Rockwell hardness test | Measuring principle of depth difference measurement
Essential criteria for the proper conduct of the ROCKWELL procedure
- The material must be suitable for the selected method. If a material is not suitable for the selected Rockwell method (the most common Rockwell method is Rockwell C with a diamond cone), a different Rockwell method must be selected (lower test load / different indenter).
- The support surface on which the test piece is placed must be in contact with the machine body without any play. If you consider that 0,002 mm of additional cupping distance changes the Rockwell result by one unit (1 HRC), it becomes clear how important the contact between the underside of the sample and the support surface of the test piece is. Even the smallest contamination, a human hair or rust particles can quickly lead to a falsification of several Rockwell counters.
Hardness testers for Rockwell hardness tests
measuring principle of depth difference measurement
The Rockwell hardness test is based on the principle of measuring the indentation of a synthetic diamond or a ball into the test piece. In order to compensate for irregularities on the sample surface and play in the system, a preload is first used up. The process is carried out according to this scheme for both simple analogue and electronic devices.
- after applying the preload
– 10 kp Rockwell
or
– 3 kp Super-Rockwell
the measuring system (digital probe / dial gauge) is set to zero
2st stage
- now the main force is applied.
- after a defined holding time (which must be observed due to possible flow behavior) this main force is relieved again
3st stage
Back at the preload, the cupping difference is now determined.
The cupping value (mm remaining penetration depth e.g. 0,080 mm) is divided (divisor 0,002).
The result is subtracted from a fixed value. The residual value represents the Rockwell hardness.
This is done for Rockwell C according to the formula 100 – (h/0,002)
Where h = remaining depth difference.
The above example therefore results in 60 HRC.
Rockwell – Minimum distance between test impressions / edge distance
The distance between the test impressions must not be too close to each other. The hardness test is based on the principle of material displacement/compaction. This leads to a hardening that increases the hardness of the material (work hardening through deformation). If the impressions were placed too close together, the actual hardness would be distorted by the work hardening of the previous hardness test impression. According to the Rockwell standard, the minimum distance between two hardness test impressions must be 3 times the hardness impression diagonal.
It must also be noted that the distance between a hardness test indentation and the edge of a test piece must not be less than 2,5 times the diagonal of the indentation, otherwise there is a risk that the material will give way (flow) towards the edge of the sample and the indenter can penetrate deeper. This would result in a hardness being determined that is lower than the actual hardness of the workpiece.
