Cory nailed it.
You inspect the indenter under a microscope and verify its condition and dimensions.
You verify the load that you are applying.
And you verify the calibration of the microscope that you are using to measure the indentions.
This system does not rely on reference standards.
The micro hardness testers, such as Mic 10 , do not use the classical Vickers visual measurement of the indentation dimension- they rely instead on the change in ultrasonic resonance of a small steel bar that terminated in a diamond tip. As the tip indents into the solid, the resonant frequency changes proportional to the hardness, but also in relation to the solid's young's modulus. Therefore, ehnever the solid has a young's modulus other than that of the default value, you must use a lab calibrated sample of the same material to be ultimately samples.
Case in point- P91 welded to 316H with a inconel 625 filler. The default calibration might be OK for the P91, but not the other 2 solids.
Microdur was not the original question, but it is my question. Experienced users say that the same grade of material being tested should be used for the calibration block. Is that strictly necessary, since the ferritic alloy steels do not vary that much in elastic modulus. Is there more to it than that?
brimstoner,
It is not necessary. The hardness tester is precalibrated for ferritic and other materials having essentially a Young's Modulus of 29,000,000. Accuracy is within specified limits. Any recalibration performed on test blocks of the same material grade as being to attempt produce the same results as stated on the test bar will most likely produce erroneous results (spoken from experience).
If the material to be tested has a different modulus than the precalibrated modulus, recalibration must be performed on calibration blocks having the same modulus.
Next MIC-10 question: when testing on tubes/pipes, do you have a rule of thumb for when bracing needs to be added? In terms of workpiece diameter and wall thickness (I understand it won't be one single answer).
I've never used bracing when using the MIC 10 or MIC 20 testers. Surface finish on the tube material and perpendicularity of the indentor to the tested surface are paramount. Only a 5 Kg or 10Kg load is applied by hand. I note that the smallest diameter and piping schedule subject to such testing under my direction has been 3/4", sch 80.
