ISO 6508-3:2023

Overview

ISO 6508-3:2023 - "Metallic materials - Rockwell hardness test - Part 3: Calibration of reference blocks" specifies the method and requirements for calibrating hardness reference blocks used for indirect and daily verification of Rockwell hardness testing machines and indenters. It also sets out the requirements and verification procedures for the calibration machines and calibration indenters (diamond and ball) used to calibrate those reference blocks. The standard notes that hard metal ball indenters are considered the standard Rockwell ball type.

Key topics and requirements

• Scope & purpose: Calibration method for reference blocks used in Rockwell verification (linked to ISO 6508-2 and ISO 6508-1).
• Reference block manufacture:
  • Blocks must be specially manufactured for hardness reference use.
  • Minimum thickness 6 mm; thicker preferred to reduce hardness change with repeated indents.
  • Surface flatness deviation ≤ 0.01 mm; parallelism ≤ 0.02 mm per 50 mm.
  • Surface roughness Ra ≤ 0.3 µm (test surface) and ≤ 0.8 µm (bottom).
  • Blocks must be demagnetized if steel and marked for thickness at calibration.
• Calibration machine and indenter requirements:
  • Calibrations/verification performed at (23 ± 5) °C.
  • Instruments traceable to national standards (e.g., ISO 376 for force proving).
  • Direct verification of the calibration machine annually (≤13 months) covering test force, measuring system, testing cycle, and hysteresis.
  • Force accuracy: preliminary force within ±0.2%, total force within ±0.1%.
  • Measuring system resolution ±0.0001 mm and expanded uncertainty 0.0002 mm (95% confidence).
• Calibration diamond indenter geometry & performance:
  • Cone included angle 120° ± 0.1° (sectional tolerance ±0.17°).
  • Tip radius mean 0.200 ± 0.005 mm (sectional tolerance ±0.007 mm).
  • Generatrix straightness and sphericity tolerances specified; axis inclination ≤ 0.3°.
  • Performance verification against reference diamond indenters with specified hardness test blocks (mean difference ≤ ±0.4 Rockwell units).
• Procedures & documentation: Includes calibration procedure, number/uniformity of indentations, marking, certificates, and validity. Annexes address uniformity, uncertainty, indenter requirements, and control verification.

Applications and users

• Useful for: calibration laboratories, quality assurance/metrology teams, hardness testing service providers, manufacturer labs of hardness-reference blocks, and industries requiring traceable Rockwell testing (automotive, aerospace, metallurgy, heat-treatment).
• Practical uses: Producing accredited calibration certificates, maintaining daily verification blocks, ensuring traceability and repeatability of Rockwell hardness testing.

Related standards

• ISO 6508-1:2023 - Rockwell test method
• ISO 6508-2:2023 - Verification and calibration of testing machines and indenters
• ISO 376 - Calibration of force-proving instruments

Keywords: ISO 6508-3:2023, Rockwell hardness test, calibration of reference blocks, calibration indenter, hardness-reference blocks, Rockwell scales, calibration machine.