Hardness testers measure material hardness using methods such as Rockwell, Brinell, Vickers, or Knoop. Calibration involves verifying the applied force, indenter geometry, and measurement system using certified hardness test blocks. Hardness testing is critical for heat treatment verification and incoming material inspection.
Inspect the tester frame, anvil, and indenter for damage or contamination. Verify the test surface and anvil are clean and seated properly. Check that the indenter is the correct type for the scale being verified (diamond cone for HRC, ball for HRB, etc.).
If the procedure requires it, verify the applied test forces using a calibrated force-measuring device (elastic proving ring or load cell). Check both preliminary (minor) and total (major) forces for Rockwell, or the full force for Brinell/Vickers.
Verify the indenter geometry using optical measurement or by substituting a certified reference indenter. For Rockwell diamond indenters, the cone angle and tip radius must meet ASTM E18 requirements.
Perform five hardness impressions on each certified test block for the scales in use. Space impressions at least three diameters apart and at least 2.5 diameters from any edge. Record each reading.
Calculate the average of the five readings and compare to the certified block value. The error is the difference between the average reading and the certified value. Also assess the repeatability (range of five readings).
If errors exceed limits, adjust the tester per manufacturer instructions (force adjustment, zero offset, etc.) and retest. Record as-found and as-left results.
Record all readings, test block IDs and certified values, indenter ID, and environmental conditions. Issue the calibration certificate with pass/fail determination for each scale tested.
Per ASTM E18 (Rockwell): the average of five readings must agree with the certified block value within ±1.0 HRC (or ±2.0 HRB). The repeatability (range of five readings) must not exceed 1.0 HRC. Similar criteria apply per ASTM E10 (Brinell) and E384 (Vickers).
12 months, with daily verification using test blocks
ASTM E18 recommends daily verification at the beginning of each shift and whenever the indenter or anvil is changed. Full calibration (including force and indenter checks) is performed annually. The daily verification with certified test blocks is a check, not a full calibration.
Direct verification measures the tester's forces and indenter geometry directly. Indirect verification tests the tester's performance by making impressions on certified test blocks. Both are required for full calibration per ASTM E18.
Test blocks have a shelf life (typically 5 years from certification) and degrade over time. Expired test blocks should not be used for calibration or verification. Surface contamination, oxidation, and handling marks can also invalidate test blocks.
CalibrationOS tracks due dates, stores certificates, and generates audit-ready reports.
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