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The Brinell Hardness Test Theory

ASTM E10 & ISO 6506: Principles, Test Forces, Indentation Measurement, Thickness, Repeatability and Practical Metrology Guidance

This page is a technical reference prepared by the ATI Metrology Engineering Team to explain the Brinell hardness test in a standards-oriented and metrology-focused manner, based on ASTM E10 and ISO 6506.
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 Brinell test principle, the role of ball diameter, applied force, indentation measurement and surface preparation, and the practical conditions that most strongly influence measurement reliability, including specimen thickness, indentation spacing, curvature effects, repeatability and sources of error.

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

Brinell hardness test after sample preparation
This image shows two Brinell hardness tests on a prepared test surface (ATI Brinell hardness testers).

📌 Table of contents

INTRODUCTION

BRINELL HARDNESS TEST SCOPE

The Brinell hardness test is an empirical indentation hardness test that provides useful information about metallic materials. The measured hardness may correlate with tensile strength, wear resistance, ductility and other physical characteristics, and is widely used for quality control, material comparison and process verification.
Brinell testing is particularly suitable for coarse-grained, cast, forged or non-homogeneous materials, because the relatively large indentation averages the material response over a wider area.

PRINCIPLE OF THE BRINELL HARDNESS TEST

The Brinell hardness test is an indentation hardness test in which a verified machine forces a tungsten carbide ball indenter, under specified conditions, into the surface of the material under test. After removal of the test force, the diameter of the resulting indentation is measured.

The general principle of the Brinell hardness test consists of two main steps:

  1. The indenter is brought into contact with the test specimen in a direction perpendicular to the surface, and the test force is applied. The force shall be applied smoothly, typically within 1 s to 8 s, then maintained for the specified dwell time, generally 10 s to 15 s unless otherwise required by the applicable standard or product specification.
  2. After removal of the force, the diameter of the indentation is measured in at least two directions perpendicular to each other. The Brinell hardness value is derived from the mean diameter of the indentation.

REPRESENTATION OF THE BRINELL HARDNESS VALUE

Brinell hardness values shall not be designated by a number alone, because it is necessary to indicate the indenter diameter and applied test force used during the test. Brinell hardness values are expressed with the symbol HBW, followed by the ball diameter and test force.

Examples:

  • 450 HBW 10/3000 = Brinell hardness number of 450 obtained with a 10 mm ball and a test force of 3000 kgf
  • 150 HBW 2.5/62.5 = Brinell hardness number of 150 obtained with a 2.5 mm ball and a test force of 62.5 kgf

SAMPLE SURFACE PREPARATION

When necessary, the test surface shall be filed, ground, milled, machined or polished so that the edge of the indentation can be clearly defined and measured to the specified accuracy. Surface preparation shall be performed in a way that minimizes any alteration of surface hardness, for example due to overheating, cold working or excessive grinding pressure.

For daily Brinell hardness tests where surface preparation is necessary, it is recommended to use a Brinell instrument with milling.

MEASUREMENT OF THE BRINELL INDENTATION

The diameter of each indentation shall be measured in two directions perpendicular (90°) to each other (ASTM). Additional measurements may be made when required. The arithmetic mean of the measured diameters is used for the calculation of the Brinell hardness value.

For tests on flat surfaces, the difference between the largest and smallest measured diameters for the same indentation shall not exceed 1% of the indenter ball diameter, unless otherwise specified for materials with anisotropic structure or special product requirements.

When indentations are made on curved surfaces, the minimum radius of curvature should be at least 2.5 times the ball diameter. In such cases, the indentation may appear slightly elliptical rather than perfectly circular, and the mean of the major and minor axes shall be used.

INDENTATION SPACING

The distance between the centers of two adjacent indentations shall be at least 3 times the mean indentation diameter. The distance from the center of any indentation to the edge of the test piece shall be at least 2.5 times the mean indentation diameter.

