The Rockwell hardness test is an empirical indentation method that measures hardness from the permanent depth of penetration left by an indenter under a defined load. A diamond spheroconical or tungsten carbide ball indenter is forced into the surface, the depth difference between a preliminary and a total test force is measured, and the result is read directly as a hardness number, with no optical measurement required. This makes Rockwell fast, repeatable and well suited to production and laboratory testing. This page is a technical reference prepared by the ATI (Affri Testing Instruments) Metrology Engineering Team for laboratory testing, audits, procedure development, troubleshooting and incoming material inspection, covering the principle, the complete test cycle, scale designations and the practical conditions that most influence measurement reliability, including specimen thickness, surface condition, support, indentation spacing, repeatability and sources of error.

Introduction and principle of the Rockwell test

The Rockwell hardness test is an empirical indentation hardness test that provides useful information about metallic materials. Its results may correlate to tensile strength, wear resistance, ductility and other physical characteristics, and are widely used in quality control and material selection. The method uses a verified machine to force a diamond spheroconical indenter or a tungsten carbide (or steel) ball indenter, under specified conditions, into the surface of the material, and measures the difference in depth of the indentation as the force is increased from a preliminary test force to a total test force and then returned to the preliminary force.

The three-step test cycle

1. Preliminary force. The indenter is brought into contact with the specimen and the preliminary test force is applied. After a specified dwell time, the baseline depth of indentation is measured.
2. Total force. The force is increased at a controlled rate by the additional test force to reach the total test force, which is held for a specified dwell time.
3. Return to preliminary force. The additional force is removed, returning to the preliminary force. After a specified dwell time the final depth is measured. The Rockwell value is derived from the difference between the final and baseline depths under the preliminary force; then the preliminary force and the indenter are removed.

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

How the Rockwell number is calculated

During the test the force is increased from the preliminary to the total force and back to the preliminary force. The difference between the two depth measurements, taken under the preliminary force, is the permanent depth increase h (in mm), from which the Rockwell number is derived. By design the scale gives a higher number for a harder material:

Regular Rockwell, diamond:  HR = 100 − h / 0.002

Regular Rockwell, ball:  HR = 130 − h / 0.002

Superficial Rockwell:  HR = 100 − h / 0.001

Here h is the permanent depth in millimetres and the scale unit is 0.002 mm for regular Rockwell and 0.001 mm for superficial Rockwell. The number 100 is the reference for diamond and superficial scales, 130 for the regular ball scales. The calculator below converts a Rockwell value into the corresponding permanent indentation depth.

Calculator: permanent indentation depth from a Rockwell value

The two classifications

Two classifications are used, differing in the test forces. The Rockwell hardness test uses a preliminary force of 10 kgf (98 N) and total forces of 60 kgf (589 N), 100 kgf (981 N) or 150 kgf (1471 N). The Rockwell superficial hardness test uses a preliminary force of 3 kgf (29 N) and total forces of 15 kgf (147 N), 30 kgf (294 N) or 45 kgf (441 N).

How Rockwell values are reported

A Rockwell value must never be designated by a number alone, because the indenter and forces used must be indicated. The number is followed by the symbol HR and the scale designation. For example, 64 HRC is a Rockwell hardness of 64 on the C scale, and 81 HR30N is a superficial Rockwell hardness of 81 on the 30N scale.

Rockwell C hardness test (diamond cone)

The Rockwell C test uses a diamond spheroconical indenter with a 120° cone angle ending in a 0.2 mm radius tip, mirror polished. A preliminary force (F0) of 98.1 N is applied first, and at this point the depth reading is zeroed to settle the test piece. An additional force (F1) is then applied up to a total force (F) of 588.6 N, 981 N or 1471.5 N depending on the scale. After the depth indicator has stabilized, the additional force is removed, leaving only the preliminary force, and the hardness is read from the residual depth. The HRC scale (1471.5 N total) is the most common for hardened steel, while HRA and HRD use the same diamond indenter at lower total forces for thin or shallow case-hardened steel.

