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The Rockwell Hardness Test Theory

ASTM E18 & ISO 6508: Principles, Test Cycle, Scales, Thickness, Repeatability and Practical Metrology Guidance

This page is a technical reference prepared by the ATI Metrology Engineering Team to explain the Rockwell and Superficial Rockwell hardness test in a standards-oriented and metrology-focused manner, based on ASTM E18 and ISO 6508 📑.
It is intended for users who require a deeper understanding of the method for laboratory testing, audits, procedure development, troubleshooting and incoming material inspection.

The content provides a detailed explanation of the Rockwell test principle, the complete test cycle (application of the minor load → application of the major load → return to the minor load), the meaning of Rockwell hardness numbers and scale designations, and the practical conditions that most strongly influence measurement reliability, including specimen thickness, surface condition, support, indentation spacing, repeatability and sources of error 🧪⚙️.

For a simple and practical introduction, please see: Rockwell hardness testing (overview) 📘
For a general overview of hardness testing methods, standards and applications, see: Hardness Testing overview 👉

Rockwell hardness test performed in laboratory conditions
This image shows a Rockwell hardness tester operating in laboratory conditions(ATI R model).

📌 Table of contents

🧭 INTRODUCTION

 

ROCKWELL HARDNESS TEST SCOPE

The Rockwell hardness test is an empirical indentation hardness test that can provide useful information about metallic materials. This information may correlate to tensile strength, wear resistance, ductility, and other physical characteristics of metallic materials, and may be useful in quality control and selection of materials.

PRINCIPLE OF THE ROCKWELL HARDNESS TEST

An indentation hardness test using a verified machine to force a diamond spheroconical indenter or tungsten carbide (or steel) ball indenter, under specified conditions, into the surface of the material under test, and to measure the difference in depth of the indentation as the force on the indenter is increased from a specified preliminary test force to a specified total test force and then returned to the preliminary test force. The test is divided into three steps of force application and removal:

  1. The indenter is brought into contact with the test specimen, and the preliminary test force is applied. After holding the preliminary test force for a specified dwell time, the baseline depth of indentation is measured.
  2. The force on the indenter is increased at a controlled rate by the additional test force to achieve the total test force. The total test force is held for a specified dwell time.
  3. The additional test force is removed, returning to the preliminary test force. After holding the preliminary test force for a specified dwell time, the final depth of indentation is measured. The Rockwell hardness value is derived from the difference in the final and baseline indentation depths while under the preliminary test force. The preliminary test force is removed and the indenter is removed from the test specimen.

ISO 6508 ROCKWELL STANDARD

Rockwell Hardness Test method ISO 6508

ASTM E18 ROCKWELL STANDARD

Rockwell Hardness Test method ASTM E18

Key: X time, Y indenter position, 1 indentation depth by preliminary force F0, 2 indentation depth by additional test force F1, 3 elastic recovery just after removal of additional test force F1, 4 permanent indentation depth h, 5 surface of specimen, 6 reference plane for measurement, 7 position of indenter, 8 indentation depth vs. time curve.

CALCULATION OF THE ROCKWELL HARDNESS NUMBER

During a Rockwell test, the force on the indenter is increased from a preliminary test force to a total test force, and then returned to the preliminary test force. The difference in the two indentation depth measurements, while under the preliminary test force, is measured in mm. From the value in mm, the Rockwell hardness number is derived. The Rockwell hardness number is an arbitrary number, which, by method of calculation, results in a higher number for harder material.

CLASSIFICATIONS OF THE ROCKWELL TEST

Two main classifications are used: the Rockwell hardness test and the Rockwell superficial hardness test. The significant difference between the two test classifications is in the test forces that are used. For the Rockwell hardness test, the preliminary test force is 10 kgf (98 N) and the total test forces are 60 kgf (589 N), 100 kgf (981 N), and 150 kgf (1471 N). For the Rockwell superficial hardness test, the preliminary test force is 3 kgf (29 N) and the total test forces are 15 kgf (147 N), 30 kgf (294 N), and 45 kgf (441 N).

