Metallic materials - Vickers hardness test - Part 3: Calibration of reference blocks
1Key Takeaways
This document specifies a method for the calibration of reference blocks to be used for the indirect verification of Vickers hardness testing machines, as specified in ISO 6507-2. The method is applicable only for indentations with diagonals ≥0,020 mm.
2Expert Interpretation
This article provides an in-depth interpretation of ISO 6507-3:2018, the international standard for the calibration of Vickers hardness reference blocks for metallic materials. It covers key technical points such as test force verification, indenter geometry requirements, and diagonal measurement system calibration. It also provides manufacturing specifications and uniformity assessment methods for reference blocks, suitable for indirect verification of hardness testing machines.
Analysis of the core content of the standard
The fourth edition of ISO 6507-3:2018 replaces the third edition of 2005. The main revisions include: refinement of the test surface area requirements, update of the uncertainty requirements for stage micrometer line spacing, harmonization of measurement system calibration with ISO 6507-2, and optimization of the standard block hardness uniformity assessment method.
Standard Block Manufacturing Specifications
| Parameters | Requirements | Technical Basis |
|---|---|---|
| Thickness | ≥5mm | Ensure test stability |
| Test surface area | ≤40cm² | Restrictions added in the new version |
| Surface roughness Ra | ≤0.05μm | Avoid scratches interfering with measurement |
| Magnetic requirements | Complete demagnetization | Steel materials require special treatment |
Key Requirements for the Calibration System
Vickers Hardness Tester Calibration must meet the following requirements:
- Test force verification: ±0.2%~0.5% (three levels of accuracy)
- Diamond indenter: 136°±0.1° cone angle, inter-face deviation ≤0.0003mm
- Diagonal measurement system: Resolution up to 0.00015mm (when d≤0.06mm)
Typical Application Case: When a laboratory uses a Mitutoyo HM-200 hardness tester to calibrate an HV30 standard block, it must first verify that the test force error is ≤±0.2% using an ISO 376 Class 0.5 dynamometer.
Key points of calibration procedure
| Test force range (N) | Force application time (s) | Indenter approach speed (mm/s) |
|---|---|---|
| F＜1.961 | 6-8 | 0.05-0.2 |
| 1.961≤F＜49.03 | 6-8 | 0.05-0.2 |
| F≥49.03 | 6-8 | 0.015-0.07 |
Temperature control requirements: 23±5℃ environment, thermal drift ≤1℃/h
Hardness uniformity evaluation
Relative non-uniformity calculation formula:
rrel = 100×(Hn-H1)/H̄
Where H1~Hn are hardness values arranged in ascending order, and H̄ is the average value. For HV30 standard blocks with 5 indentations:
- ≤250HV: rrel≤8% or d1-dn≤0.001mm
- ＞250HV: rrel≤4%
Standard Evolution and Technical Background
This edition of the standard strengthens the vibration control requirement for microhardness testing (F < 1.961N): acceleration ≤ 0.005gn. This is due to the higher demand for micro-force testing stability driven by the development of nanoindentation technology.
The validity period of standard blocks is 5 years for steel and 2-3 years for aluminum/copper alloys, reflecting the differences in material aging properties.
Implementation Suggestions
1. The Calibration laboratory should establish:
- Annual direct verification system (test force, indenter, measurement system)
- Five-interval verification method using stage micrometer
2. Production site recommendations:
- Standard block thickness marking accuracy 0.01mm
- Test surface anti-repair marking management