General information
The hardness of carbon and graphite materials is determined according to the DIN 51917 standard. The DIN 51917 standard defines the procedure for determining the Rockwell hardness. The test method described in this standard is a modified Rockwell test method for testing carbon materials with balls in accordance with ISO 6508-1. The Rockwell hardness of metal-graphite materials, e.g. carbon brushes in motors and generators, can also be determined using this method.
Categorisation of the carbon test method
The procedure for carbon testing is based on the Rockwell method. The hardness testing method in this case is also static, with similar characteristics to the Rockwell method:
- The procedure is standardised (DIN 51917, ASTM C886).
- Hardness is tested in the macro range with this method, with a test force between 29.42 and 1471 N.
- It is a differential-depth method. This means that the residual depth of indentation left by the indenter is measured to determine the hardness value of a test specimen.
- Indenter shape and material: a carbide metal ball with different ball diameter depending on the method.
Test procedure
The procedure for carbon testing is identical to that of the test procedure.
Methods
The individual methods are distinguished from each other by:
- Diameter of the indenter ball
- Magnitude of total test force (total force or main load);
Scale division (basis h0 for the residual indentation depth (h) to be measured is 100 or 130 Units (depending on the scale: 1 unit E = 0.002 mm or 0.001 mm)); see also table in illustration "Rockwell hardness test".
The resulting test methods use three different ball diameters (hard metal ball made from tungsten carbide with diameters of: 2.5 mm, 5 mm, 10 mm) and six different total test forces (68.65 N, 196.1 N, 392.3 N, 588.4 N, 980.7 N, 1471 N).
The following table shows the test methods standardised according to DIN 51917.
| Hardness symbol | Carbide metal ball indenter [Ø in mm] | Scale division [mm] | Test preforce [N] | Total test force [N] |
| HR 10/20 | 10 | 0.002 | 98.7 | 196.1 |
| HR 5/20 | 5 | 0.002 | 98.7 | 196.1 |
| HR 10/40 | 10 | 0.002 | 98.7 | 392.3 |
| HR 5/40 | 5 | 0.002 | 98.7 | 392.3 |
| HR 10/60 | 10 | 0.002 | 98.7 | 588.4 |
| HR 5/60 | 5 | 0.002 | 98.7 | 588.4 |
| HR 10/100 | 10 | 0.002 | 98.7 | 980.7 |
| HR 5/100 | 5 | 0.002 | 98.7 | 980.7 |
| HR 10/150 | 10 | 0.002 | 98.7 | 1471 |
| HR 5/150 | 5 | 0.002 | 98.7 | 1471 |
| HR 5/7 | 5 | 0.001 | 29.42 | 68.65 |
| HR 2.5/7 | 2.5 | 0.001 | 29.42 | 68.65 |
How is the hardness value read and represented?
The carbon testing hardness value consists of four components:
- A numeric hardness value;
- The two letters "HR", standing for "Hardness according to Rockwell";
- The ball diameter in mm;
- The applied test load in kgf;
Example of how to represent and read a hardness value:
33 HR 2,5/7
33 …hardness value
HR …according to Rockwell
2.5/7 …with a ball diameter of 2.5 mm and a main load of 7 kgf