REPRESENTATION OF ROCKWELL HARDNESS VALUE

Rockwell hardness values shall not be designated by a number alone because it is necessary to indicate which indenter and forces have been employed in making the test. The hardness number is followed by the symbol HR and the scale designation. Examples: 64 HRC = Rockwell hardness number of 64 on the Rockwell C scale; 81 HR30N = Rockwell superficial hardness number of 81 on the Rockwell 30N scale.

ROCKWELL HRC HARDNESS TEST

INDENTER HRC

Diamond cone with an angle of 120° at the summit and ending with a radius of 0,2 mm. Mirror polished.
Rockwell indenter HRC

DEFINITION HRC

An initial force called “pre-load” of 98,1 N ( Fo) is applied; in this phase the indent depth reader must be zeroed. This phase is necessary in order to settle the test piece, after which a supplementary force is applied (F1) until a full load (F) of 588,6 or 981 or 1471,5 N is applied. Once the time required for the depth indicator to stop has passed, the supplementary load (F1) is removed leaving only the initial load (Fo).

Rockwell HRC hardness test principle

ROCKWELL HRBW HARDNESS TEST

Diagram illustrating hardness test procedure with carbide spheres, featuring measurements and scales in Italian.

INDENTER HRBW

Tungsten carbide W ball with a hardness not less than 850 HV and a diameter of 1/16” (1,5875mm).
Rockwell indenter HRBW

DEFINITION HRBW

The procedure follows the same principle as the diamond cone test, but with a reference zero set at 130 scale units.
This configuration provides an additional measurement range, allowing reliable testing of very soft materials where the ball indenter produces deeper penetration.
The hardness value is obtained by reading the value indicated by the measuring system and subtracting it from 130.
On the Rockwell ball scale, a value of 130 corresponds to the maximum hardness, while lower values indicate increasing penetration and softer material.

ROCKWELL AND SUPERFICIAL ROCKWELL MINIMUM THICKNESS

WHAT IS THE SUPERFICIAL ROCKWELL HARDNESS TEST?

Superficial Rockwell hardness testing follows the same basic principles and test sequence as the standard Rockwell hardness test.
The key difference lies in the lower preliminary and total test forces, which make the method more sensitive to surface conditions.

Superficial Rockwell scales HRN (diamond indenter) and HRTW (ball indenter) use a preliminary test force of 3 kgf (29 N) and total test forces of 15 kgf (147 N), 30 kgf (294 N), and 45 kgf (441 N).

Due to the reduced test forces, the Superficial Rockwell hardness test is particularly suitable for:

  • thin components
  • surface-hardened materials
  • coatings and layered structures
  • materials with hardness gradients near the surface
  • testing small or delicate areas

Superficial Rockwell scales HRN and HRTW are commonly used for metallic materials, while HRW, HRX, and HRY scales are applied to nonmetallic materials and coatings.

MINIMUM THICKNESS MEASURABLE

To obtain valid Superficial Rockwell hardness results, the specimen thickness should generally exceed ten times the depth of indentation.
As a practical verification, no visible deformation should appear on the opposite surface of the test piece after testing; however, minor marking does not always indicate an invalid result.

Additional care is required when testing case-hardened components, such as those treated by carburizing, carbonitriding, nitriding, or induction hardening, where the hardened layer thickness and hardness gradient must be considered to avoid substrate influence.

Minimum thickness of the test piece in relation to the Rockwell hardness (ISO 6508)
X Rockwell hardness, Y minimum thickness of the test piece (mm)

Rockwell Minimum Thickness

Minimum thickness of the test piece in relation to the Rockwell hardness (ASTM E18)
Locate a point corresponding to the thickness-hardness combination to be tested. Only scales falling to the left of this point may be used to
test this combination.

Rockwell Minimum Thickness ASTM

ROCKWELL AND SUPERFICIAL ROCKWELL SCALES

The Rockwell and Superficial Rockwell hardness testing methods include a wide range of scales, each defined by a specific combination of indenter type, preliminary test force and total test force.
Each scale is intended for particular materials, thicknesses and hardness ranges, ensuring reliable measurements while avoiding excessive penetration, anvil effects or indenter damage.

The table below summarizes the most commonly used Rockwell (HR) and Superficial Rockwell (HRN / HRT) scales, indicating the corresponding indenters, applied forces and typical industrial applications.
Correct scale selection is essential to obtain accurate, repeatable and standards-compliant hardness values.

