The Rockwell hardness test is often used as a ‘quick test’ on the production floor or in the laboratory, mainly for metallic materials. However, it can also be used for more advanced tests, such as the Jominy test (End Quench Test). Due to the relatively high loads used, application of the Rockwell hardness test is generally limited to larger sample geometries.

The Rockwell test calculation

The Rockwell hardness test calculation (HR) is made by measuring the depth of an indentation made with a diamond or tungsten carbide ball.

HR = E - e
E: is a constant of 100 (diamond) or 130 (ball) units
e: is the penetration depth in units of 0.002 mm (Regular) or 0.001 mm (Superficial scales)

Indenters
The Rockwell hardness test utilizes 1 of 5 different indenters:

• Diamond cone
• Tungsten ball in sizes 1/16", 1/8", 1/4", or 1/2"

Main loads
As with any hardness scale, Rockwell testing follows the principle that higher test loads reduce the influence of surface imperfections, roughness, and material inhomogeneities on the measurement result. Consequently, the Rockwell scales can be divided into regular scales, which use higher test loads and are generally preferred whenever possible, and superficial scales, which use lower loads and are particularly suitable for thin materials such as coatings, sheet metal, and other applications where indentation depth must be minimized.​

• Superficial scales: 15 kgf, 30 kgf and 45 kgf,
• Regular scales: 60 kgf, 100 kgf and 150 kgf

In total, this gives 30 different Rockwell scales. Each one is characterized by a different combination of indenter type and main load, and is suitable for a particular material or application (for example, HRA, HRB, HRC, HR30N).

Specimen​

Before performing a Rockwell hardness test, the test surface must be sufficiently prepared. The required surface finish depends on the
test scale and load being used. Since Rockwell testing is considered a macro-hardness method, utilizing test forces above 1 kgf. As a
result, a ground surface is generally sufficient, and for some applications, little or no surface preparation may be required.​

The specimen must be well-supported during testing. Ensure that the back surface is flat, clean, and free from oxides, dirt, or
contaminants. Any movement, flexing, or instability of the specimen during testing can lead to inaccurate results, typically causing
hardness values to appear lower than the actual material hardness.​

Equipment​

To ensure reliable and traceable results, the hardness tester should undergo direct verification and calibration at least annually by an
accredited calibration laboratory. Between direct calibrations, operators are encouraged to perform regular indirect verifications using certified reference test blocks with known hardness values.​

Because the Rockwell method measures indentation depth, the condition of the support system is critical. The test anvil should be flat, free from indentations, damage, and contamination, and should be free from grease or debris. Similarly, the anvil seat and spindle
interface must be kept clean.​

For C-frame testers with a movable spindle, always keep the dust protector in place to prevent dirt, oxide particles, and other
contaminants from accumulating on the spindle. Clean spindles only with light machine oil and wipe away any excess oil after cleaning.​

Environment​

Like all hardness testing methods, Rockwell testing is sensitive to shock and vibration. The instrument should be installed on a stable
and rigid surface, and testing should not be performed while the machine is exposed to significant vibration from nearby equipment or
other external sources. ​

ISO and ASTM standards specify an operating temperature between 10 °C and 35 °C.​

Operator​

The operator is responsible for ensuring that the specimen is correctly positioned and remains stable throughout the test cycle.
Indentations should be placed sufficiently far apart to avoid interference from material deformation caused by previous tests.​

As a general guideline, the distance between the centers of two adjacent indentations, or between the center of an indentation and the edge of the test piece, should be at least 5 mm. For specific applications, always consult the relevant Rockwell testing standard to confirm the required spacing and testing conditions.​

The Rockwell Method​

The Rockwell hardness test is defined in ASTM E18, ISO 6508, and JIS Z 2245. While there are minor differences between these standards, the core testing principles are largely aligned.​

• Use the correct indenter and test force according to the selected Rockwell scale (see below table).
• Preliminary force application time: <2 seconds​
• Total test force application time : 2-6 seconds​
• Sample thickness:​
  • ASTM E18: At least 10 times the indentation depth for both indenters​
  • ISO 6508 - Diamond indenter: At least 10 times the indentation depth. ​
  • ISO 6508 - Tungsten carbide indenter: At least 15 times the indentation depth.​

Download our Application note about Hardness Testing

Regular test methods

Superficial test methods

ISO 6508 and ASTM E18 = A B C D E F G H K N T
ASTM E18 = L M P R S V W X Y

Download our brochure covering our Rockwell hardness testers

Whether you're just starting with hardness testing or are an experienced professional, this curated selection of resources is here to assist you in your work.

Take advantage of downloadable reference posters for easy use in your lab, and dive into expert-led webinars hosted by our application specialists to deepen your understanding of hardness testing techniques and methods.