Germination of fatigue crack of bolt:
The first place where the fatigue crack begins is conveniently called the fatigue source, and the fatigue source is very sensitive to the bolt microstructure and can initiate fatigue cracks at a very small scale. Generally speaking, within three to five grain sizes, the bolt surface quality problem is the main fatigue source and most fatigue begins at the bolt surface or subsurface.
However, there are a large number of dislocations and some alloying elements or impurities in the crystal of bolt material, and the grain boundary strength is very different, and these factors may lead to fatigue crack initiation. The results show that fatigue cracks are prone to occur at grain boundaries, surface inclusions or second phase particles and voids, which are all related to the complexity and changeability of materials. If the microstructure of bolts can be improved after heat treatment, its fatigue strength can be increased to some extent.
Effects of decarbonization on fatigue:
The decarburization of bolt surface can reduce the surface hardness and wear resistance of bolt after quenching, and can effectively reduce the fatigue strength of bolt. GB/T3098.1 standard for bolt performance of decarbonization test. A large number of documents show that improper heat treatment can reduce the fatigue strength of bolts by decarburizing the surface and reducing the surface quality. When analyzing the failure cause of high strength bolt fracture, it is found that the decarbonization layer exists at the junction of head rod. However, Fe3C can react with O2, H2O and H2 at high temperature, resulting in the reduction of Fe3C inside the bolt material, thus increasing the ferritic phase of the bolt material and reducing the strength of the bolt material.
Post time: Dec-26-2022
