How much does the fatigue resistance of high strength bolts improve after heat treatment?
Fatigue strength of high-strength bolts has always been an issue of concern. Data show that most of the failures of high-strength bolts are caused by fatigue failure, and there are almost no signs of bolts during fatigue failure, so major accidents are easy to occur when fatigue failure occurs.
So, can heat treatment improve the performance of fastener materials? How much does it increase fatigue strength? In view of the increasingly high use requirements of high-strength bolts, it is very important to increase the fatigue strength of bolt materials through heat treatment.
Material fatigue crack of high strength bolt
1. The place where fatigue cracks begin first is called the source of fatigue. The fatigue source is very sensitive to the microstructure of the bolt, and can initiate fatigue cracks on a small scale.
2. Generally within 3 to 5 grain sizes, the bolt surface quality problem is the main source of fatigue, and most of the fatigue starts from the bolt surface or subsurface.
3. A large number of dislocations inside the crystal of the bolt material, some alloying elements or impurities, and differences in grain boundary strength. These factors may cause fatigue crack initiation.
Studies have shown that fatigue cracks are prone to occur at grain boundaries, surface inclusions or secondary phase particles, and voids. These positions are 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.
Effect of heat treatment on fatigue strength
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, and 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 increase its fatigue strength.
1. Hardness is an indicator of the softness and hardness of a metal material. It is the material's ability to resist the intrusion of harder objects than it. The hardness also reflects the strength and plasticity of the metal material.
2. The stress concentration on the bolt surface will reduce its surface strength. When subjected to an alternating dynamic load, the process of micro-deformation and recovery continues at the notched stress concentration portion, and the stress it receives is far greater than the portion without stress concentration It is easy to cause fatigue cracks.
Heat treatment
Improve fatigue strength of bolt materials
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 also obtain higher impact toughness.
Reasonable heat treatment to refine the grains and shorten the grain boundary distance can prevent the occurrence of fatigue cracks.
If a certain amount of whiskers or second particles exist in the material, these added phases can prevent the slip of the resident slip zone to a certain extent, thereby preventing the initiation and propagation of microcracks.
Heat treatment
Effect on fatigue strength of bolt material
In the heat treatment process, the heat treatment process should be determined according to the bolt performance. The initial fatigue crack is caused by the stress concentration caused by the microstructure defect 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 increase the life of products.
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.
1. GB / T3098.1 standard has decarburization test for bolt performance, and specifies the maximum depth of decarburization layer. A large amount of literature data shows that due to improper heat treatment, the surface of the bolt is decarburized and the surface quality is reduced, which reduces its fatigue strength.
2. When analyzing the cause of the fracture failure of the high-strength bolts of the 42CrMoA wind turbine, it was found that the decarburization layer was present at the junction of the head bars. Fe3C can react with O2, H2O, and H2 at high temperatures, leading to a decrease in 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.
In the heat treatment process, the heating temperature is controlled, and the controlled atmosphere protection heating must be used to solve this problem. In the long run, it can save resources and meet the sustainable development strategy.
