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.
