35°C: a temperature that dissolves all certainty

Thermal expansion: an enemy for your hardness tests!

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 ).

>>>Why is this all relevant for you?

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.

>>What are the tangible problem that we may experience, related to the rise of the temperature?

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.

>>But how can you decrease this error?

Click below to find it out!