Factors affecting spring fatigue strength:
A. Yield strength There is a certain relationship between the yield strength of the material and the fatigue limit. Generally speaking, the higher the yield strength of the material, the higher the fatigue strength. Therefore, in order to improve the fatigue strength of the spring, we should try to increase the yield of the spring material. Strength, or use 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.
B. The larger the size of the size effect material, the higher the possibility of defects caused by various cold and hot working processes, and the greater the possibility of surface defects. These reasons will lead to a decrease in fatigue performance. Therefore, when calculating the spring The influence of size effect should be considered when fatigue strength.
C. When the corrosive medium spring works in corrosive medium, it becomes a source of fatigue due to pitting corrosion on the surface or corrosion of the surface grain boundary, and it will gradually expand under the action of variable stress and cause fracture. For example, spring steel working in fresh water, The fatigue limit is only 10% to 25% in the 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 variable loads, but also to the working life. Therefore, when designing and calculating a spring affected by corrosion, you should The working life is taken into consideration.
For springs that work under corrosive conditions, in order to ensure their fatigue strength, materials with high corrosion resistance, such as stainless steel, non-ferrous metals, or surface protection layers, such as plating, oxidation, spraying, painting, etc. can be used. Cadmium plating can greatly increase the fatigue limit of the spring.
D. Metallurgical defects Metallurgical defects refer to non-metallic inclusions, bubbles, element segregation in the material, etc. Inclusions existing on the surface are the source of stress concentration, which will cause premature fatigue between the inclusion and the substrate interface Cracks. The use of vacuum smelting, vacuum casting and other measures can greatly improve the quality of steel.
E. The maximum stress of the surface state mostly occurs on the surface of the spring material. Therefore, the surface quality of the spring has a great influence on the fatigue strength. The defects such as cracks, defects and scars caused by the spring material in the rolling, drawing and coiling process It is often the cause of the fatigue fracture of the spring.
The smaller the surface roughness of the material, the smaller the stress concentration, and the higher the fatigue strength. It is the influence of the surface roughness of the material on the fatigue limit. As can be seen from the figure, as the surface roughness increases, the fatigue limit decreases. In the same In the case of roughness, different steel grades and different coiling methods have different fatigue limit reduction degrees. For example, the reduction degree of cold coil springs is smaller than that of hot coil springs. Because steel hot coil springs and their heat treatment are heated, due to Oxidation makes the surface of the spring material rough and produces decarburization, which reduces the fatigue strength of the spring. Figure 2 shows the effect of the depth of the decarburization layer on the fatigue strength. The surface of the material is ground, pressed, shot blasted and rolled. Both can improve the fatigue strength of the spring.
F. The fatigue strength of temperature carbon steel decreases from room temperature to 120°C, rises from 120°C to 350°C, and drops again after the temperature is higher than 350°C. There is no fatigue limit at high temperatures. Springs that work under high temperature conditions, Consider the use of heat-resistant steel. Under conditions below room temperature, the fatigue limit of steel increases.