There are many methods for hardness testing, and these methods vary in principle. Even the same hardness testing method will have different testing forces, indenters, and scales depending on the material of the workpiece. Choosing an appropriate hardness testing method and conditions based on the material, processing technology, shape, size, and testing requirements of the workpiece is something that needs to be carefully considered when performing hardness testing.
Generally, the correct selection of a hardness testing method should follow these principles:
Estimate the hardness value of the workpiece to be tested. Before testing the hardness, you should make a rough estimate of the hardness value of the workpiece. If the approximate hardness value cannot be estimated, choose based on a higher hardness. (Note: You can refer to the reference table of hardness ranges for commonly used metal materials.)
After selecting a testing method, if the hardness range, thickness, size, etc. of the tested workpiece allow, choose a larger testing force. This helps to reduce the relative error of the testing results.
Based on the thickness of the sample and the processing technology, such as for thin samples or samples with a coating, or for samples with different depths of hardened layers after strengthening treatment, the corresponding testing method and force should be chosen based on the thickness of the sample, the coating or hardened layer depth (thickness), and material hardness.
Generally, for thin samples and samples with coating or hardened layers, small load Vickers or superficial Rockwell and Knoop testing methods are often used.
In experimental and material research work, if the test data is to be compared with hardness values in literature, the same testing method as the reference material should be used as much as possible to avoid errors introduced by conversions.
For metal materials with HBS < 450, including annealed, normalized, tempered steels, various cast irons, and many non-ferrous metals and their alloys (including aluminum, copper, magnesium, lead, zinc, tin, etc.), the Brinell hardness testing method is more reasonable. The Brinell method has various testing forces and indenters to choose from, it produces large indentation diameters, and it can measure the comprehensive properties of the metal's components over a large area, minimizing the influence of individual phases or local structures, thus providing accurate, stable, and reproducible data.
The hardness values of metal materials measured by the Rockwell hardness testing method are comparatively less accurate than those measured by Brinell and Vickers methods. This is because the Rockwell method measures the depth of the indentation indirectly reflecting the hardness value, with a small testing point, and each Rockwell unit is only 0.002 mm deep, which can easily result in errors. However, the Rockwell method is convenient and quick, particularly suitable for heat treatment processes and final product testing of steel materials.
For large castings and parts already assembled into machines, methods like Shore, Leeb, and hammer impact Brinell hardness testing methods are often used. The magnetic series Brinell Rockwell hardness testers produced by Shenyang Tianxing are also well-applied for hardness testing of large castings. This series of instruments are portable hardness testers with the same testing principle as benchtop machines, providing high reliability in hardness values, and are currently the preferred instruments for testing large workpieces in the boiler and power industries.
In some special cases, scratch, file, Knoop, and micro-hardness testing methods can be respectively selected.
In the same system experimental and research work, using the same material but different processing technologies may result in significant hardness differences. In this case, it's generally not advisable to switch testing methods. To obtain comparable results on a unified scale without conversion, the Vickers hardness testing method can be chosen, as it can measure from very low to very high hardness values, thus providing ideal results for comparison.
Based on the size, quantity, testing accuracy requirements, and heat treatment processes of the parts, the appropriate hardness testing method should be correctly selected.
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