What is the decarburization and carburizing test of the socket button head self tapping screw?

The test requirements for socket button head self tapping screw generally include chemical analysis of raw materials, tensile strength and other performance tests, hardness tests, grain size tests, brittle fracture tests, decarburization and carburizing tests.

And Shi Shi Tong mainly talks about the decarburization and carburizing test of the socket button head self tapping screw:

1) Sampling, sample: Take a longitudinal section of the sample along the center line of the socket button head self tapping screw. After the sample is taken, it is embedded in the resin powder, and the sample is fixed by heating. Then, the surface of the sample was ground and polished, and the surface of the sample was etched with a 3% nitrate solution to meet the test requirements.

2) Metallographic microscopy: A sample that meets the requirements is placed under a metallographic microscope, magnified 100 times, and observed and compared to determine the minimum height of the undecarburized layer and the maximum depth of the fully decarburized layer. The partially decarburized layer is ferrite plus tempered sorbite, and the height of the body tissue should be measured from the point where the ferrite (white) is not visible to the low distance.

3) Microhardness method: Microhardness measurement should be carried out on unetched specimens, using a diamond indenter with a Vickers hardness of HV136, or a 200 gf load of the indenter. The hardness values ​​of the test areas in these four positions are: small diameter, 0.75h from the bottom of the tooth to the crest (h – thread height), 0.003 in side of the thread, and the bottom of the tooth. According to the Vickers hardness values ​​of the above four positions, it is judged whether the thread is decarburized and decarburized.

The hardness from 0.7 to 5h from the base of the tooth should be equal to or greater than the hardness of the small diameter minus 30 Vickers hardness values. The hardness of the base should be equal to or greater than the hardness of the small diameter minus 30 Vickers hardness values. From the base of the tooth to the crest of 0.75h (h – thread height), the side of the thread is 0.003in, the hardness of the root should not be greater than the hardness of the small diameter plus 10 Vickers hardness values.

4) The above two methods for testing the decarburized layer can be used. However, in the case of controversy, the results of the microhardness test should be used as the basis for arbitration.

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