ISO 4545-3:2017

Overview

ISO 4545-3:2017 - "Metallic materials - Knoop hardness test - Part 3: Calibration of reference blocks" specifies the method for calibrating hardness reference blocks used for the indirect verification of Knoop hardness testing machines (as per ISO 4545‑2). The procedure is applicable to Knoop indentations with long diagonals ≥ 0.020 mm and defines manufacturing, measurement, environmental and documentation requirements to ensure traceability and repeatability of Knoop hardness measurements.

Key topics and technical requirements

• Scope and traceability
  • Calibration method for reference blocks used in indirect verification of Knoop hardness machines.
  • Verification instruments and calibration must be traceable to national standards (see ISO 376).
• Reference block manufacture
  • Minimum thickness: 5 mm.
  • Maximum test surface area: 40 cm².
  • Blocks must be non-magnetic (steel blocks should be demagnetized).
  • Flatness: max deviation 0.005 mm; parallelism: max 0.010 mm in 50 mm.
  • Surface roughness: Ra ≤ 0.05 μm; no scratches that interfere with indentations.
  • Thickness marked at calibration to prevent accidental regrind.
• Calibration machine and measurement systems
  • Direct verification of the calibration machine at intervals ≤ 12 months (force, indenter, diagonal measuring system, testing cycle).
  • Test force verification at three plunger positions, three measurements each, within ±0.5% of nominal (ISO 376 Class 0.5 or better).
  • Indenter geometry and finish tightly specified (angle β = 130° ± 0.1°, face flatness and junction tolerances down to 0.0003 mm).
  • Diagonal measurement resolution: estimation to 0.00015 mm or 0.25% (whichever greater); stage micrometer line interval uncertainty ≤ 0.0002 mm or 0.04%.
• Calibration procedure and environment
  • Temperature (23 ± 5) °C, thermal drift ≤ 1 °C.
  • Approach velocity 0.015–0.07 mm/s; time to full force ≈ 7 s; dwell time ≈ 14 s.
  • Maximum vibrational acceleration < 0.005 g.
• Sampling and uniformity
  • Minimum of 5 indentations per block (recommended 10, 15 or 25 to reduce uncertainty).
  • Uniformity assessed by relative non‑uniformity r (%) and compared with maximum permissible values specified by block hardness.
• Marking and certification
  • Blocks must be marked with arithmetic mean hardness (e.g., “249 HK 1”), supplier, serial number and accompanied by a calibration certificate.

Applications and who uses it

• Calibration laboratories and national metrology institutes performing Knoop reference block calibration.
• Manufacturers of hardness reference blocks and hardness testing machine suppliers.
• Quality/inspection engineers, metallurgists and production labs that rely on validated Knoop hardness measurements for material characterization, incoming inspection and process control.
• Accreditation bodies and labs seeking traceable hardness verification and compliance with ISO 4545 series.

Related standards

• ISO 4545-1: Knoop hardness test - Test method
• ISO 4545-2: Knoop hardness test - Verification and calibration of testing machines
• ISO 376: Calibration of force-proving instruments

Keywords: ISO 4545-3:2017, Knoop hardness, calibration of reference blocks, hardness testing machines, Knoop test calibration, metallic materials, indenter, diagonal measurement, traceability.