Effect of heat treatment on increase fatigue strength of bolt

The fatigue strength of the bolt is always paid attention to the problem, data showed that the failure of the bolts is mostly due to damage caused by fatigue, and the fatigue damage of bolts almost no signs of major accidents, so easily lead to fatigue failure occurred in the. Heat treatment can optimize the performance of the fastener material and improve its fatigue strength. It is more important to improve the fatigue strength of the bolt material by heat treatment.

Keywords: fatigue crack initiation

The fatigue crack of the first place known as the source of fatigue, the fatigue source is sensitive to bolt microstructure, fatigue crack initiation can in a very small scale, generally 3 to 5 grain size, the problems of bolt surface quality is the main source of fatigue, most of the surface or sub surface fatigue in bolt. There are a large number of dislocations in the crystal and some alloy elements or impurities, grain boundary strength differences, these factors may lead to fatigue crack initiation. The results show that the fatigue cracks are prone to occur at the same time, such as grain boundary, surface inclusion or second phase particles and voids. If the microstructure can be improved after heat treatment, the fatigue strength of the bolt can be improved to some extent.

Keywords: effect of decarburization on fatigue strength

The surface decarburization bolt will reduce the surface hardness and wear resistance of the bolt after quenching, and significantly reduce the fatigue strength of bolt. The GB/T3098.1 standard is to test the performance of decarburization bolt, and the maximum depth of decarburized layer. A large number of literature shows that due to improper heat treatment, the surface of the bolt and the decline in the quality of surface decarburization, so the fatigue strength reduction. In the analysis of the failure cause of the high strength bolts of 42CrMoA wind turbine, found in the head at the junction is because of the existence of decarburized layer. Fe3C can react with O2, H2O and H2 at high temperature, which leads to the decrease of Fe3C in the bolt material, which increases the ferrite phase of the bolt material, reduces the strength of the bolt material, and easily leads to the micro crack. In the process of heat treatment, it is necessary to control the heating temperature, and it is necessary to solve the problem.

Key words: heat treatment effect on fatigue strength

In the analysis of the fatigue strength of the bolt, it is found that the static load bearing capacity of the bolt can be improved by increasing the hardness. Because of the stress of the bolt, the stress concentration will be caused, and the hardness of the sample without stress concentration can improve the fatigue strength.

Hardness is a measure of the degree of hardness of the metal material, the material is more difficult to resist the pressure of the object, the hardness also reflects the strength of the metal material, the size of the plastic. The surface of the bolt stress concentration will reduce its surface strength, the dynamic load by alternating, at the notch stress concentration occurred continuously process micro deformation and recovery, and it was affected by the stress is far greater than the no stress concentration positions, thus easily lead to fatigue crack.

Fasteners by quenching and tempering heat treatment to improve the microstructure, and has excellent mechanical properties, can improve the fatigue strength of bolt material, reasonable control of grain size to ensure the low temperature impact toughness, high impact toughness can be obtained. Grain refinement and reasonable heat treatment, shorten the distance of grain boundary can prevent fatigue cracks inside the material, if there is a certain amount of whisker or second particles, the join phase can slip persistent slip band stop in a certain extent, so as to prevent the initiation and propagation of micro cracks.

Keywords: conclusion

Adorable was born in material fatigue crack always the weakest link, because bolt surface or subsurface defects are prone to crack, the material is easy to produce persistent slip bands, grain boundaries, surface inclusion or second phase particles and empty, because these locations easily lead to stress concentration.

The heat treatment has great influence on the fatigue strength of the bolt material. In the process of heat treatment, the heat treatment process should be determined according to the bolt's performance. The initial fatigue crack is caused by the stress concentration caused by the microstructure defects. Heat treatment is a method to optimize the structure of the fastener, which can improve the fatigue performance of the bolt material and improve the life of the product to a certain extent. In the long run, it can save resources and meet the sustainable development strategy

Engaged in fasteners for many years, do you know the history of the bolt?