How much is the fatigue resistance of high-strength bolts increased after heat treatment

The fatigue strength of high-strength hardware bolts has always been a concern. Data shows that most of the failures of high-strength bolts are caused by fatigue failure, and there is almost no sign of fatigue failure of the bolts, so major accidents are likely to occur when fatigue failure occurs.

So, can heat treatment improve the performance of fastener materials? How much does it increase its fatigue strength? In view of the increasing use requirements of high-strength bolts, it is more important to improve the fatigue strength of bolt materials through heat treatment.

1. Material fatigue cracks of high-strength bolts:

The place where the fatigue crack first starts is called the fatigue source. The fatigue source is very sensitive to the microstructure of bolts, and fatigue cracks can be induced at a very small scale. Generally within 3 to 5 grain sizes, the surface quality of the bolt is the main source of fatigue, and most of the fatigue starts on the bolt surface or subsurface. A large number of dislocations, some alloying elements or impurities in the crystal of bolt material, and the difference in grain boundary strength may all lead to fatigue crack initiation. Studies have shown that fatigue cracks are prone to occur in the following locations: grain boundaries, surface inclusions or second-phase particles, and cavities. These locations are all related to the complex and changeable microstructure of the material. If the microstructure can be improved after heat treatment, the fatigue strength of the bolt material can be improved to a certain extent.

2. The influence of heat treatment on fatigue strength (Guide: What are the parameters of ball screws in general)

When analyzing the fatigue strength of bolts, it was found that improving the static load bearing capacity of bolts can be achieved by increasing the hardness, while the improvement of fatigue strength cannot be achieved by increasing the hardness. Because the notched stress of the bolt will cause a larger stress concentration, increasing the hardness of the sample without stress concentration can improve its fatigue strength. Hardness is an index to measure the degree of hardness of a metal material. It is the ability of a material to resist the intrusion of objects harder than it. The level of hardness also reflects the strength and plasticity of the metal material. The stress concentration on the surface of the bolt will reduce its surface strength. When subjected to alternating dynamic loads, the process of micro-deformation and recovery will continue to occur at the stress concentration part of the notch, and the stress it receives is much greater than the part without stress concentration, which is easy to Lead to the generation of fatigue cracks.

3. The influence of decarburization on fatigue strength

Decarburization of the bolt surface will reduce the surface hardness and wear resistance of the bolt after quenching, and significantly reduce the fatigue strength of the bolt. There is a decarburization test for bolt performance in the GB/T3098.1 standard, and the maximum decarburization depth is specified. A large amount of literature shows that due to improper heat treatment, the surface of the bolt is decarburized and the surface quality is reduced, thereby reducing its fatigue strength. When analyzing the reasons for the failure of the high-strength bolts of the 42CrMoA wind turbine, it was found that there was a decarburized layer at the junction of the head and rod. Fe3C can react with O2, H2O, and H2 at high temperature to reduce the Fe3C inside the bolt material, thereby increasing the ferrite phase of the bolt material, reducing the strength of the bolt material, and easily causing micro-cracks. In the heat treatment process, the heating temperature must be controlled well, and at the same time, the controlled atmosphere protection heating must be used to solve this problem.

Fasteners improve the microstructure through heat treatment and tempering, and have excellent comprehensive mechanical properties, which can improve the fatigue strength of the bolt material, reasonably control the grain size to ensure low-temperature impact energy, and also obtain higher impact toughness . Reasonable heat treatment to refine the grains and shorten the distance between the grain boundaries can prevent the occurrence of fatigue cracks. If there is a certain amount of whiskers or second particles inside the material, these added phases can prevent the resident slip to a certain extent. The slip of the belt prevents the initiation and expansion of microcracks.

Heat treatment has a great influence on the fatigue strength of bolt materials. During the heat treatment process, the heat treatment process should be determined according to the bolt properties. The initial fatigue cracks are caused by the stress concentration caused by the microstructure defects of the bolt material. Heat treatment is a method to optimize the organization of fasteners, which can improve the fatigue performance of bolt materials to a certain extent and increase the life of the product. In the long run, it can save resources and conform to the strategy of sustainable development.

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