TESTING OF THE AMBIENT TEMPERATURE

Brinell hardness tests should be carried out at an ambient temperature within the limits of 10 °C to 35 °C (50 °F to 95 °F). Users should be aware that the temperature of both the test specimen and the hardness tester may influence the result. The test shall therefore be performed under conditions that do not adversely affect measurement reliability.

PRACTICAL BRINELL TEST CONDITIONS

Parameter Requirement / Practical Rule Why It Matters
Minimum specimen thickness The specimen thickness should be at least 10 times the indentation depth. Prevents influence from the opposite surface and reduces anvil effect.
Back-side condition No visible bulge, mark or deformation should appear on the opposite side after testing. Visible deformation may indicate that the specimen is too thin for a valid Brinell result.
Distance between indentations The distance between the centers of two adjacent indentations shall be at least 3 times the mean indentation diameter. Avoids interaction between adjacent plastic deformation zones.
Distance from the edge The distance from the center of the indentation to the edge of the specimen shall be at least 2.5 times the mean indentation diameter. Prevents edge influence and distortion of the indentation shape.
Surface preparation The test surface should be smooth, clean and free from oxide scale, coatings, dirt and lubricants, unless otherwise specified. Ensures a clearly measurable indentation diameter and improves measurement accuracy.
Curved surfaces The minimum radius of curvature should be at least 2.5 times the ball diameter. Excessive curvature can produce elliptical indentations and affect the result.
Indentation measurement The indentation diameter should be measured in at least two perpendicular directions. Improves accuracy and compensates for slight asymmetry in the impression.
Ambient temperature Testing should generally be performed between 10 °C and 35 °C. Temperature may influence both the specimen response and the measuring system.

PRINCIPLE OF THE BRINELL HARDNESS TEST

INDENTER BRINELL

Brinell testing uses tungsten carbide balls with standard diameters of 1 mm, 2.5 mm, 5 mm and 10 mm.

DEFINITION

The Brinell test consists of pressing a ball indenter of diameter D, under a defined force F, into the test specimen and then measuring the resulting indentation diameter d after unloading.

The Brinell hardness value, designated as HBW, is calculated by dividing the applied force by the curved surface area of the indentation, which is considered as a segment of a sphere. Because the result is based on indentation diameter rather than penetration depth, the method is particularly suitable for materials where a more representative average hardness value is required.

Diagram of Brinell hardness test showing formula and indentation from a spherical indenter.

BRINELL HARDNESS SCALES

The combinations of ball diameter and test force define the Brinell hardness scales. Standard Brinell test conditions are based on specific force-to-diameter ratios, commonly 1, 2.5, 5, 10 and 30, depending on the material being tested.

The correct scale shall be selected according to:

  • material type and expected hardness
  • specimen thickness
  • required indentation size
  • applicable product or test standard

In practice, larger balls and higher loads are typically used for coarse or heavy materials, while smaller balls and lower loads are used where indentation size must be limited.

Brinell hardness scales are defined by the combination of ball diameter and applied test force. The force-to-diameter ratio (F/D²) is the key parameter that determines the indentation geometry and the applicable hardness range.

