Page 43 - Metalforming Magazine April 2022

heat-affected zones, differences in microstructural phase hardness and metallurgical phase segregation, among other properties. ASTM E384 and ISO 6507 cover the standard test method for Vickers microhardness testing.
Converting Between Scales
Ideally, investigators should perform hardness tests using the appropriate scale for the material type and thickness.
Most hardness tests involve pressing an indenter of a defined shape into the surface of the test piece using a defined load. The resulting hardness reading relates to the size or depth of the indentation.
When possible, avoid converting between different hardness scales. The thickness of the test material must be at least 10 times greater than the expected indentation depth. Other- wise, the hardness of the support structure influences the test reading.
Conversion charts, specific to each metal alloy, have been determined experimentally over time. While these charts prove valid for the tested datasets, those products no longer may be widely used. Consider, for example, the simple for- mula relating ultimate tensile strength (UTS, measured in psi) to Brinell hardness number (BHN): UTS = 500 BHN. This formula traces its roots to a publication from 1926, when researchers selected from then-current alloys.
If you must convert from one scale to another, consult ASTM E140 and select the table that contains the specific metal alloy of interest. Different conversion factors exist for materials with varying elastic moduli or strain-hardening capacity. MF
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