Rockwell B hardness test (ball indenter)

The Rockwell B test uses a tungsten carbide ball indenter with a hardness of not less than 850 HV and a diameter of 1/16 in (1.5875 mm). The procedure follows the same principle as the diamond cone test, but the reference zero is set at 130 scale units instead of 100. This provides an additional measurement range, allowing reliable testing of soft materials, where the ball produces deeper penetration. The hardness value is obtained by subtracting the measured penetration from 130: a value of 130 corresponds to maximum hardness, while lower values indicate increasing penetration and softer material. The same ball indenter, in different sizes and at different forces, defines the other ball scales (HRE, HRF, HRG, HRH, HRK).

Rockwell and Superficial Rockwell minimum thickness

Superficial Rockwell testing follows the same basic principle and sequence as the standard Rockwell test, but uses lower preliminary and total forces, which makes it more sensitive to surface conditions. The superficial scales HRN (diamond indenter) and HRTW (ball indenter) use a 3 kgf (29 N) preliminary force and 15 kgf (147 N), 30 kgf (294 N) or 45 kgf (441 N) total forces. Because of the reduced forces, the superficial test is particularly suitable for:

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

The HRN and HRTW scales are commonly used for metallic materials, while HRW, HRX and HRY scales are applied to nonmetallic materials and coatings. As a practical rule, the specimen thickness should generally exceed ten times the depth of indentation, and no visible deformation should appear on the opposite surface after testing (although minor marking does not always mean the result is invalid). Extra care is required on case-hardened components (carburized, carbonitrided, nitrided or induction-hardened), where the hardened layer thickness and hardness gradient must be considered to avoid substrate influence.

Calculator: minimum thickness (ISO 6508-1 and ASTM E18)

Rockwell and Superficial Rockwell scales and applications

The Rockwell and Superficial Rockwell methods include a wide range of scales, each defined by a specific combination of indenter type, preliminary force and total force, and intended for particular materials, thicknesses and hardness ranges. Correct scale selection is essential to obtain accurate, repeatable and standards-compliant values, while avoiding excessive penetration, anvil effects or indenter damage. The table summarizes the most commonly used Rockwell (HR) and Superficial Rockwell (HRN / HRT) scales, with indenters, applied forces and typical applications.

Maximum allowable repeatability and error

The accuracy of Rockwell and Superficial Rockwell 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 so that results remain reliable, comparable and traceable across laboratories and production environments. These limits are used during machine verification, performance checks and periodic calibration. The calculator returns the ISO and ASTM limits for a chosen scale and hardness, and the tables below give the full ASTM E18 and ISO 6508 limits.

ASTM E18, Table A1.3 — maximum allowable repeatability and error

* For the L, M, P, R, S, V, W, X and Y scales, the ranges of standardized test blocks are determined by dividing the usable range into two ranges where possible. ** High, medium and low range blocks may be commercially unavailable for some scales; where so, one or two standardized blocks may be used. High-range blocks for ball-indenter scales should be below 100 HR units.

ISO 6508, Table 2 — permissible repeatability range and bias

H_avg is the mean hardness value. ** The greater value becomes the permissible repeatability range of the testing machine. Note: the requirements for permissible repeatability range r and permissible bias b may differ between ASTM E18 and ISO 6508.

Corrections for tests on curved (convex) surfaces

When a Rockwell test is made on a convex cylindrical or spherical surface, the indenter is less laterally supported than on a flat surface, so it penetrates slightly more and the reading comes out lower than the true value. ISO 6508-1 therefore gives corrections to be added to the reading: Annex C for convex cylinders (by indenter type, hardness reading and radius of curvature) and Annex D for convex spheres on the HRC scale (by reading and sphere diameter). ASTM E18 gives equivalent corrections for cylinders indexed by diameter. For radii or diameters between the tabulated values the correction is obtained by linear interpolation, and where the curvature is sharp and the hardness low the correction becomes too large to be acceptable. The calculator applies these corrections for ISO and ASTM; always report the correction and the curvature on the test report.