Scale Ball Ø (mm) Test Force (kgf) Test Force (N) F/D² Ratio Typical Hardness Range (HBW) Typical Applications
HBW 10/3000 10 3000 29420 30 95 – 650 Steels, forgings, cast steels
HBW 10/1500 10 1500 14710 15 48 – 327 Medium hardness steels
HBW 10/1000 10 1000 9807 10 32 – 218 Aluminum, copper alloys
HBW 10/500 10 500 4903 5 16 – 109 Soft non-ferrous alloys
HBW 10/250 10 250 2452 2.5 8 – 55 Very soft metals
HBW 10/125 10 125 1226 1.25 4 – 27 Thin soft materials
HBW 10/100 10 100 981 1 3 – 22 Very soft alloys and coatings
HBW 5/750 5 750 7355 30 95 – 650 Medium parts, steels
HBW 5/250 5 250 2452 10 32 – 218 Non-ferrous alloys
HBW 5/125 5 125 1226 5 16 – 109 Soft metals
HBW 5/62.5 5 62.5 613 2.5 8 – 55 Thin sections
HBW 5/31.25 5 31.25 306 1.25 4 – 27 Small components
HBW 5/25 5 25 245 1 3 – 22 Very thin materials
HBW 2.5/187.5 2.5 187.5 1839 30 95 – 650 Localized steel testing
HBW 2.5/62.5 2.5 62.5 613 10 32 – 218 Small non-ferrous parts
HBW 2.5/31.25 2.5 31.25 306 5 16 – 109 Thin materials
HBW 2.5/15.625 2.5 15.625 153 2.5 8 – 55 Very thin materials
HBW 2.5/7.8125 2.5 7.8125 77 1.25 4 – 27 Micro-components
HBW 2.5/6.25 2.5 6.25 61 1 3 – 22 Very soft materials
HBW 1/30 1 30 294 30 95 – 650 Micro steel testing
HBW 1/10 1 10 98 10 32 – 218 Precision testing
HBW 1/5 1 5 49 5 16 – 109 Small soft parts
HBW 1/2.5 1 2.5 24.5 2.5 8 – 55 Thin materials
HBW 1/1.25 1 1.25 12.3 1.25 4 – 27 Very thin coatings
HBW 1/1 1 1 9.81 1 3 – 22 Ultra-light testing

BRINELL MINIMUM THICKNESS (mm)

MINIMUM THICKNESS MEASURABLE

The thickness of the specimen shall be such that no bulge, mark or other visible effect appears on the side opposite the indentation due to the applied force. As a general rule, the specimen thickness should be at least ten times the depth of the indentation.

Because Brinell indentations are relatively large, specimen thickness and support conditions are critical to avoid anvil influence and loss of validity.

CAUTION: Under certain test conditions, particularly when testing relatively thin materials, brittle materials or hard coatings on harder substrates, there is a risk that the test specimen may crack, break or shatter under load. Users are strongly advised to exercise care when testing any material that could fail mechanically under the applied force. If there is doubt, the test should not be performed without further evaluation.

Indentation Diameter (mm) Ball Ø 10 mm Ball Ø 5 mm Ball Ø 2.5 mm Ball Ø 1 mm
0.2 0.1
0.3 0.2
0.4 0.4
0.5 0.7
0.6 0.4 1.0
0.7 0.5
0.8 0.7
0.9 0.8
1.0 1.0
1.1 1.3
1.2 0.7 1.5
1.3 0.9 1.8
1.4 1.0 2.1
1.5 1.2 2.5
1.6 1.3
1.7 1.5
1.8 1.7
1.9 1.9
2.0 2.1
2.2 2.6
2.4 1.5 3.1
2.6 1.7 3.6
2.8 2.0 4.3
3.0 2.3 5.0
3.2 2.6
3.4 3.0
3.6 3.4
3.8 3.8
4.0 4.2
4.2 4.6
4.4 5.1
4.6 5.6
4.8 6.1
5.0 6.7
5.2 7.3
5.4 7.9
5.6 8.6
5.8 9.3

BRINELL MAXIMUM ALLOWABLE REPEATABILITY AND ERROR

The accuracy of Brinell hardness measurements depends not only on correct execution of the test, but also on the repeatability and permissible error of the testing machine.

International standards define maximum allowable limits to ensure that hardness values remain reliable, comparable and traceable across laboratories and industrial environments.

