Numerous aspects of the Rockwell hardness test can influence the measurement result. These include the function and calibration of individual components of the hardness machine, variations in the indenter, the testing cycle that is used, the testing environment, the condition of the test material, and the operator. When considering all of these influences, it seems remarkable that the Rockwell test has provided such a reliable test throughout its long usage. Much of the test’s reliability may be attributed to the common practice of performing periodic verifications of the testing machine, often several times during a day.
When a high level of accuracy is important, it is usually necessary to put more effort into a measurement process than is specified by test method standards (10)
As with any method of measurement, it is beneficial to identify the significant sources of error in a Rockwell hardness measurement so that an attempt can be made to reduce the errors and, thus, improve accuracy. Through an understanding of how the various test influences can affect a Rockwell hardness measurement, it becomes evident that a considerable difference in hardness results can be obtained for the same test sample merely by varying one or more of the test parameters. The difference in test results can be significant, even while remaining within the individual parameter tolerances specified by test method standards. It is also likely that many Rockwell machines are adjusted to offset one error with another error in order to correctly measure reference standards.
The ASTM and ISO test method standards specify the general procedures to use when performing a Rockwell hardness test. In addition, the instruction manual supplied with most testing machines normally provides supplementary details on specific operational procedures. This section will discuss procedures and precautions to be applied to general Rockwell hardness testing. It will not cover specialized procedures for testing the vast varieties of materials and part geometries for which Rockwell hardness may be used. It should be noted that there are many specialized fixtures, indenters, anvils, and testing machine configurations that are commercially available for the testing of large parts, long parts, inner surfaces, curved surfaces, and other complex shaped parts(1,11,12,13)
This section will also discuss several of the more significant sources of error of the Rockwell hardness test. These include the influences of the hardness machine, indenters, testing cycle, testing environment, and other factors that may affect the reproducibility of the test.