In the Rockwell hardness test, a differential-depth method, the residual depth of the indent made by the indenter is measured. In contrast, the indentation size is measured in the Brinell, Vickers and Knoop optical test methods.
The deeper a defined indenter penetrates at a defined test force into the surface of a workpiece (specimen), the softer the tested material.
In the Rockwell method, the total test force is applied in two stages. This allows the impact of specimen surface roughness (e.g. grooves in the specimen) and measuring errors caused by backlash in indentation depth measurement to be eliminated.
The sequence of the Rockwell test (HR) according to ISO 6508 can be described as follows (see also illustration below):
1. test step
First, the indenter is pressed with the test pre-force (also referred to as pre-force or preload) to a penetration depth of h0 in the specimen to be tested. h0 defines the reference level (basis) for subsequent measurement of the residual indentation depth (h).
2. test step
Next, the additional test force is applied for a dwell period defined in accordance with the standard (several seconds), whereby the indenter penetrates into the specimen to a maximum indentation depth of h1. The test pre-force plus the additional test force gives the total test force (also referred to as total force or main load).
3. test step
After the dwell period, the additional test force is removed, the indenter moves up by the elastic proportion of the penetration depth in the total test force and remains at the level of the residual indentation depth (h - expressed in units of 0.002 or 0.001 mm). This is also referred to as the depth differential (difference in indentation depth before and after application of the total test force). Now the Rockwell hardness (HR) can be calculated, using the residual indentation depth (h) and a formula defined in the standard, taking account of the applied Rockwell scale.