Brinell hardness testing per IS 1500, ASTM E10 and ISO 6506

Definition: Brinell hardness (HBW) is the test force applied through a tungsten carbide ball of diameter D divided by the curved surface area of the impression it leaves, calculated from the mean diameter d of the impression measured after the force is removed, as specified in IS 1500 (Part 1), ASTM E10 and ISO 6506-1.

1. The Brinell principle and the HBW formula

A hard ball of diameter D is pressed into the test piece with a force F, held for a fixed dwell time and withdrawn. The impression is measured across two diameters at right angles and the mean d taken. Since the ball geometry is known, the surface area of the impression follows from D and d, and hardness is force divided by that area:

HBW = 0.102 × 2F / [ π D ( D − √(D² − d²) ) ]   with F in N, D and d in mm

The factor 0.102 is 1/9.807 — it converts newtons to kilogram-force so the number matches the historical kgf/mm² scale; if you work in kgf, drop it. The depth of the impression, needed for the thickness rule below, is h = (D − √(D² − d²)) / 2.

Unlike Rockwell, which reads depth under load, Brinell measures the recovered impression optically. It is slower, but the large ball averages over many grains and the result does not depend on frame deflection. IS 1500 (Part 1) is identical to ISO 6506-1; ASTM E10 uses the same formula, balls and forces.

2. Scales: force–diameter ratio, ball sizes and forces

Results from different ball sizes are comparable only if the impressions are geometrically similar, which requires the same ratio of force to ball diameter squared. The standards define this as 0.102·F/D² and allow five values. Pick the ratio for the material, then the ball size for the specimen:

Four ball diameters are standardised — 10, 5, 2.5 and 1 mm — and each ratio gives a corresponding force. The 10 mm ball with 3000 kgf is the reference condition for steel and iron; the smaller balls exist for thin or small parts and for machines that cannot reach 29 kN.

A result is written as HBW ball/force, with the dwell time added if it is not the standard 10–15 s — for example 210 HBW 5/750/20. The ratio 15 is listed in the standards but rarely called for in Indian material specifications.

3. Specimen, indent size and spacing rules

The formula is only valid when the impression is neither a shallow dimple nor a near-hemisphere. IS 1500 (Part 1) and ISO 6506-1 require the mean diameter d to lie between 0.24 D and 0.6 D — 2.4 to 6.0 mm for a 10 mm ball. Outside this band the test is invalid and must be repeated with a different ball or force at the same ratio.

• Thickness. At least 8 times the depth of indentation h. A 200 HBW steel tested at HBW 10/3000 gives d ≈ 4.27 mm and h ≈ 0.48 mm, so the minimum thickness is about 3.8 mm. No deformation may be visible on the back face.
• Edge distance. Centre of the impression at least 2.5 d from any edge — 10.7 mm in the example above.
• Spacing. Centres of adjacent impressions at least 3 d apart — 12.8 mm in the example.
• Support. The piece must sit rigidly on the anvil; curved pieces need a V-anvil or a spot face.
• Surface preparation. Remove scale, decarburised skin, paint and grease and finish the surface so the rim of the impression is clearly visible — a ground or fine-filed face is normal for castings and forgings. Preparation must not heat or cold-work the surface.

4. Running the test: force, dwell and reading the indent

The force is raised smoothly, without shock or vibration, to its nominal value. ISO 6506-1 requires the time from first contact to full force to be 2–8 s, and the full force to be held for 10 to 15 s. For metals that continue to creep under the ball — lead, tin, zinc, some aluminium alloys — the dwell is extended by agreement and recorded in the designation. Test temperature is normally 10–35 °C.

After unloading, the impression is measured across two diameters at right angles and the mean taken. The standards require the measuring system to read the diameter to within ±0.5 % of d. On a 4 mm impression that is 0.02 mm — roughly the width of the fuzzy rim seen through a hand microscope. Because HBW varies with about the square of d, a 1 % error in diameter gives about a 2 % error in hardness.

1. Manual microscope. A 20× or 40× reading microscope with a graduated reticle. The operator judges where the rim ends; on a bright ground surface this is repeatable, but on cast surfaces or where the impression has a piled-up ridge, two operators will differ by several hundredths of a millimetre.
2. Camera and image analysis. A digital camera images the impression, software fits a circle to the edge from the contrast gradient and computes d and HBW directly. The result is independent of the operator, the image is saved with the report, and the same algorithm is used on the test block during verification and on production parts. ASTM E10 explicitly permits automated measuring systems that meet the same accuracy requirement.

