1. Metallographic and hardness test
(1) During routine inspection, the surface hardness may be measured at the end, shank or head of the screw (as long as the depth of the effective hardened layer of case carburization and the geometry of the screw allow). For screws with a nominal thread diameter of not less than M4, the surface hardness should be measured using a micro-Vickers hardness tester (test force 0.3HV). On the tooth profile of the longitudinal section specimen, the distance from the edge of the specimen is at least 0.05mm. For screws with a nominal thread diameter smaller than M4, it should be on the plane, and preferably on the head. The core hardness should be carried out on the 1/2 radius of the transverse plane with sufficient distance from the end of the screw (should have a complete thread minor diameter).
(2) Observe the microstructure under a metallographic microscope, and there should be no banded hypoeutectoid ferrite between the effective carburizing hardened layer and the core. The depth test of the effective hardened layer of carburizing shall be carried out on the flank of the thread, and the measuring point shall be half of the distance between the tooth crest and the tooth bottom. However, for screws not larger than M4, the test should be carried out on the bottom of the tooth. When measured with a micro-Vickers hardness tester with a test force of 3N, the depth of the effective hardened layer of carburization shall be calculated from the test point exceeding the actual core hardness by 30HV0..3.
2. Mechanical properties test
(1) The screw-in performance test is to screw the screw sample into the test plate until one complete thread completely passes the test without breaking.
(2) The destructive torque test is to clamp the shank of the screw specimen in a screw mold or other device that matches the screw thread, and use a calibrated torque-measuring device to torque the screw until it breaks. , the fracture should not occur in the clamped threaded part.
(3) Carry out a tensile test on the screw sample to check the minimum tensile load for failure. The fracture should be within the length of the rod or the unthreaded thread, and should not occur at the junction of the nail head and the rod. Before the sample breaks, it should be It can reach the minimum tensile load specified by the corresponding performance class.
(4) Hydrogen embrittlement is a problem that must be strictly paid attention to in the surface treatment process of self-tapping locking screws. In the pickling process, the screw is stirred in dilute hydrochloric acid, and the amount of hydrogen absorbed by the pickling steel increases linearly with the square root of time and reaches the saturation value. Less than 100%, a large number of hydrogen atoms will be produced, which will be attached to the surface of the screw, resulting in hydrogen infiltration, and the steel will become brittle due to the absorption of hydrogen.
The self-tapping locking screw has a hydrogen driving time of 6~8h, and the temperature is 160~200℃ (phosphating) and 200~240℃ (electroplating). However, in the production process, the hydrogen drive time should be determined according to many production conditions such as core hardness, surface roughness, electroplating time, coating thickness, pickling time, and acid concentration. It is best to do it before passivation and just after electroplating.
The hydrogen embrittlement resistance test should be strictly specified, and once hydrogen embrittlement occurs, the hydrogen flooding process must be improved.
