Several factors affecting the fatigue strength of spring
1. There is a certain relationship between the yield strength and the fatigue limit of the yield strength material. Generally, the higher the yield strength of the material, the higher the fatigue strength. Therefore, in order to improve the fatigue strength of the spring, try to increase the yield strength of the spring material. Or a material with a high ratio of yield strength to tensile strength. For the same material, the fine grain structure has a higher yield strength than the coarse grain structure.
2. The surface state stress often occurs on the surface layer of the spring material, so the surface quality of the spring has a great influence on the fatigue strength. Defects such as cracks, flaws and scratches caused by the spring material during rolling, drawing and rolling are often the cause of spring fatigue fracture.
The smaller the surface roughness of the material, the smaller the stress concentration and the higher the fatigue strength. The effect of surface roughness of the material on the fatigue limit. As the surface roughness increases, the fatigue limit decreases. In the case of the same roughness, different steel grades and different rolling methods have different degrees of reduction in fatigue limit, such as the reduction of the cold coil spring is smaller than that of the hot coil spring. Because of the steel hot coil spring and its heat treatment heating, the surface of the spring material is roughened and decarburized due to oxidation, which reduces the fatigue strength of the spring.
Grinding, pressing, blasting and rolling the surface of the material. Both can increase the fatigue strength of the spring.
3. The larger the size of the size effect material, the higher the possibility of defects due to various cold working and hot working processes, and the greater the possibility of surface defects, which may lead to a decrease in fatigue performance. Therefore, the effect of the size effect should be considered when calculating the fatigue strength of the spring.
4. Metallurgical Defects Metallurgical defects refer to non-metallic inclusions, bubbles, segregation of elements, etc. in materials. The inclusions present on the surface are sources of stress concentration, which may cause fatigue cracks to occur prematurely between the inclusions and the interface of the substrate. Vacuum smelting, vacuum casting and other measures can greatly improve the quality of steel.
5. When the corrosive medium spring works in corrosive medium, it will become a fatigue source due to pitting corrosion on the surface or corrosion of the surface grain boundary. Under the action of variable stress, it will gradually expand and cause fracture. For example, spring steel working in fresh water has a fatigue limit of only 10% to 25% in air. The effect of corrosion on the fatigue strength of the spring is not only related to the number of times the spring is subjected to the variable load, but also to the working life. Therefore, when designing a spring that is affected by corrosion, the working life should be taken into account.
In order to ensure the fatigue strength of the spring working under corrosive conditions, materials with high corrosion resistance, such as stainless steel, non-ferrous metals, or surface-protected layers such as plating, oxidation, spray coating, painting, etc., may be used. Practice has shown that cadmium plating can greatly increase the fatigue limit of the spring.
6. The fatigue strength of temperature carbon steel decreases from room temperature to 120 ° C, rises from 120 ° C to 350 ° C, and decreases after the temperature is higher than 350 ° C. There is no fatigue limit at high temperatures. For springs operating under high temperature conditions, heat resistant steel should be considered. At temperatures below room temperature, the fatigue limit of steel increases.
For specific values of these factors affecting fatigue strength, please refer to the relevant information.