Scale Indenter Main Load (N) Pre Load (N) Application
HRB Ball 1/16″ 981 98,1 Copper alloys, soft steels, aluminum alloys, malleable iron, etc.
HRC Diamond cone 120° 1471,5 98,1 Steel, hard cast irons, pearlitic malleable iron, titanium, deep case hardened steel, and other materials harder than HRB100.
HRA Diamond cone 120° 588,6 98,1 Cemented carbides, thin steel, and shallow case-hardened steel.
HRD Diamond cone 120° 981 98,1 Thin steel and medium case hardened steel, and pearlitic malleable iron.
HRE Ball 1/8″ 981 98,1 Cast iron, aluminum and magnesium alloys, bearing metals.
HRF Ball 1/16″ 588,6 98,1 Annealed copper alloys, thin soft sheet metals.
HRG Ball 1/16″ 1471,5 98,1 Malleable irons, copper-nickel-zinc and cupro-nickel alloys. Upper limit G92 to avoid possible flattening of ball.
HRH Ball 1/8″ 588,6 98,1 Aluminum, zinc, lead.
HRK Ball 1/8″ 1471,5 98,1 Bearing metals and other very soft or thin materials. Use smallest ball and heaviest load that does red not give anvil effect.
HR 45 N Diamond cone 120° 441,4 29,43 Thin components and layers
HR 30 N Diamond cone 120° 294,3 29,43 Thin components and layers
HR 15 N Diamond cone 120° 147,1 29,43 Thin components and layers
HR 45 T Ball 1/16″ 441,4 29,43 Thin components and layers
HR 30 T Ball 1/16″ 294,3 29,43 Thin components and layers
HR 15 T Ball 1/16″ 147,1 29,43 Thin components and layers

MAXIMUM ALLOWABLE REPEATABILITY AND ERROR

The accuracy of Rockwell and Superficial Rockwell hardness measurements depends not only on correct test execution, but also on the repeatability and permissible error of the testing machine.
International standards define maximum allowable limits to ensure that hardness results remain reliable, comparable and traceable across different laboratories and production environments.

The tables below specify the maximum allowable repeatability and error for Rockwell and Superficial Rockwell hardness testing, as established by ASTM E18 and ISO 6508.
These limits are used during machine verification, performance checks and periodic calibration to confirm that the hardness tester operates within acceptable tolerances.

ASTM E-18
TABLE A1.3 Maximum Allowable Repeatability and Error of Testing Machines for Ranges of Standardized Test Blocks

Scale Hardness Range
**		 Max. Repeatability
(Rockwell units)		 Max. Permissible Error
(Rockwell units)
HRA < 70 2 ± 1
≥ 70 and < 80 1.5 ± 1
≥ 80 1 ± 0.5
HRBW < 60 2 ± 2.5
≥ 60 and < 88 1.5 ± 2.5
≥ 88 1.5 ± 1
HRC < 35 2 ± 1
≥ 35 and < 60 1.5 ± 1
≥ 60 1 ± 0.5
HRD < 51 2 ± 1
≥ 51 and < 71 1.5 ± 1
≥ 71 1 ± 0.5
HREW < 84 1.5 ± 1
≥ 84 and < 93 1.5 ± 1
≥ 93 1 ± 1
HRFW < 80 1.5 ± 1
≥ 80 and < 94 1.5 ± 1
≥ 94 1 ± 1
HRGW < 55 2 ± 1
≥ 55 and < 80 2 ± 1
≥ 80 2 ± 1
HRHW < 96 2 ± 1
≥ 96 2 ± 1
HRKW < 65 1.5 ± 1
≥ 65 and < 85 1 ± 1
≥ 85 1 ± 1
HRLW / HRMW / HRPW /
HRRW / HRSW / HRVW *		 — 2 ± 1
HR15N < 78 2 ± 1
≥ 78 and < 90 1.5 ± 1
≥ 90 1 ± 0.7
HR30N < 55 2 ± 1
≥ 55 and < 77 1.5 ± 1
≥ 77 1 ± 0.7
HR45N < 37 2 ± 1
≥ 37 and < 66 1.5 ± 1
≥ 66 1 ± 0.7
HR15TW < 81 2 ± 1.5
≥ 81 and < 87 1.5 ± 1
≥ 87 1.5 ± 1
HR30TW < 57 2 ± 1.5
≥ 57 and < 70 1.5 ± 1
≥ 70 1.5 ± 1
HR45TW < 33 2 ± 1.5
≥ 33 and < 53 1.5 ± 1
≥ 53 1.5 ± 1
HR15WW / HR30WW / HR45WW * — 2 ± 1
HR15XW / HR30XW / HR45XW * — 2 ± 1
HR15YW / HR30YW / HR45YW * — 2 ± 1