Parameter Definition How It Is Evaluated Practical Meaning
Error (E) The deviation between the measured Brinell hardness value and the certified reference value of a standardized test block. Calculated during machine verification by comparing the average measured hardness value with the certified value of the reference block. Indicates the accuracy of the hardness tester.
Repeatability (R) The variability between repeated measurements obtained under the same verification conditions. Estimated from the range of the measured indentation diameters or hardness values obtained on a standardized test block. Indicates the consistency of the hardness tester.
Certified test block A reference block with a certified Brinell hardness value used for verification and calibration. Used during direct and indirect verification procedures. Provides traceability and comparability of results.
Verification Procedure used to confirm that the hardness tester operates within the limits defined by the applicable standard. Performed periodically using certified reference blocks and defined tolerances. Confirms that the machine remains compliant over time.
Calibration Comparison of the machine parameters with certified reference instruments or standards. Performed to establish traceability and quantify machine performance. Supports accuracy, compliance and audit readiness.

The tables below define the maximum allowable repeatability (R) and maximum permissible error (E) for Brinell hardness testing machines, expressed as a percentage of the measured hardness value (H).
The limits depend on the force-to-diameter ratio (HB30, HB15, HB10, HB5, HB2.5, HB1) and are used during machine verification in accordance with ISO 6506 and ASTM E10.

ASTM E10-23 – TABLE A1.2 Maximum Allowable Repeatability and Error of Testing Machines (HB30, HB15, HB10, HB5, HB2.5, HB1) 

Reference Block Hardness Maximum Repeatability R
(% of d)		 Maximum Error E
(% of H)
HBW ≤ 125 3.0 ±3.0
125 < HBW ≤ 225 2.5 ±2.5
HBW > 225 2.0 ±2.0

ISO 6506-2:2017 – Table 2 — Repeatability and error of the testing machine for force-diameter index = 30 (HB30)

H Range Max Repeatability R
(% of H)		 Max Error E
(% of H)
Hc < 250 HBW 3 ±3
250 ≤ Hc ≤ 450 HBW 3 ±3
Hc > 450 HBW 3 ±3

ISO 6506-2:2017 – Table 3 — Repeatability and error of the testing machine for force-diameter index = 15 (HB15)

H Range Max Repeatability R
(% of H)		 Max Error E
(% of H)
Hc < 100 HBW 3 ±3
100 ≤ Hc ≤ 250 HBW 3 ±3
Hc > 250 HBW 3 ±3

ISO 6506-2:2017 – Table 3 — Repeatability and error of the testing machine for force-diameter index = 10 (HB10)

H Range Max Repeatability R
(% of H)		 Max Error E
(% of H)
Hc < 100 HBW 3 ±3
100 ≤ Hc ≤ 200 HBW 3 ±3
Hc > 200 HBW 3 ±3

ISO 6506-2:2017 – Table 3 — Repeatability and error of the testing machine for force-diameter index = 5 (HB5)

H Range Max Repeatability R
(% of H)		 Max Error E
(% of H)
Hc < 70 HBW 3 ±3
70 ≤ Hc ≤ 100 HBW 3 ±3
Hc > 100 HBW 3 ±3

ISO 6506-2:2017 – Table 3 — Repeatability and error of the testing machine for force-diameter index = 2.5 and 1 (HB2.5, HB1)

H Range Max Repeatability R
(% of H)		 Max Error E
(% of H)
Hc < 70 HBW 3 ±3

Note: Hc represents the certified hardness value of the reference test block used during verification.

TERMINOLOGY & TIPS

CALIBRATION

Calibration is the determination of the values of the significant measurement parameters of a Brinell 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 Brinell hardness calibration services in accordance with ISO/IEC 17025.

INDENTERS

Indenters for the Brinell hardness test shall be tungsten carbide balls of the allowed diameters (1 mm, 2.5 mm, 5 mm and 10 mm). Dust, dirt and other foreign materials shall not be allowed to accumulate on the indenter, as contamination or wear may affect the test result.

