Rockwell Hardness Test — ISO 6508 / ASTM E18

What Is Rockwell?

The Rockwell hardness test is a method in which hardness is read directly from a scale by measuring the permanent indentation depth remaining after a preliminary load and a major load are applied. A diamond cone or hard metal ball indenter is used.

Purpose of Rockwell

The purpose is to perform heat-treatment verification and series-production quality control with a fast, repeatable hardness measurement.

Which Products Is Rockwell Applied To?

Heat-treated steel parts
Fasteners
Non-ferrous metals
Hard plastics (M and R scales)

In Which Sectors Is Rockwell Used?

MetalSteelMachineryIndustrial EquipmentPlastics

Standards Related to Rockwell

ISO 6508-1
ASTM E18

Rockwell Conditions

Scale: Chosen according to the material; HRC (diamond cone, 150 kgf) and HRB (1.5875 mm ball, 100 kgf) are common
Preliminary load: 10 kgf (98 N) in standard Rockwell
Dwell time: Defined in the standard, typically 1–5 s
Surface: Smooth, free of burrs, and perpendicular to the indenter

Equipment for Rockwell

Rockwell hardness testing machine
Diamond-cone and ball indenters
Reference hardness blocks
Specimen support anvils

Sample Requirements for Rockwell

Thickness must be at least ten times the indentation depth
The surface must be free of oxide and grease
Indentations must be at the distance from the edge and from each other specified in the standard
Generally at least three measurements are taken

Duration of Rockwell

A few seconds per measurement; the duration extends if surface preparation is required.

Rockwell Procedure

1The machine is verified with reference blocks.
2The scale and indenter appropriate to the material are selected.
3The specimen is seated firmly on the anvil.
4The preliminary load is applied followed by the major load, held for the defined duration.
5The major load is removed and the hardness value is read directly.
6The measurement is repeated and averaged.

Rockwell Acceptance Criteria

The acceptance range is given in the product or material standard. The result is always stated together with the scale (e.g. 45 HRC).

Evaluation of Results

The average and scatter of the measurements are evaluated. The result is invalid if the specimen moved or support was inadequate.

Rockwell Report

Standard applied
Scale and indenter used
Individual measurements and the average
Specimen identification

Accreditation

EUROLAB holds internationally recognised accreditation obtained from accreditation bodies party to the IAF and ILAC multilateral recognition arrangements, under ISO/IEC 17025 for laboratory testing, ISO/IEC 17020 for inspection and surveillance, and ISO/IEC 17021-1 and ISO/IEC 17065 for system and product certification. This chain ensures the reports and certificates issued are accepted worldwide.

Whether this test can be offered within accredited scope depends on your product and the standard applied; since accreditation is method-based, the scope is confirmed in writing at the quotation stage.

Scope and authorisations

Related Regulations

Material product standards
Heat-treatment specifications

Mekanik Testler — What Does This Testing Field Cover?

The section below describes the field that the Rockwell test belongs to; the conditions and acceptance criteria of this particular test are given in the sections above. The scope specific to your product is confirmed in writing at the quotation stage.

Mechanical testing measures how a material or a part behaves under force. The family ranges from static loading (tension, compression, bending) to time-dependent behaviour (fatigue, creep), and from hardness and wear to fracture toughness.

What these tests have in common is that the result is reported as a MATERIAL PROPERTY. Yield strength or hardness is not a conformity decision; it is numerical data against which the design and the product standard will compare. Mechanical test reports are therefore usually an input to an engineering decision rather than to a product approval — material selection, safety factor, life prediction.

The distinction between static and dynamic behaviour is the key to understanding the field. A part may appear entirely unstressed under a static load yet break under repeated loading; fatigue fracture occurs at stresses well below the yield strength and gives no warning. For rotating, vibrating or cyclically loaded parts, a tensile test alone is therefore not sufficient evidence.

Test conditions determine the result, and this is critical for the readability of the report: specimen geometry, loading rate, temperature, conditioning and the gripping arrangement. The same material gives different values under different conditions. In plastics the rate effect is very pronounced, in metals the temperature effect; in composites the specimen orientation changes the result entirely.

Hardness is the most misused measurement in the field. Because it is fast, close to non-destructive and inexpensive, it is often used in place of strength. Conversion tables from hardness to strength exist, but they are APPROXIMATE and depend on the material group; using them as the sole evidence in material acceptance is wrong.

The fracture mechanics side requires a specific competence. Fracture toughness and fatigue crack growth rate measurements make it possible to calculate how long a cracked structure will last; they are the basis of damage-tolerant design in fields such as pressure equipment, aircraft structures and pipelines.

Method groups in this field

Static tests
Tension, compression, bending, shear and torsion. Yield and tensile strength, elongation and modulus of elasticity are obtained.
Impact tests
Charpy and Izod notch impact; the brittle-to-ductile transition behaviour of the material is determined as a function of temperature.
Hardness
Brinell, Vickers, Rockwell, Shore and micro hardness. Valuable for surface treatment depth and local differences in strength.
Fatigue
Force- and strain-controlled axial fatigue, fatigue in bending; S-N curves and the endurance limit.
Creep and stress relaxation
Behaviour under long-term load at high temperature; decisive for boiler, turbine and pressure equipment materials.
Fracture mechanics
Plane-strain fracture toughness, the J integral, CTOD and fatigue crack growth rate.
Wear and friction
Taber and rotating drum abrasion, coefficient of friction measurements.

Commonly confused in this field

Calculating strength from hardness is approximate; it cannot be used as the sole evidence in material acceptance.
Adequate static strength does not demonstrate fatigue strength; a cyclically loaded part requires a separate test.
If test rate and temperature are not reported, the result cannot be compared — especially for polymers and composites.
A test specimen and the actual part may not give the same value; moulding orientation, weld lines and differences in surface treatment all have an effect.
A strength value given for a composite without stating the specimen orientation is meaningless.
ASTM and ISO counterparts measure the same quantity but use different specimen geometries; the results are not treated as directly equivalent.

Which decisions these results support

Design verification and material selection
Incoming material control and supplier approval
Meeting the mechanical requirements of the product standard
Failure analysis, life prediction and damage tolerance calculation
Verifying heat treatment and surface treatment processes

Related Tests

Frequently Asked Questions About Rockwell

Why is the Rockwell result written with the scale?

Different scales use different indenters and loads; the same material gives a different number on different scales. The value is therefore only meaningful together with the scale letter.

Can Rockwell be applied to thin sheet?

There is a thickness limit for standard Rockwell. Superficial Rockwell (such as HR15N) or the Vickers method is preferred for thin sections.