Rockwell hardness testing per IS 1586, ASTM E18 and ISO 6508

Definition: The Rockwell hardness test (IS 1586 Part 1, ASTM E18, ISO 6508-1) measures the permanent depth of an indentation made by a diamond cone or a ball indenter under a preliminary force of 10 kgf (98.07 N) followed by a total force of 60, 100 or 150 kgf; each 0.002 mm of depth is one Rockwell unit.

1. The three standards and how they relate

ISO 6508 is published in three parts: Part 1 is the test method, Part 2 the verification and calibration of testing machines, Part 3 the calibration of reference test blocks. IS 1586 (Bureau of Indian Standards) follows the same three-part structure and is technically aligned with ISO 6508. ASTM E18 is a single document that contains the method, machine verification (Annex A1), indenter requirements (A3) and test-block standardisation (A4). For a laboratory in India that supplies both domestic and export customers, testing to IS 1586 / ISO 6508-1 and verifying the machine to ISO 6508-2 satisfies almost every specification; ASTM E18 differs in a few tolerances and in permitting a shorter dwell, so check the customer's drawing.

2. Test cycle

1. Seat the specimen on a clean anvil so that the surface is normal to the indenter axis. Remove scale, paint and burrs; the surface finish should be ground or fine-machined for the diamond scales.
2. Apply the preliminary test force F₀ = 98.07 N (10 kgf) for the regular scales or 29.42 N (3 kgf) for the superficial scales. Its dwell should not exceed 3 s. The depth reference is set at this point (the dial is zeroed or the electronics tare).
3. Apply the additional force F₁ smoothly, without shock, to reach the total force F = F₀ + F₁ within 1–8 s. Hold the total force for 4 s ± 2 s (ISO 6508-1; ASTM E18 allows a shorter dwell in its default cycle — refer to the current edition).
4. Remove F₁ while keeping F₀; the permanent depth h is read under the preliminary force.
5. The hardness number is N − h/S: for diamond scales A, C, D it is 100 − h/0.002; for ball scales B, E, F, G, H, K it is 130 − h/0.002; for all superficial scales (N and T) it is 100 − h/0.001. Digital and dial testers compute this automatically.

Report the result as, for example, 58 HRC or 85 HRB or 72 HR30N. ISO 6508 also allows the ball type to be stated (HRBW for a tungsten-carbide ball). Make at least three indentations per specimen and report the mean where the material is not uniform.

3. Scales, indenters and loads

4. Minimum thickness and spacing

ISO 6508-1 requires the specimen to be at least 10 times the indentation depth for the diamond indenter and 15 times for ball indenters; no deformation may be visible on the back face. Depth is (100 − HR) × 0.002 mm for diamond scales and (130 − HR) × 0.002 mm for ball scales (× 0.001 mm for superficial scales).

Keep the centre of each indentation at least 3 indentation diameters from any other indentation and at least 2.5 diameters from an edge. On cylindrical surfaces apply the curvature corrections tabulated in ISO 6508-1 / ASTM E18 (they add up to several Rockwell points for small diameters), or use a V-anvil and note the correction on the report.

5. Verifying and calibrating the machine

ISO 6508-2 (and ASTM E18 Annex A1) describe two levels. Direct verification checks the test forces with a class-1 force-proving instrument (each force within ±1.0 %), the depth-measuring system (±0.001 mm over the range, ASTM ±0.5 units), the indenter geometry (cone 120° ± 0.35°, tip radius 0.200 ± 0.010 mm, ball diameter 1.5875 ± 0.0035 mm) and the test cycle timing. It is done at installation, after repair or relocation, and typically every 12 months. Indirect verification uses certified reference blocks: for each scale in use, blocks from at least two hardness ranges (three for HRC: about 20–30, 35–55 and 59–65 HRC), five indentations per block, with the mean error and the repeatability within the limits of the standard — of the order of ±1 to ±1.5 HRC depending on the range and the standard edition. A daily check with one block per scale, close to the hardness being tested, is required by ASTM E18 and recommended by ISO 6508-2 before the machine is used on production parts.

The blocks themselves are calibrated to ISO 6508-3 / IS 1586 Part 3 on a reference machine with traceability to national standards; see the guide on hardness test-block calibration for what the certificate must contain.

6. Common sources of error

• Dirty or worn anvil, swarf under the specimen, or a specimen that rocks — reads low and scatters.
• Chipped diamond or flattened ball — reads high on hard materials; verify the indenter on a block after any impact.
• Testing too close to an edge or to a previous indentation — reads low.
• Insufficient thickness — the anvil supports the material and the reading is high; check the back face.
• Decarburised or scaled surface on heat-treated parts — grind 0.2–0.3 mm before testing.
• Uneven loading speed on manual dial testers — a shock during load application overshoots the depth.

Frequently asked questions

Which ETS machine

• RASN dial Rockwell testerManual dead-weight machine with 60/100/150 kgf loads, supplied with diamond and ball indenters and certified test blocks.
• RASN-D digital Rockwell testerDigital readout with scale conversion and printer output; motorised RSNE for hands-off load application.
• RASN-TSFA fully automatic touch-screen testerLoad-cell controlled cycle with programmed dwell, statistics and PC export for production lines.
• RASN-Twin Rockwell-cum-SuperficialAdds 15/30/45 kgf superficial scales for case-hardened and thin parts; digital and touch-screen versions available.
• All Rockwell hardness testersDial, digital, touch-screen and fully automatic models.