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To better understand the Rockwell hardness testing cycle, imagine it divided
into eight parts or steps, as illustrated previously in Figure 3. These eight testing
cycle steps are defined as either a time period or an indentation velocity, each
of which can be varied. They are:
1. the contact velocity of the indenter at the point of contact with the test
material;
2. the preliminary force application rate as the preliminary force is applied;
3. the preliminary force dwell time, the time period from the onset of reaching
the preliminary force until the first baseline depth of indentation is
measured;
4. the additional force application rate as the additional force is added to the
preliminary force to obtain full application of the total force;
5. the total force dwell time, the time period during which the total force is
fully applied;
6. the additional force removal rate as the additional force is removed,
returning to the preliminary force level;
7. the recovery dwell time, the time period from when the additional force is
fully removed, until the second and final depth of indentation is measured;
and
8. the preliminary force removal rate as the preliminary force is removed.
With the exception of step 8, which has no influence on the hardness
measurement, all of the testing cycle steps can affect the hardness result to
some degree, some considerably more than others. The extent of the possible
range in hardness values depends on which steps of the testing cycle are
varied and the amount of the change, and on the hardness level and flow
properties of the material under test. Both an increase and decrease in the
hardness measurement value can occur by changing any one of the testing
cycle steps. It is also possible that by varying two or more steps of the testing
cycle, the respective effects can offset the others and result in essentially no
change in the measured hardness. The testing cycle steps that are considered
to have the greatest effect on the hardness measurement result are typically
specified by the Rockwell hardness test method standards. Also, newer
commercial hardness testers are often pre-programmed by the manufacturer
with a default testing cycle, defining two or more of these variables.
The effect that each of the eight test-cycle steps has on the hardness result
can be divided into two categories: (1) indenter velocity or force application
rate effect; and (2) dwell time effect. Steps 1, 2, 4, 6, and 8 fall under the first
category. The effect of obtaining different measurement values by varying one
or more of these five testing cycle steps is due either to rate sensitivity of the
material under test or to the dynamics of the hardness tester. The remaining
three parts of the testing cycle, steps 3, 5, and 7, fall into the second category
defined as dwell times. Each of the three dwell time steps affect the hardness
result because of creep and elastic recovery of the test material which occurs
during these periods of constant force levels. The relative effect that each of
the eight test-cycle steps has on the Rockwell hardness result are discussed
below by presenting data from actual Rockwell hardness measurements.
The information is presented to illustrate trends only since the effect of each
testing cycle step will vary depending on the hardness scale and the specific
material tested.
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