ROCKWELL HARDNESS

The Rockwell hardness is different from the other ones since it is based on a difference in the depth of penetration of the material. Therefore, there is no optical measurement of the indentation left by the indenter. These are simple tests requiring little surface preparation. This simplicity and speed make it a very popular test in production. We speak of macro-hardness for the Rockwell hardness because, due to the applied loads, any differences in hardness that could be related to the microstructure of the material are avoided. The Rockwell hardness is generally read directly from the Hardness Tester.

Indenters and Rockwell scales

There are several types of indenters. Consisting of a diamond cone or a tungsten carbide ball of different sizes, these different indenters give access to different Rockwell hardness scales (HR from A to G, the HRC scale, applicable to steel, cast iron and titanium, is the most common one).

The principle of Rockwell tests

In order to obtain a Rockwell hardness value, a pre-load force (F0) is applied, which deforms the material. This position is the origin of the test. The Hardness Tester then applies the test load (F0 + F1) which continues the deformation of the material and then returns to the pre-load force (F0). The measured hardness is directly related to the depth of the ‘h’ tests in the following diagram weighted by a unit-specific factor (S) and a constant

HRX = C – h/S.

(C takes the value 130 for the HR B, E F, G, H K scale and 100 for the A, C, D, N and T scales)

S takes the value 0.001 for the N and T scales and 0.002 for all other ones.

A Rockwell hardness unit consists of a residual depth corresponding to the S value in mm.

1 Indentation depth by preliminary force F₀
2 Indentation depth by additional test force F₁
3 Elastic recovery just after removal of additional test force F₁
4 Permanent indentation depth h
5 Surface of specimen
6 Reference plane for measurement
7 Position of indenter

The Rockwell hardness and superficial Rockwell hardness

A distinction exists between Rockwell hardness tests and superficial Rockwell tests, which are applicable to coatings, to thin samples; these are the N and T scales.

The difference between the two types of tests lies in the loads and pre-loads used (lower in the case of the superficial Rockwell tests). A superficial Rockwell hardness unit corresponds to a residual depth of 1 μm (0.001 mm).