Because of its wide use, the Rockwell hardness test is often used to measure the hardness of round or cylindrical objects, such as shafts and metal tubes.
However, when testing hardness on non-flat surfaces, it is important to take into account the effect of non-flatness of the samples on the results.
This is because a not perfectly flat object can cause errors in the test results due to uneven force distribution. The hardness values will consequently not be true and accurate.
Reference standards help us in this. ASTM-E18 and ISO-6508 show how to easily correct results.
Let’s see how together: just 3 steps are needed.
First of all, keep in mind that the correction coefficient does not apply indifferently on all Rockwell hardness test scales: those mentioned by ISO-6508 and ASTM-E18 are A, C, D, B, F, G, 15N, 30N, 45N, 15T, 30T and 45T.
So, if you want to apply the correction on other scales not mentioned here, know that you are not respecting international reference standards.
In that case, we suggest you consider other hardness detection methods.
Do you need advice?
To apply the correction coefficient on cylindrical or spherical samples, it is essential to verify that the minimum diameter/radius is respected with reference to the Rockwell hardness test scale chosen.
For your convenience we have summarized below the minimum and maximum thicknesses for each scale following the ISO-6508-1 standard.
Is everything clear so far? Let’s continue then.
Now that we have all the basic elements of application of the correction coefficient on the rounds, the only thing we have to do now is to calculate it.
In the latest generation of digital Rockwell hardness testers, the calculation is superfluous: Simply indicate in the software settings the diameter/radius of the test sample and the correction is automatically applied by returning the correct HR value at the end of the test cycle.
In the analog versions (i.e. those with mechanical comparator), unfortunately you will have to calculate it.
According to ASTM-E18 and ISO-6508 standards, the application of the correct correction coefficient depends on 3 essential parameters:
1. radius or diameter of test sample
2. type of indenter used
3. type of Rockwell hardness test scale used
Once this information is collected, it is sufficient to perform a Rockwell hardness test cycle, obtain the value and consult the table provided by the regulations
Let’s take an example:
– Scale used: HRA
– Test sample radius: 8 mm
– Value obtained: 20HRA
– Correction coefficient to be applied: 20HRA + 2,5 = 22,5 HRA
