Rockwell hardness testing according to ISO 6508: Procedure & scales

Q: Which indenters are used in Rockwell hardness testing?

Depending on the scale, two types of indenters are used in Rockwell hardness testing: a diamond cone with a tip angle of 120° (for hard materials) and carbide (tungsten carbide) balls with diameters of 1/16‘, 1/8’, 1/4‘ or 1/2’. The choice depends on the expected hardness and material properties in accordance with ISO 6508.

Q: What is the difference between Rockwell and Super Rockwell?

The main difference lies in the test force intensity. While the classic Rockwell method uses a preload of 10 kgf, the Super Rockwell (Superficial Rockwell) method uses only 3 kgf. This enables hardness testing on extremely thin sheets or surface-hardened layers without the indenter piercing the material or affecting the core.

Q: Which Rockwell scale is the most commonly used?

The HRC (Rockwell C) scale is the most commonly used method worldwide for hardened steels and tools. It uses a diamond cone and a total test force of 150 kgf. For softer metals such as aluminium or brass, however, the HRB (Rockwell B) scale with a carbide ball and a load of 100 kgf is the industrial standard.

Q: What does the abbreviation HRC mean in hardness testing?

HRC stands for Hardness Rockwell Scale C. The “HR” is the abbreviation for the hardness testing method, while the “C” defines the specific scale (diamond cone/150 kgf). A correct hardness value according to this method is always given as a combination of a numerical value and a scale, for example 60 HRC.

Q: What is the minimum thickness of a sample for Rockwell hardness testing?

As a rule of thumb, the material thickness must be at least ten times the permanent indentation depth. If the sample is too thin, the so-called ‘anvil effect’ occurs, in which the hardness of the base material distorts the measurement result. In such cases, the Super Rockwell method with a lower test load should be used.

Q: Why must a preload be applied during Rockwell hardness testing?

The test preload (usually 10 kgf or 3 kgf) serves to eliminate influences caused by the surface condition of the sample (e.g. roughness or slight contamination). Only after applying the preload is the dial gauge set to zero, making the subsequent depth difference measurement extremely precise and reproducible.

Q: How is Rockwell hardness correctly documented in accordance with ISO 6508?

A standardised hardness value consists of the numerical value, the abbreviation HR and the scale designation. Since 2015, a ‘W’ (for tungsten carbide) has been added when using a carbide ball. Example: 60 HRC (diamond cone) or 90 HRBW (carbide ball).

Rockwell hardness test procedure:
Determination of Rockwell hardness HR

The Rockwell hardness test is a globally standardised depth difference method (ISO 6508 / ASTM E18) for determining the hardness of metallic materials. In contrast to optical methods such as Brinell, Vickers or Knoop, the Rockwell hardness test measures the permanent indentation depth of a test specimen.

The principle of Rockwell hardness is simple: the deeper the indenter penetrates the workpiece surface at a constant test force, the lower the material hardness. The hardness value (HR) is calculated directly from this measured difference. Due to its speed and automatability, this Rockwell hardness testing method is one of the most efficient methods in modern quality assurance.

Rockwell hardness testing procedure according to ISO 6508 – 3-step method

Maximum efficiency through fast depth difference measurement

The right Rockwell hardness tester for your application

DuraJet G5 Series

Specialised in the Rockwell range and plastics testing (9.8 N to 2450 N). This system is the ideal solution for rapid series testing directly in production and precise measurements on components with minimal sample preparation.

DuraVision G5 Serie

Das robuste Universalgenie für den Makrolastbereich (3 kgf bis 3000 kgf). Die Serie ist für die Prüfung großer, schwerer Bauteile sowie für hochautomatisierte Rockwell-Prüfaufgaben im Labor und in der Fertigung optimiert.

DuraVision Hardness testing machine

DuraJet G5 Hardness tester

Important features and standards of Rockwell hardness testing

Rockwell hardness testing is defined as a static hardness testing method and can be characterised by specific technical parameters. Compared to other methods in the macro range, it is distinguished by the following features:

Global standardisation: The method complies with the international standards ISO 6508 and ASTM E18.
Test force range: The method is used to test hardness in the macro range (test force >= 49.03 N) with a test force of 29.42 N to 1471 N.
Measuring principle: As a classic depth difference method, Rockwell hardness is not determined optically, but rather by the permanent indentation depth created by the indenter.
Shape and material of the indenter: Depending on the Rockwell method, a diamond cone (120°) or carbide balls are used.

Rockwell vs. Super Rockwell: Differences between the methods

There are basically two types of methods: the classic Rockwell (Regular Rockwell) and the Super Rockwell (Superficial Rockwell). The main difference lies in the amount of load and the sensitivity of the measurement.

