Rockwell hardness test to ISO 6508 / ASTM E18

Knowledge of the Rockwell test method (determination of Rockwell hardness HR)

General knowledge on Rockwell hardness testing

In the Rockwell hardness test, a differential-depth method, the residual depth of the indent made by the indenter, is measured. In contrast, the size of the indentation is measured in the Brinell, Vickers and Knoop optical test methods.

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 hardness testing according to Rockwell, 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.

Advantages and disadvantages of hardness testing using the Rockwell method

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.

Examples of Rockwell methods and applications

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 Rockwell test methods (e.g.: HRA, HRBW, HRC, HR30N, HR15TW), each covering different hardness ranges and consequently the widest variety of materials and applications (see table below).

Rockwell is often used as a quick test in production or in the laboratory, as well as for other processes, such as the Jominy end quench test / Jominy test.

The most common Rockwell method in practice is HRC. In principle, ball indenters are used for the hardness tests on softer materials and diamond indenters for testing harder materials. The diamond would destroy softer materials or pierce through them.