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Hardness Test: The Rockwell Test

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In this article we analyze the Rockwell hardness test, explaining how it is performed and what the difference is between the two Rockwell hardness scales: HRB and HRC

In the previous article, we defined what hardness is, and how it is measured and analyzed some hardness tests, in particular the Brinell test.

If you haven’t read it, you can retrieve it at this link: Quality Control and Hardness: Brinell Test

Rockwell Hardness

In this article we are going to analyze a further hardness test: the Rockwell test. This one, unlike those analyzed in the previous article, consists of a:

  • An initial load, or pre-load
  • A second load, after which release occurs.

Going deeper: a pre-load is applied to the indenter in order to overcome the first layer of material, thus overcoming any constraining reactions, oxides, and surface defects. With the application of the second load, the part is brought to plastic deformation.

Afterwards, the indenter is released, so that the elastic part of the deformation is retracted. The residual difference between the initial force and the force after the release of the load is called e and is measured to determine the final hardness.

The Hardness Scales: Rockwell B and Rockwell C

There are different Rockwell hardness scales, defined by different tests. The 2 most widely used are Rockwell B ( HRB hardness) and Rockwell C ( HRC hardness).

These differ in:

  • The indenter used: in test B a polished and hardened steel ball is used, while in test C a diamond cone with a rounded tip and a 120° angle at the vertex is used.
  • The magnitude of the load: On the B scale –> F0 = 98N, F0 + F1 = 980N On the C scale –> F0 = 98N, F0 + F1 = 1470N
  • The final hardness: on the B scale –> HRB = 130 – e ; on the C scale –> HRC = 100 – e

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