* Appropriate ranges of standardized test blocks for the L, M, P, R, S, V, W, X, and Y scales shall be determined by dividing the usable range of the scale into two ranges, if possible.
** The user may find that high, medium and low range test blocks are unavailable commercially for some scales. In these cases one or two standardized blocks where available may be used. It is recommended that all high range test blocks for Rockwell scales using a ball indenter should be less than 100 HR units.

ISO 6508
Table 2 — Permissible repeatability range and bias of the testing machine

Scale Hardness Range Permissible Bias
(Rockwell units)		 Maximum Permissible Repeatability
HRA 20 to 75 ± 2 ≤ 0.02 (100 − Havg) or 0.8 HRA units **
> 75 to 95 ± 1.5
HRBW 10 to 45 ± 4 ≤ 0.04 (130 − Havg) HRBW units
> 45 to 80 ± 3
> 80 to 100 ± 2
HRC 10 to 70 ± 1.5 ≤ 0.02 (100 − Havg) or 0.8 HRC units **
HRD 40 to 70 ± 2 ≤ 0.02 (100 − Havg) or 0.8 HRD units **
> 70 to 77 ± 1.5
HREW 70 to 90 ± 2.5 ≤ 0.04 (130 − Havg) HREW units
> 90 to 100 ± 2
HRFW 60 to 90 ± 3 ≤ 0.04 (130 − Havg) HRFW units
> 90 to 100 ± 2
HRGW 30 to 50 ± 6 ≤ 0.04 (130 − Havg) HRGW units
> 50 to 75 ± 4.5
> 75 to 94 ± 3
HRHW 80 to 100 ± 2 ≤ 0.04 (130 − Havg) HRHW units
HRKW 40 to 60 ± 4 ≤ 0.04 (130 − Havg) HRKW units
> 60 to 80 ± 3
> 80 to 100 ± 2
HR15N / HR30N / HR45N — ± 2 ≤ 0.04 (100 − Havg) or 1.2 HR-N units **
HR15T / HR30T / HR45T — ± 3 ≤ 0.06 (100 − Havg) or 2.4 HR-T units **

H avg is the mean hardness value.
** The one with a greater value becomes the permissible repeatability range of the testing machine.
NOTE The requirements for permissible repeatability range, r, and/or permissible bias, b, might be different in ASTM E 18.

TERMINOLOGY & TIPS

CALIBRATION

Calibration is the determination of the values of the significant measurement parameters of a Rockwell hardness testing machine by comparison with values indicated by a reference instrument or by certified reference standards.
Calibration establishes traceability to national or international standards and is essential to ensure reliable and comparable hardness results.

👉 Ensure traceability and compliance with our Rockwell Hardness calibration services in accordance with ISO/IEC 17025.

INDENTERS

Standard Rockwell indenters consist of either a diamond spheroconical indenter or tungsten carbide ball indenters with nominal diameters of 1.588 mm (1⁄16 in.), 3.175 mm (1⁄8 in.), 6.350 mm (1⁄4 in.), or 12.70 mm (1⁄2 in.).
Indenters shall be kept clean and free from dust, dirt, oil or other foreign materials, as contamination or damage can significantly affect penetration behavior and test results.

PORTABLE ROCKWELL HARDNESS TESER

A portable Rockwell hardness testing machine is designed to be transported, positioned and operated directly by the user, and to perform hardness measurements based on the Rockwell indentation hardness test principle.
Portable machines are typically used when test pieces cannot be moved to a laboratory environment.

👉 Explore our Portable Rockwell Hardness Testers for on-site and large-part testing.

