Hardness test blocks: verification, calibration and certificates

Definition: A hardness test block (reference block) is a piece of uniformly heat-treated steel or non-ferrous alloy whose hardness has been certified on a calibrated reference machine to ISO 6508-3, ISO 6506-3, ISO 6507-3 or ASTM E18 / E10 / E92 Annexes; it is the only practical way to verify that a hardness tester in daily use still reads correctly.

1. Why blocks, and why more than one

A hardness tester can drift in three independent ways: the applied force (springs, dead weights, load cell), the indenter (worn ball, chipped diamond) and the depth or optical measuring system. A reference block checks the whole chain at once — force, indenter, measurement, cycle and operator — against a value traceable to a national standard. Because the errors are not linear across the scale, each scale in use needs blocks at least at two hardness levels, ideally three: ISO 6508-2 uses roughly 20–30, 35–55 and 59–65 HRC for the C scale, and 40–60, 60–80 and 80–100 HRB for the B scale. A laboratory that reports HRC, HRB, HBW 2.5/187.5 and HV 30 therefore holds eight to ten blocks.

2. Daily verification versus periodic calibration

The daily check is not calibration — it is a pass/fail gate. If the mean reading falls outside the permitted error (of the order of ±1 HRC for a well-kept machine; the exact limit per scale and range is tabulated in ISO 6508-2 and ASTM E18 Annex A1), stop testing, check the anvil, indenter and seating, retest, and if it still fails, quarantine the results since the last successful check and call for an indirect verification.

3. Indenter checks

1. Diamond cone (Rockwell). Inspect the tip under 10–20× magnification for chips and flats after any suspected impact. The functional test is a performance check on blocks: readings with the suspect indenter and with a reference indenter on the same block should not differ by more than the limit in ISO 6508-2 (typically ±0.8 HRC at 60 HRC). A new indenter is supplied with its own certificate of geometry and performance.
2. Ball indenters. Balls deform; a 1.5875 mm steel ball used on HRB material harder than about 100 HRB flattens and reads high. Replace the ball rather than measure it — carbide balls (minimum 1500 HV) are now the standard in ISO 6508 and ASTM E18 ("HRBW").
3. Vickers pyramid. Check the 136° ± 0.5° face angle and the line of junction (≤ 0.001 mm for micro, ≤ 0.002 mm macro) on the certificate; verify performance on HV blocks.
4. Brinell balls. Carbide, 2.5 / 5 / 10 mm, diameter tolerance ±0.005 mm (10 mm ball); replace when the impression is no longer circular or the surface is pitted.

4. How to use and store a test block

• Test only on the certified (marked) surface; the underside is not calibrated and is often engraved or ground concave so it cannot be used.
• Keep indentations at least 3 diameters apart and 2.5 diameters from the edge; when the surface is full, the block is finished — regrinding invalidates the certificate.
• Never test a block on a magnetic chuck or through a film of oil; clean it and the anvil with a solvent and wipe dry before and after use.
• Store in the case with a light film of oil; corrosion pits read low and a rusty block must be retired.
• Record every indentation (date, reading, operator) so the block's history can be shown to an auditor.

5. What the calibration certificate must contain

A certificate from a laboratory working to ISO 6508-3 (or the corresponding Brinell / Vickers part) and ISO/IEC 17025 will state, as a minimum:

1. Identification of the block: serial number (also engraved on the block), scale, nominal hardness and manufacturer.
2. The certified mean hardness value and the number of indentations used (at least five, distributed over the surface).
3. The non-uniformity (range of the five readings), which must not exceed the limit in the standard — for example 1.0 HRC at 60 HRC and above, 2 % of the mean for Brinell, 2–3 % for Vickers depending on force.
4. The expanded uncertainty of the certified value at k = 2 (typically 0.3–0.5 HRC for a good reference machine).
5. The reference machine, its last direct verification and the chain of traceability to a national metrology institute (in India, CSIR-NPL; internationally NIST, PTB or an equivalent).
6. The standard and edition used, the test conditions (temperature 23 ± 5 °C, indenter type and serial), the date of calibration and the signature of the authorised person.
7. A statement of the accreditation status of the calibrating laboratory, if it is accredited (in India by NABL). Ask for the accreditation scope: a laboratory accredited for Rockwell may not be accredited for Vickers.

Blocks do not carry an expiry date in ISO 6508-3; their validity ends when the certified surface is used up or damaged. Many quality systems nonetheless fix a recertification interval of two to five years, and customer audits often expect it — check what your own ISO 9001 / ISO/IEC 17025 procedure requires and refer to the current edition of the standard.

6. Choosing blocks

Frequently asked questions

Which ETS product

• Accessories and test blocksCertified Rockwell, Brinell and Vickers test blocks, diamond and ball indenters, anvils and Brinell microscopes.
• Calibration serviceVerification of hardness testers with certified blocks and traceable certificates, on site.
• Extension calibratorBench instrument for checking the depth-measuring system of hardness testers and extensometers.
• Rockwell hardness testersEvery ETS tester ships with the test blocks needed for the daily check.