For camera systems, illumination matters as much as resolution. A ring light that leaves the rim in shadow on one side biases the fitted circle; check the system against a stage micrometer or a certified reference indentation, not only against a block's hardness value.

5. HBW vs HBS, where Brinell fits and its limits

Brinell tests were long made with hardened steel balls (HBS) up to about 450 HB. The steel ball flattens measurably at high loads, so it gave different numbers from a carbide ball on hard material. ISO 6506 dropped the steel ball in its 1999 revision and ASTM E10 followed; current editions of IS 1500, ASTM E10 and ISO 6506 recognise only the tungsten carbide composite ball, and the correct symbol is always HBW. Old drawings marked "HB" or "HBS" should be tested with a carbide ball and reported as HBW.

Where Brinell is the better choice:

• Castings and forgings. The large impression averages over grains, graphite flakes and minor porosity that would scatter a Rockwell or Vickers reading. Grey and SG iron, steel castings, rolled bar and forged stock are the classic Brinell materials.
• Coarse-grained and inhomogeneous material. Weld deposits, hard-facing overlays, sintered parts, non-ferrous castings.
• Rough surfaces. A 3000 kgf impression is far less sensitive to finish and scale than a 150 kgf Rockwell C depth reading.
• Correlation with tensile strength for steel (section 7).

Its limits: the scale is not used above about 650 HBW, where the ball begins to deform and the impression becomes too shallow to measure well, so hardened parts go to Rockwell C or Vickers. The impression is large (up to 6 mm) and deep, ruling out finished surfaces, thin sections and small parts, and a manual cycle takes 30–60 s against 10 s for Rockwell. See our Rockwell vs Brinell vs Vickers guide for the full comparison.

6. Machine verification per IS 1500 Part 2 / ISO 6506-2

IS 1500 (Part 2), identical to ISO 6506-2, and the verification annex of ASTM E10 define two levels. Direct verification checks each element of the machine separately and is required on installation, after repair or relocation, and when indirect verification fails. Indirect verification uses certified hardness reference blocks to check the machine as a whole and is required at least every 12 months. A daily check on one block before use is also expected.

Indirect verification uses at least two blocks spanning the range in use for each scale — for a steel foundry, one around 150–200 HBW and one around 300–400 HBW at HBW 10/3000. The blocks must be calibrated per IS 1500 (Part 3) / ISO 6506-3; see our guide to test block calibration and certificates. ASTM E10 uses the same hardness bands and closely similar tolerances.

The measuring device is the element most often left out. Checking the hardness value on a block does not prove the microscope reads to 0.5 % — an error in the reticle can be masked by an opposite error in force. Direct verification of the diameter scale is a requirement, not an option.

7. Converting Brinell to tensile strength

For carbon and low-alloy steels in the normalised, annealed or quenched-and-tempered condition, the Brinell number is roughly proportional to ultimate tensile strength:

R_m (MPa) ≈ 3.45 × HBW   (carbon and low-alloy steels, roughly 100–400 HBW)

A 200 HBW forging is thus expected to show about 690 MPa on the tensile machine, a 300 HBW Q&T bar about 1030 MPa. The tables in ASTM E140 and ISO 18265 differ somewhat at the ends of the range and are the documents to quote in a report.

The rule is a screening tool, not a substitute for a tensile test. It does not hold for austenitic stainless steel, cold-worked material, cast iron, aluminium or copper alloys, and says nothing about yield strength or elongation. Where a specification calls for both hardness and tensile properties, both must be tested. Our hardness conversion guide covers the ASTM E140 tables and their limits.

Frequently asked questions

Which ETS machine

• B3000-PC3000 kgf Brinell tester with PC software and camera measurement of the impression — the full HBW 10/3000 scale for foundries and forge shops.
• ETS-LB3000Load-cell force measurement with an industrial PC, so the applied force is read directly rather than inferred from weights and levers.
• ETS-LCB-3000ATClosed-loop automatic Brinell: motorised force application, timed dwell and camera reading in one cycle, built for production lines.
• ETS-BIMSPortable camera and software kit that measures 1–7 mm impressions to the ±0.5 % requirement and retrofits any make of existing Brinell tester.
• RASNB and RSNE-250Rockwell-cum-Brinell machines for HBW 2.5/187.5 and HBW 5/250 on small parts, alongside the regular Rockwell scales.