For example, some drawings will call out a Vickers hardness value when only a Rockwell C scale unit is available to conduct the final hardness test. Remember that when the Rockwell C scale test is conducted, the depth of penetration is being achieved by the application of a 150-kg load with a diamond indenter. Whereas the load for the Vickers (particularly if it is a microhardness value) has only a load application between 10 grams and 1,000 grams, not kilograms!!
The Vickers macro-test unit will apply a load to the indenter of 1 kg up to 120 kg. So what the writer is suggesting is that you cannot readily and accurately compare the two readings of Vickers to Rockwell. The depth of indentation by the Rockwell will only be giving you an “average” hardness reading throughout its depth of penetration into the heat-treated surface.
If the component is carburized, quenched and hardened steel, remember that as the carbon is diffusing into the steel surface there is a dilution effect occurring into the diffused and newly formed case. Thus, the resulting hardness value (Rockwell C scale) will not be a true reflection of the actual hardness.
The most effective method of determining both surface and core hardness values would be to “slice and dice” a coupon from the batch of steel being processed that is of the same (or similar) heat as that steel used for the manufacture of the component. Mount the sample (as one would mount a sample for metallographic examination). Then, pre-grind and polish followed by a transverse microhardness test across the now-exposed and polished surface into the core. The test could be conducted using either Knoop 300-gram load or Vickers-300 gram load.
If the component was through hardened, the same test-coupon orientation could be used. The Rockwell C scale at 150-kg load would be used across the polished and exposed face of the component to obtain a comparative hardness value. Surface preparation is extremely important to obtaining (and interpreting) an accurate hardness test.
At times, a Brinell hardness test value will be converted into a Rockwell reading. The comparative result is not accurate because the Brinell hardness value is the measurement of displacement of material in relation to the load applied (3,000 kg) as opposed to the measurement of depth with the Rockwell C scale (150 kg).
Be sure to utilize the most appropriate hardness test method to obtain the most representative test result of the component being tested.