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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.
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