Overview
EN ISO 4545-3:2018 (ISO 4545-3:2017) specifies the method for the calibration of hardness reference blocks used for the indirect verification of Knoop hardness testing machines (see ISO 4545‑2). The procedure is intended for indentations with long diagonals ≥ 0.020 mm and defines manufacturing, dimensional, surface and calibration-machine requirements to ensure traceable, repeatable Knoop hardness measurements.
Key topics and requirements
- Scope and applicability
- Calibration method for reference blocks used in indirect verification of Knoop hardness testers.
- Applicable only for Knoop indentations with long diagonals ≥ 0.020 mm.
- Reference block manufacture
- Thickness: ≥ 5 mm.
- Maximum test surface area: 40 cm².
- Magnetism: blocks shall be free of magnetism (steel blocks recommended to be demagnetized before calibration).
- Flatness and parallelism: flatness deviation ≤ 0.005 mm; parallelism error ≤ 0.010 mm per 50 mm.
- Surface roughness: Ra ≤ 0.05 µm; surface free of scratches that interfere with indentation measurement.
- Marking: thickness recorded to the nearest 0.01 mm or an identifying mark on the test surface to prevent regrind.
- Calibration machine and verification
- Must meet ISO 4545‑2 general requirements plus additional checks in this part.
- Direct verification interval: not exceeding 12 months.
- Direct verification includes: calibration of test force, indenter verification, diagonal measurement system calibration/verification, and verification of the testing cycle (or force vs. time behavior).
- Instruments used for verification must be traceable to national standards.
- Test force verification performed at three plunger positions (covering travel limits).
- Uniformity and uncertainty
- Procedures and acceptance criteria for uniformity of hardness across the block and for estimating measurement uncertainty (see Annex B).
- Supporting material
- Annex A: guidance on adjustment of Köhler illumination systems.
- Annex B: calculation of uncertainty of mean hardness value.
- Documentation
- Requirements for marking and issuing a calibration certificate and statements on calibration validity.
Applications and users
- Calibration and testing laboratories performing Knoop hardness measurements.
- Quality assurance teams in metallurgy, aerospace, automotive and precision manufacturing.
- Manufacturers of hardness testers and reference blocks.
- Accreditation bodies and calibration service providers ensuring traceability, repeatability and compliance with ISO standards.
Related standards
- ISO 4545-1: Knoop test method
- ISO 4545-2: Verification and calibration of testing machines
- ISO 376: Calibration of force‑proving instruments
Keywords: EN ISO 4545-3:2018, Knoop hardness, calibration, reference blocks, hardness testing, metallic materials, traceability, uncertainty, ISO 4545-2.