PORTABLE BRINELL HARDNESS TESTING MACHINE

A portable Brinell hardness testing machine is designed to be transported, positioned and operated directly by the user, and to perform measurements according to the Brinell indentation hardness test principle. Portable systems are particularly useful when large or installed components cannot be brought to the laboratory.

👉 Explore our portable Brinell hardness testers for on-site and large-part testing.

BRINELL HARDNESS MACHINE

Equipment for Brinell hardness testing generally consists of a testing machine capable of supporting the specimen, applying the required force to the ball indenter, and measuring the mean diameter of the resulting indentation in accordance with the Brinell test principle.

The design of the testing machine shall ensure that no rocking or lateral movement of the specimen or indenter occurs during force application, and that the force is applied smoothly and without impact.

👉 Discover our range of Brinell 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.

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 or equivalent suitable support. Special anvils or fixtures may be required for parts that cannot be adequately supported by standard anvils.

STANDARDIZATION

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

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. Surface preparation shall be carried out in such a way that any alteration of the surface hardness, for example due to heating or cold working, is minimized.

BIAS

Bias represents the systematic deviation of the measured Brinell hardness value from the certified reference value of a standardized test block.
It is evaluated during verification procedures by comparing the average measured hardness with the certified value of the reference block.
Bias is typically expressed as a percentage of the hardness value (% of H) and indicates the accuracy of the testing machine.
A positive or negative bias may result from factors such as incorrect force application, indenter wear, calibration drift or errors in the optical measurement system.
Controlling bias is essential to ensure traceability, compliance with standards and comparability of results across different laboratories and production environments.

DWELL TIME

Dwell time is the specified duration during which the test force is maintained while the indenter is in contact with the test specimen.
In Brinell hardness testing, the full test force is typically applied within a defined time and maintained for a dwell time generally ranging from 10 to 15 seconds, depending on the material and applicable standard.
Proper dwell time is essential to allow the material to undergo complete plastic deformation and to stabilize the indentation geometry before measurement.
Insufficient dwell time may lead to underestimation of the indentation diameter, while excessive dwell time may affect the material response, especially in time-dependent materials.
Consistent application of dwell time improves repeatability, measurement stability and compliance with ASTM E10 and ISO 6506.

THE ERROR E

The error in the performance of a Brinell hardness machine at each hardness level, under the specified verification conditions, is estimated by the percent error of the average of n indentation measurements made on a standardized test block relative to the certified average hardness value of that block.

THE REPEATABILITY R

Repeatability, denoted as R, represents the variability of results obtained under specified verification conditions. In Brinell testing, it is typically estimated by the percent range of the diameter values of n indentations made on a standardized test block, relative to the average of the measured diameters.

VERIFICATION

Verification is the process of checking a Brinell hardness testing machine to ensure continued conformance with applicable specifications. Brinell testing machines shall be verified periodically using certified test blocks.

ENVIRONMENTAL CONDITIONS

Brinell hardness testing shall be performed under controlled environmental conditions. External vibrations, unstable supports, temperature fluctuations and inadequate optical conditions may affect indentation formation and diameter measurement. The hardness tester should therefore be installed on a stable base and used under suitable laboratory or production conditions.

👨‍🔬👩‍🔬 AUTHOR & TECHNICAL RESPONSIBILITY

Technical content provided by the ATI Srl Metrology Engineering Team, with expertise in hardness testing methods, international standards and industrial quality control.

🔎 ARE YOU LOOKING FOR A BRINELL HARDNESS TESTER?

Selecting the correct Brinell hardness tester depends on several factors, including material type, expected hardness range, specimen size and required level of automation.

ATI provides a complete range of Brinell hardness testing solutions for laboratory use, production environments and heavy-duty industrial applications. Our systems are designed to ensure high accuracy, repeatability and full compliance with ASTM E10 and ISO 6506, even in demanding conditions.

Whether you need a bench tester, a portable solution or a fully automatic system with integrated sample preparation, our metrology experts can help you identify the most suitable configuration for your application.

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