As known, there are several standard Rockwell and Superficial Rockwell scales: the choice of a certain scale depends on the hardness of the material, minimum thickness of the work piece or of the hardened layer (in case of surface treatment as carburising, nitriding, etc.)
The hardness of the material determines the penetrator to use: cone diamond indenter or ball penetrator. The diamond indenter is used for tempered steel or hard metal; it is not suitable for testing of steel having a resistance lower than 785 N/mm2.
Steel ball penetrators are used for soft metals: the softer the material, lighter the load will be and larger the ball diameter.
For example, the HRB scale (1/16” ball and 100 kgf test load) does not allow testing of soft material as the HRL scale does (1/4” ball and 60 kgf load). Generally, the largest balls are used for testing of plastics or similar only. The Rockwell method allows testing of plastic even under load.
When testing very thin sheets or hardened layers, the indentation made by the penetrator during load application will influence a large area of the material, all around the indentation self. If deformation appears on the opposite side of the work piece, the test results will be wrong.
Therefore, for testing of thin pieces, the load to be applied should not give a deformation higher than the minimum thickness of the part, avoiding in this way that the indentation breaks through on the opposite side of the specimen. This is a general rule for all methods of hardness testing.
For each kind of testing, the minimum measurable thickness must be evaluated and considered.
However, there are no specific rules, because these depend on the kind of material to be tested. Conventionally, the minimum measurable thickness is intended to be 10 times the penetration depth (see table below).
The same is valid also for hardened surface layers (carburising, etc.), where normally the scale HRA is used (cone diamond-60 kgf).
The most common Rockwell hardness scales are:
HRC (cone diamond – 150 kgf)
This is the most typical Rockwell scale, used for testing of tempered, hardened or deeply carburised parts. When speaking of Rockwell hardness, we generally mean the HRC scale.
This can generate some confusion, as sometimes the HRC hardness is requested for testing of parts that require different Rockwell scales or hardness methods. The HRC scale can be converted in other scales using conversion tables that must be considered an approximation only.
HRA (cone diamond – 60 kgf)
Principally used for cemented and hard metal parts, where the hardness of carbide could generate splintering of the diamond. For this reason, heavy loads are not advisable.
HRB (1/16” ball penetrator – 100 kgf)
Generally used in Europe for brass alloys (copper, bronze, etc) and in the USA for ferrous alloys until approx. 686 N/mm²
SUPERFICIAL ROCKWELL or SUPER ROCKWELL
The scales: HR15N, HR30N, HR45N (cone diamond) are suitable for testing of parts having thin layers.
The scales: HR15T, HR30T, HR45T (1/16” ball penetrator) are suitable for testing of thin samples.