How Much Is The Fatigue Resistance Of High-strength Bolts Improved After Heat Treatment?

           The fatigue strength of high-strength bolts has always been an important issue. Data show that most of the failure of high-strength bolts is caused by fatigue damage, and there are almost no signs of bolt fatigue damage, so major accidents are likely to occur when fatigue damage occurs.

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

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   1. Material fatigue cracks of high-strength bolts:

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

   2. Effect of heat treatment on fatigue strength

  When analyzing the fatigue strength of bolts, it is found that improving the static load bearing capacity of bolts can be achieved by increasing the hardness, but the improvement of fatigue strength cannot be achieved by increasing the hardness. Because the notch stress of the bolt will cause a large stress concentration, increasing the hardness of the sample without stress concentration can improve its fatigue strength. Hardness is an index to measure the softness and hardness of metal materials, and it is the ability of materials to resist the intrusion of objects harder than it. The level of hardness also reflects the strength and plasticity of metal materials. 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 that of the part without stress concentration, so it is easy to lead to the formation of fatigue cracks.

  3. Effect 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. In the GB/T3098.1 standard, there is a decarburization test for bolt performance, and the maximum decarburization depth is specified. A large number of literatures show that due to improper heat treatment, the bolt surface is decarburized and the surface quality is reduced, thereby reducing its fatigue strength. When analyzing the cause of fracture failure of high-strength bolts in 42CrMoA wind turbines, it was found that there was a decarburization layer at the junction of the head and rod. Fe3C can react with O2, H2O, and H2 at high temperature, resulting in the reduction of Fe3C inside the bolt material, thereby increasing the ferrite phase of the bolt material, reducing the strength of the bolt material, and easily causing microcracks. Controlling the heating temperature in the heat treatment process and using a controlled atmosphere to protect the heating can solve this problem well.

  Fasteners improve the microstructure through heat treatment and tempering, and have excellent comprehensive mechanical properties, which can improve the fatigue strength of bolt materials, reasonably control the grain size to ensure low-temperature impact energy, and can also obtain high impact toughness. Reasonable heat treatment refines the grains and shortens the grain boundary distance to 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 slippage of the belt prevents the initiation and propagation 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 performance of the bolt. The generation of initial fatigue cracks is caused by the stress concentration caused by the defects in the microstructure of the bolt material. Heat treatment is a method to optimize the structure of fasteners, which can improve the fatigue performance of bolt materials to a certain extent and improve the life of products. In the long run, it can save resources and conform to the strategy of sustainable development.