Rockwell terminology and practical tips

Correct Rockwell testing depends not only on the indenter and forces, but also on the condition of the machine, the specimen support, the surface preparation and the verification procedure. The following terminology and notes summarize the main elements that influence the reliability of HR results.

Calibration, verification and standardization

Calibration is the determination of the significant measurement parameters of a Rockwell machine by comparison with a reference instrument or certified reference standards; it establishes traceability to national or international standards. Verification is the periodic checking of the machine for continued conformance with the applicable specification, performed with certified test blocks in accordance with ASTM E18 and ISO 6508. Standardization is the process of bringing the machine into conformance with a known standard through verification and, where necessary, calibration adjustments. ATI provides Rockwell calibration services in accordance with ISO/IEC 17025.

Indenters

Standard Rockwell indenters are 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 must be kept clean and free from dust, dirt, oil or other foreign material, since contamination or damage can significantly affect penetration and results.

Specimen support and test piece

A suitable specimen support, commonly called an anvil, must support the test piece during testing. Seating and supporting surfaces of anvils must be clean, smooth and free from pits, deep scratches or foreign material, and damaged anvils must be repaired or replaced; common support anvils should have a minimum hardness of 58 HRC. Flat specimens are tested on a flat anvil whose bearing surface is perpendicular to the indenter axis; small-diameter cylinders on a hardened V-grooved anvil directly under the indenter, or on aligned, clamped twin cylinders. Both the test surface and the supporting surface should be smooth, flat and free from oxide scale, foreign matter and lubricants. For certain reactive materials that adhere to the indenter, a suitable lubricant (for example kerosene) may be used and must be reported. Preparation must minimize any alteration of surface hardness from excessive heat, cold working or improper grinding.

Repeatability, bias and error

Repeatability (R) is the variability of measurements obtained under specified verification conditions at a given hardness level, typically estimated as the range of n measurements on a standardized test block. Bias is the systematic deviation of measured values from the reference value of a certified test block, and error is the difference between the indicated value and the reference value, evaluated during verification against defined tolerance limits.

Dwell time and environmental conditions

Dwell time is the specified period during which the preliminary and total forces are maintained; consistent dwell times let elastic and plastic deformation stabilize and ensure repeatable, comparable results. Testing should be performed under controlled environmental conditions: external vibrations, unstable supports, temperature fluctuations and air drafts can influence penetration and depth measurement, particularly at low forces and on superficial scales. The machine should be installed on a stable base and allowed to reach thermal equilibrium before testing.

Portable Rockwell testing

A portable Rockwell machine is designed to be transported, positioned and operated directly by the user to perform measurements based on the Rockwell principle, typically when test pieces cannot be moved to a laboratory. See ATI portable Rockwell hardness testers for on-site and large-part testing.

Common sources of error

• Insufficient thickness — too thin a specimen lets the anvil influence the reading (anvil effect).
• Curved surface not corrected — convex surfaces read low unless the appropriate correction is applied.
• Poor support or unstable base — movement during the cycle changes the measured depth.
• Dirty or damaged indenter or anvil — contamination and damage alter penetration.
• Surface condition — oxide scale, roughness or a non-perpendicular surface distort the result.
• Wrong scale selection — a scale unsuitable for the material or thickness gives unreliable values.
• Indentations too close together or to an edge — prior deformation affects the next reading.

Frequently asked questions

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Author and technical responsibility

Technical content reviewed by the ATI (Affri Testing Instruments) Metrology Engineering Team, with expertise in hardness testing methods, international standards, accredited calibration and industrial quality control.

Need help choosing a Rockwell scale or tester?

Our metrology engineers can help you choose the Rockwell or Superficial Rockwell scale, the tester, the anvils and fixtures, the test blocks and the calibration program for your application. For broader method selection, see the Hardness Testing Academy or the general hardness testing overview.