Measurement method of Vickers hardness of precision casting parts

Precision casting Vickers hardness test is a kind of indentation hardness test. The value of Vickers hardness has nothing to do with the size and load value of the indenter. There is no need to change the indenter according to the soft and hard of the material. The edge of the square indentation contour is clear and easy to measure. Vickers hardness is applied to all metals and is one of the most widely used hardness standards. As long as the texture of the tested material is uniform, Vickers hardness test can obtain reliable hardness value with low load and small indentation, which can reduce the degree of material damage. It can also be used for thin and small test materials.

Although Vickers hardness of precision casting can measure both soft and hard materials, it has certain requirements for samples. In order to obtain accurate Vickers hardness value, some precautions in sample preparation are introduced.

(1) The surface of precision casting Vickers hardness sample shall be smooth and flat without oxide scale, sundries and oil stain. The surface roughness parameters of the sample shall not affect the measurement accuracy of surface hardness.

(2) During the preparation of Vickers hardness samples for precision casting, the effects of overheating or cold hardening on the surface should be minimized.

The measurement engineer grinds the nitrided precision casting parts to be tested to obtain better surface roughness, and corrodes the surface with HNO3 and alcohol solution to detect whether the white bright layer is damaged. The white bright layer is 10 thick μ The compound composed of iron and nitrogen of M, HNO3 and alcohol solution can not corrode the stable compound composed of iron and nitrogen, so the white bright layer presents metallic luster, and the position without white bright layer will be corroded into black by HNO3 and alcohol solution. The roughness of the position with white bright layer after grinding is tested, and the test result RZ is 14 μ m. Considering the influence of roughness on the accuracy of measuring surface hardness, it is necessary to confirm the repeatability of roughness.

Test 1: repeatedly grind the precision casting parts before nitriding, record the roughness value after each grinding, and use hv0 5 load to confirm the hardness of white bright layer.

Test 2: repeatedly grind the precision casting parts before nitriding, record the roughness value after each grinding, and use hv0 2. Confirm the hardness of white bright layer.

The nitrided precision casting parts are polished without damaging the white bright layer, and the measured roughness Rz is 14 μ m. As shown in the figure. Therefore, the test to verify the accuracy of the measurement method also starts from RZ = 14 μ M start, see table for data record. It can be seen from the recorded data that hv0.5 is used for 5 times 2 the maximum fluctuation range of hardness results measured by load is 14 hv0 2。 Due to hv0 2 the fluctuation range of the confirmed hardness results is small, so the measurement load of the surface hardness is finally determined to be 0.2 kg. Communicate with the customer to change the drawing load requirements from 0.5 kg to 0.2 kg, and the polished precision casting parts need HNO3 and alcohol solution for corrosion to check whether the white bright layer is damaged, and the hardness test points need to be at the position with white bright layer.

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