Metallographic and hardness test of self-tapping screw

(1) During routine inspection, the surface hardness can be measured at the end, shaft or head of the screw (as long as the effective hardened layer depth of the surface carburization and the geometry of the screw allow). For screws with a nominal thread diameter of not less than M4, a micro Vickers hardness tester (test force 0.3HV) should be used to determine the surface hardness. On the tooth profile of the longitudinal section sample, the distance from the edge of the sample is at least 0.05mm. Screws with a nominal thread diameter of less than M4 should be on the plane, and the head should be given priority. The core hardness should be carried out at 1/2 radius of the transverse load surface with a sufficient distance from the end of the screw (there should be a complete thread minor diameter). (Guide: Encyclopedia of fastener standards and specifications)

(2) Observe the microstructure under a metallographic microscope. There should be no band-shaped hypoeutectoid ferrite between the carburized effective hardened layer and the core. The carburizing effective hardened layer depth test should be carried out on the side of the thread, and the measuring point should be half of the distance between the top of the tooth and the bottom of the tooth. But for screws not larger than M4, the test should be performed on the bottom of the tooth. When measuring with a micro Vickers hardness tester with a test force of 3N, the effective hardened layer depth of carburizing should be calculated from the test point that exceeds the actual core hardness of 30HV0.3.

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