For hardness testing to Rockwell, a differential depth method, the residual indentation depth created by the indenter is measured. In contrast, the optical Brinell, Vickers and Knoop methods measure the size of the indentation left behind by the indenter.
The deeper a defined indenter penetrates the surface of a specimen with a specified test load, the softer the material that is being tested. The Rockwell hardness (HR) is then determined from the residual indentation depth, along with a few other factors (see below).
In the Rockwell method, the total test force is applied in two steps. This is intended to eliminate effects from the roughness of the specimen surface (e.g., grooves on the specimen) as well as measurement errors caused by the play of the indentation depth measurement.
In the Rockwell method, the test indentations must be set in such a way that there is sufficient distance to the edge of the specimen and between the individual test indentations. The minimum values that must be adhered to in the Rockwell hardness test according to standard ISO 6508 can be found in the diagram on the right.
The Rockwell method offers the following advantages:
- No specimen preparation required (cutting, grinding, embedding)
- Direct reading of the hardness value; no optical evaluation required (measurement of diagonals as with optical methods)
- Quick (short test cycle) and economical method (hardness testers are comparatively cost-effective, since they don’t have to be equipped with elaborate optical systems like the machines for the optical Brinell, Vickers and Knoop methods)
- Non-destructive test; the specimen can be reused
The Rockwell method has the following disadvantages:
- It is not always the most accurate hardness testing method, as even a small differential depth measurement error can lead to a significant error in the calculated hardness value.
- The test location must be free of any dirt or debris (e.g. scale and cinder, foreign bodies or oil) to achieve a meaningful test result.
- The indenter has unknown effects on the test result, e.g. when the indenter is worn and the cone tip no longer adheres to the standard requirements (standard requirement: only use certified and calibrated indenters to minimize unwanted effects!).
- With increasing hardness, the materials are more difficult to differentiate.
To achieve as broad an application range as possible for the Rockwell test, several methods were developed for both the Rockwell and the Super Rockwell processes.
The individual Rockwell methods are differentiated by:
- The type of indenter (material, shape and size or ball diameter);
- Magnitude of the total test force (total force or main load);
- The scale division (basis h0 for the residual indentation depth h to be measured is 100 or 130
units (depending on the scale, the following applies: 1 unit E = 0.002 mm or 0.001 mm).
The resulting Rockwell methods use five different indenters (diamond cone with 120 degree curvature or a hard metal ball made of tungsten carbide with diameters: 1/16", 1/8", 1/4", 1/2") and six different total test forces (15, 30, 45, 60, 100, 150 kgf).
This results in 30 different Rockwell scales standardized according to ISO 6508 and ASTM E18 (e.g., A, B, C, 30N, 15T) or test methods (z.B.: HRA, HRBW, HRC, HR30N, HR15TW), each covering different hardness ranges and consequently the widest variety of materials and applications (see table below and poster “Hardness Testing of Metallic Materials”).
Rockwell is often used as a “quick test” in production or in the laboratory, as well as for other processes, such as the Jominy test.
The most common Rockwell method in practice is HRC. In principle, ball indenters are used for the hardness testing of softer materials and diamond indenters for testing harder materials. The diamond would destroy softer materials or pierce through them.
The table shows the Rockwell methods standardized according to ISO 6508 and their applications. The preload for all methods is 10 kgf.
|Method||Indenters||Main load (kgf)||Applications|
|HRA||Diamond 120°||60||Case-hardened steels and alloys, hard metals|
|HRBW||1/16" ball||100||Copper (Cu) alloys, unhardened steels (in the USA, also for steel up to approx. 686 N/mm²)|
|HRC||Diamond 120°||150||Case-hardened steels and alloys, hard metals|
|HRD||Diamond 120°||100||Case-hardened steels and alloys, hard metals|
|HREW||1/8" ball||100||Aluminum (Al) alloys, copper (Cu) alloys|
|HRFW||1/16" ball||60||Thin, soft sheet steel|
|HRGW||1/16" ball||150||Bronze, copper (Cu), cast iron|
|HRHW||1/8" ball||60||Aluminum (Al), zinc (Zn), lead (Pb)|
|HRKW||1/8" ball||150||Bearing metals and other very soft or thin materials, including plastics (see ASTM D785)|
The following overview shows all Super Rockwell methods standardized according to ASTM E18, and their applications. The test pre-force for all methods is 3 kg.
|Method||Indenters||Main load (kgf)||Applications|
|HR15N||Diamond 120°||15||Workpieces with think case hardening|
|HR15TW||1/16" ball||15||Thin sheet metal|
|HR15WW||1/8" ball||15||Aluminum (Al), zinc (Zn), lead (Pb), tinplate|
|HR15XW||1/4" ball||15||Aluminum (Al), zinc (Zn), lead (Pb), tinplate|
|HR15YW||1/2" ball||15||Aluminum (Al), zinc (Zn), lead (Pb), tinplate|