1. Sample Surface
The surface of the sample must be smooth, flat, and free from dirt, scale, or any contaminants. A rough surface can lead to lower test values and increased variability in results. According to national standards, the surface roughness (Ra) should not exceed 0.8 μm. This ensures accurate and reliable measurements during hardness testing.
2. Sample Thickness
The thickness of the specimen is crucial for accurate hardness testing. If the sample is too thin, strain hardening may occur at the indentation’s bottom, affecting the anvil and leading to inaccurate readings. National standards specify that for diamond indenters, the specimen thickness should be at least 10 times the depth of the residual indentation. For more precise results, it should be 15 times the indentation depth. After testing, there should be no visible deformation on the back of the sample. The U.S. standard also provides a relationship between minimum thickness and Rockwell hardness levels.
3. Curved Specimens
Hardness measured on curved surfaces tends to be lower than on flat surfaces. To account for this, a correction value must be added. Appendix F provides correction values based on the diameter and hardness of cylindrical specimens. These adjustments help ensure that the measured values are as accurate as possible when testing on curved surfaces.
4. Sample Placement and Anvil Selection
Proper placement of the sample and selection of the anvil are essential for accurate results. The test surface must be perpendicular to the indenter's axis. Care must be taken to prevent displacement, partial elastic deformation, or incorrect force application during the test process.
4.1 Flat Specimens
For flat samples, the bearing surface must be parallel to the test surface. A non-parallelism of 1° can affect the results, especially on the C scale. If the specimen is warped, such as a strip, the convex side should face the anvil to avoid floating and elastic deformation under load, which could cause measurement errors.
4.2 Cylindrical Specimens
A V-shaped anvil is recommended for cylindrical samples. When testing a convex cylindrical surface, a convex anvil should be used. For concave cylindrical surfaces, a concave anvil is required. This ensures proper contact and accurate measurement.
4.3 Other Curved Specimens
For other curved specimens, the cross-section at the test point should be perpendicular to the indenter's axis. This helps maintain consistent pressure and reduces the risk of measurement errors due to improper alignment.
4.4 Pipe Specimens
V-shaped anvils are suitable for thick-walled pipes. However, thin-walled pipes may experience elastic deformation, leading to inaccurate indentation depth measurements. In such cases, small-diameter pipes (with an inner diameter larger than 4.8 mm) should use the PHR-1ST instrument, while large-diameter pipes require a special pipe anvil. The test result is based on the outer diameter and includes a correction value (see Appendix G). Some instruments have a special feature to test the inner surface of pipes, allowing the head to be inserted into the pipe's hole for accurate readings, especially for pipes with an inner diameter over 30 mm.
4.5 Sheet Materials
Thin sheet materials, such as strips, may not meet the minimum thickness requirements. Even with a surface Rockwell test, visible deformation marks may appear on the support surface. This highlights the importance of selecting appropriate samples for testing.
5. Hardness Block Failure
Only the front side of the hardness block should be used. It cannot be used indefinitely, and a new block must be replaced when it becomes "filled up." If the block is rusted, it will compromise test accuracy, and a new one should be used immediately.
6. Avoiding Afterburner
The test should be applied slowly and evenly. The force indicator must align precisely with the corresponding tick mark. If the mark is missed or exceeded, the accuracy of the test will be affected.
7. Reading Accuracy
The operator's head position should remain consistent during the test. The reading position should match the one used during drum adjustment to ensure consistency and reduce human error.
8. Indenter Damage
Over time, indenters may become damaged due to prolonged use, impacts, or material hardening. If the tester is used frequently or if test accuracy decreases, the indenter should be inspected using an 8–10x magnifying lens. Any cracks, streaks, or defects in the diamond indenter should prompt immediate replacement. Similarly, if the ball indenter becomes deformed, it should be replaced to maintain testing accuracy.
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