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The Rockwell hardness test is one of several common indentation
hardness tests used today, other examples being the Brinell hardness test
and Vickers hardness test. Most indentation hardness tests are a measure of
the deformation that occurs when the material under test is penetrated with a
specific type of indenter. In the case of the Rockwell hardness test, two levels
of force are applied to the indenter at specified rates and with specified dwell
times, as illustrated for the Rockwell C scale (HRC) test in Figure 1. Unlike
the Brinell and Vickers tests, where the size of the indentation is measured
following the indentation process, the Rockwell hardness of the material is
based on the difference in the depth of the indenter at two specific times during
the testing cycle, indicated by the X
marks in Figure 1. The value of hardness
is calculated using a formula that was derived to yield a number falling within
an arbitrarily defined range of numbers known as a Rockwell hardness scale.
Because the hardness value is dependent on the definition of the test method,
there are no alternative measurement systems to directly or independently
measure Rockwell hardness, nor are there intrinsic artifacts to reference.
The general Rockwell test procedure is the same regardless of the Rockwell
scale or indenter being used. The indenter is brought into contact with the
material to be tested, and a preliminary force (formally referred to as the minor
load) is applied to the indenter. The preliminary force is usually held constant
for a set period of time (dwell time), after which the depth of indentation is
measured. After the measurement is made, an additional amount of force is
applied at a set rate to increase the applied force to the total force level
(formally referred to as the major load). The total force is held constant for
a set time period, after which the additional force is removed, returning to the
preliminary force level. After holding the preliminary force constant for a set
time period, the depth of indentation is measured for a second time, followed
by removal of the indenter from the test material. The measured difference
between the first and second indentation depth measurements, h, (see
Figure 1) is then used to calculate the Rockwell hardness number. For many
older models of Rockwell hardness machines, the operator must manually
control most or all of the steps of the test procedure. Many of today's newer
machines automatically perform the entire Rockwell test.
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