condition verifications fall outside specified tolerances, it is an indication that hardness tests made since the last indirect verification may be suspect.
