Let’s get straight to the point: the thermal expansion is a phenomenon that causes an increasing in the volume of the metal, strictly related to the rise of the temperature.
But I guess you already knew that.
Clearly, the coefficient of thermal expansion changes, based changes from a metal to another. For example, the aluminum (72 X 10−6 °C−1 ) or iron and steel (both of them 36 X 10−6 °C−1 ).
In the hottest months of the year, like the one we are experiencing right now, the thermal expansion does not pass unobserved, in particular if we’re talking about hardness of material located in places with temperatures that rise over 35°C.
The hardness tester is an instrument that calculates the geometrical dimensions of the indentation based on microns (whether it is a Vickers, Brinell or Rockwell). That’s why even the smallest variation, both chemical or physical, found in the metallic parts of the hardness tester or even in the specimen we’re testing could invalidate the whole metrological repeatability.
For instance, let’s think about the problems it could cause to the metallic support of the indenter or to various metallic parts of which is made the optical support.
Straight to the point, the indentation will always result larger and deeper, due to the increase of both the diameter/diagonal and the height of the indentation. Plus, we have to consider the alteration that the specimen will be subjected too, which is related to the coefficient of thermal expansion as well.
That’s why we can’t really talk about repeatability of hardness values in this case.
Make sure you’re not working outside the range of temperature between 10°C and 35°C, possibly working inside the range between 18°C and 28°C, as established by the referring standards (e.g. ISO 6508-1 [7.1]).
Don’t expose your hardness tester directly to the heat.
If the location where you’ve been using your instrument has been over 35°C for a while, we’d warmly suggest you to re-calibrate your hardness tester, both directly and indirectly.
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