Rockwell (regular): The standard preload force is 10 kgf. The total force varies between 60, 100 or 150 kgf, depending on the scale.
Super Rockwell (superficial): This method uses a significantly lower preload force of only 3 kgf. The main load here is 15, 30 or 45 kgf.

Test methods	Rockwell			Super Rockwell
pre force	10kgf	10kgf	10kgf	3kgf	3kgf	3kgf
main force	150kgf	100kgf	60kgf	45kgf	30kgf	15kgf

The hardness values determined by Super Rockwell can be converted to Rockwell values and vice versa. The Superficial Rockwell method was invented in the USA, where its use is significantly more common than in Europe. It is particularly suitable for use in hardness testing of thin components and layers, or with specimens whose calculated hardness value is outside the Regular Rockwell scale.

Rockwell Tabelle - Methoden und Anwendungen

Various Rockwell methods have been developed so that the Rockwell method can be used in practice for a wide range of testing requirements. This applies to both the classic Rockwell method and the Super Rockwell method.

The individual Rockwell test methods differ in the following points:

Type and nature of the indenter (material, shape, geometry and ball diameter)
Amount of the total test force applied (also called main load or main force)
Scale division for determining the indentation depth - depending on the Rockwell scale, the basic unit h0 is either 100 or 130 units, whereby one unit (E) corresponds to 0.002 mm or 0.001 mm

Rockwell scale	Indenter	Total test force (kgf)	Typical applications & materials
HRA	Diamond cone 120°	60	Case-hardened steels and alloys, carbides
HRBW	1/16" Carbide ball	100	Copper (Cu) alloys, unhardened steels (in the USA also for steel up to approx. 686 N/mm²)
HRC	Diamond cone 120°	150	Case-hardened steels and alloys, carbides (most commonly used scale)
HRD	Diamond cone 120°	100	Case-hardened steels and alloys, carbides
HREW	1/8" Carbide ball	100	Aluminium (Al) alloys, copper (Cu) alloys
HRFW	1/16" Carbide ball	60	Thin, soft sheet steel
HRGW	1/16" Carbide ball	150	Bronze, copper (Cu), cast iron
HRHW	1/8" Carbide ball	60	Aluminium (Al), zinc (Zn), lead (Pb)
HRKW	1/8" Carbide ball	150	Bearing metals and other very soft or thin materials, including plastics (according to ASTM D785)
HRLW	1/4" Carbide ball	60
HRMW	1/4" Carbide ball	100
HRPW	1/4" Hartmetallkugel	150
HRRW	1/2" Hartmetallkugel	60
HRSW	1/2" Hartmetallkugel	100
HRVW	1/2" Hartmetallkugel	150

How do you read and express a Rockwell hardness value?

A Rockwell hardness value consists of three components:

a numerical hardness value
the two capital letters HR, which stand for ‘Rockwell hardness’
the designation of the Rockwell scale, which defines the combination of the main load (total test force) and the type of indenter used in the corresponding Rockwell method.

Note: Since the change to the ISO 6508 series of standards in March 2015, test methods using a spherical indenter must also be marked with a ‘W’. The W provides information about the ball material (tungsten carbide).

Examples of how a Rockwell hardness value is represented and read:

According to the ‘normal’ Rockwell method (Regular Rockwell), method HRC:

45 HRC:

45 … hardness value
HR … according to Rockw
C … with main load 150 kgf and diamond indenter 120°

According to the Super Rockwell method (Superficial Rockwell), method HR30T:

80 HR30TW:

80 … hardness value
HR … according to Rockwell
30 … with main load 30 kgf
TW … and indenter 1/16 inch ball made of tungsten carbide

Advantages and disadvantages of the Rockwell method

Rockwell testing is characterised by numerous advantages that make it particularly popular in industrial practice:

Complex sample preparation such as cutting, grinding or embedding is not required.
The determined hardness value can be read directly - there is no need for additional optical evaluation, as is necessary with Brinell, Vickers or Knoop methods.
The method impresses with its speed and cost-effectiveness: the test cycle is short and Rockwell hardness testers are more cost-effective than optical testers as no complex optics are required.

Despite its many advantages, Rockwell hardness testing also has some disadvantages:

The accuracy of the method can be compromised - even small measurement errors in the depth difference can lead to significant deviations in the determined hardness value.
The test site must be absolutely clean and free from contamination such as scale, oil or foreign bodies to ensure a reliable test result.
The condition of the indenter plays a decisive role: wear or deviations from the standard specifications influence the result. Therefore, only certified and calibrated indenters may be used.
With very hard materials, it is difficult to differentiate between differences in hardness using the Rockwell test method, even with an exact reading on the Rockwell scale.