ROCKWELL HARDNESS MACHINE

A Rockwell hardness machine is an instrument capable of performing a Rockwell hardness test and/or a Rockwell superficial hardness test, and of displaying the resulting Rockwell hardness number according to the selected scale.

👉 Discover our range of Rockwell Hardness Machines for laboratories and industrial applications.

SPECIMEN SUPPORT

A suitable specimen support, commonly referred to as an anvil, shall be used to properly support the test piece during hardness testing.
The seating and supporting surfaces of anvils shall be clean, smooth, and free from pits, deep scratches or foreign material. Damaged anvils shall be repaired or replaced.
Common specimen support anvils should have a minimum hardness of 58 HRC.

Flat specimens shall be tested on a flat anvil with a smooth bearing surface perpendicular to the axis of the indenter.
Small-diameter cylindrical specimens shall be tested using a hardened V-grooved anvil positioned directly under the indenter, or on hardened, parallel twin cylinders properly aligned and clamped.
Special anvils or fixtures, including clamping fixtures, may be required for parts that cannot be adequately supported by standard anvils.

STANDARDIZATION

Standardization is the process of bringing a Rockwell hardness testing machine into conformance with a known standard through verification and, when necessary, calibration adjustments.

TEST PIECE

For accurate and repeatable results, both the test surface and the supporting surface of the test piece shall be smooth, flat and free from oxide scale, foreign matter and lubricants.
An exception may be made for certain reactive materials that tend to adhere to the indenter; in such cases, a suitable lubricant (for example, kerosene) may be used. Any use of lubricant shall be reported in the test documentation.

Specimen preparation shall be performed in a manner that minimizes any alteration of surface hardness, such as changes caused by excessive heat, cold working or improper grinding.

REPEATABILITY (R)

Repeatability, denoted as R, represents the variability of hardness measurements obtained under specified verification conditions at a given hardness level.
It is typically estimated as the range of n hardness measurements performed on a standardized test block during a performance verification.

VERIFICATION

Verification is the process of checking a Rockwell hardness testing machine to ensure continued conformance with applicable specifications.
Rockwell hardness testing machines shall be verified periodically using certified test blocks, in accordance with the requirements of ASTM E18 and ISO 6508.

ENVIRONMENTAL CONDITIONS

Rockwell hardness testing shall be performed under controlled environmental conditions.
External vibrations, unstable supports, temperature fluctuations and air drafts may influence indenter penetration and depth measurement, particularly for low test forces and superficial scales.
The hardness testing machine should be installed on a stable base and allowed to reach thermal equilibrium before testing.

BIAS AND ERROR

Bias represents the systematic deviation of measured hardness values from the reference value of a certified test block.
Error is the difference between the indicated hardness value and the true or reference value, and is evaluated during machine verification in accordance with defined tolerance limits.

DWELL TIME

Dwell time is the specified period during which the preliminary and total test forces are maintained.
Consistent dwell times are essential to allow elastic and plastic deformation to stabilize and to ensure repeatable and comparable hardness results.

ROCKWELL HARDNESS TESTERS

Click one of the products below to see the features!

Modern hardness testing machine with digital display for precise material assessment.

ATI HTR

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ATI MRS

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ATI IN

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DAKOMASTER 300

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Fully automatic hardness testing machine with test loads from 1 to 250 kgf. Rockwell, Superficial Rockwell and Brinell HBWT. Touchscreen PC and high level software.
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MRS Serie

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Fully automatic hardness testing machine with test loads from 1 to 250 kgf. Rockwell, Superficial Rockwell and Brinell HBWT. Different frames and supports.
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RSD Serie

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Fully automatic hardness testing machine without motors, with test loads from 3 to 187,5 kgf. Rockwell, Superficial Rockwell and Brinell HBWT. Different frame stand supports…
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DRM / EX / RT

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Semi-automatic very simple hardness testing machines with test loads from 3 to 250 kgf. Rockwell, Superficial Rockwell and Brinell HBWT. Suitable for small samples.
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Hardness testing machine with digital display and metal wheel attachment.

PRS

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High-speed automatic hardness tester for independent use of the measuring head in the production department and lines. Rockwell – Superficial Rockwell – Brinell/ Test loads…
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