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Factors Affecting The Accuracy Of The Hardness Tester
Sep 13, 2018

Factors affecting the accuracy of the hardness tester

Let's talk about the factors that affect the accuracy of the hardness tester.

First, the degree of roughness of the surface to be tested directly affects the accuracy of the hardness measurement (the effect of the Rockwell hardness tester is relatively small).

When measuring the hardness value of the workpiece with a cloth hardness tester, the pressure head of the hardness tester is a steel ball indenter, which is pressed into the surface to be tested under a certain pressure to obtain a circular indentation, and then the circular indentation is measured by a reading microscope. The diameter, then find the corresponding hardness value in the Brinell hardness table, that is, the hardness value of the sample to be tested. As the roughness value increases, the resistance of the measured surface to the indenter is smaller, the plastic deformation is larger, the larger the circular indentation is, and the corresponding hardness value is smaller, resulting in the measured value being lower than the actual value. Therefore, when using the Brinell hardness tester to measure the hardness of the workpiece, try to treat the surface of the workpiece to be roughed. The roughness of the surface to be tested can be measured by the hand grinding wheel or other methods to achieve the accuracy of the hardness tester. Hardness value.

Second, the surface hardened layer of the workpiece to be tested will directly affect the hardness measurement value.

When some workpieces are processed by the finishing machine, the turning tool has a pressing (rolling) effect on the surface of the test piece at the same time, so that the metal grains on the surface layer of the finished car surface are refined and refined, and the metal grains in the deep layer of the test piece are finer. This produces a thin layer of hard layer. The thickness of the hard layer is generally around 0.3 mm. This hard layer causes the hardness measurement to be higher than the true value. The Rockwell hardness tester is a diamond cone with a small contact area (0.4 mm diameter) and a small contact area to the surface to be tested. When loading, the indenter easily penetrates the hard layer, so the measurement of hardness is less biased. Tests have shown that the measurement deviation is generally within 5HRC. The Brinell hardness tester is a steel ball indenter, and the contact area between the indenter and the surface to be tested is large. When loading, the indenter must overcome the large resistance of the pressing layer to press into the surface to be tested, which makes the pressing of the durometer indenter insufficient, and the circular indentation is also reduced, resulting in corresponding The hardness value is higher than its true value. Moreover, the measurement of hardness is highly variable. Tests have shown that the measurement deviation of hardness is around 20HB. Therefore, in order to make the measured hardness value more accurate, the surface hardened layer should be disposed of before measurement.