​1. 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, try to increase the yield strength of the spring material, 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 and fine grain structure.

2. Surface condition

The maximum stress mostly occurs on the surface 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 scars caused by spring materials during rolling, drawing and coiling are often the causes of spring fatigue fracture.

The smaller the surface roughness of the material, the smaller the stress concentration and the higher the fatigue strength. The influence of material surface roughness on fatigue limit. As the surface roughness increases, the fatigue limit decreases. In the case of the same roughness, different steel grades and different coiling methods have different degrees of fatigue limit reduction. For example, the degree of reduction of cold coil springs is smaller than that of hot coil springs. Because the steel hot coil spring and its heat treatment are heated, the surface of the spring material becomes rough and decarburized due to oxidation, which reduces the fatigue strength of the spring.

Grinding, pressing, shot blasting and rolling the material surface. Both can improve the fatigue strength of the spring.

3. Size effect

The larger the size of the material, the higher the possibility of defects caused by various cold working and hot working processes, and the greater the possibility of surface defects. These reasons will cause the fatigue performance to decrease. Therefore, the influence 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 in the material, and so on. The inclusions present on the surface are the source of stress concentration, which will cause premature fatigue cracks between the inclusions and the substrate interface. The use of vacuum smelting, vacuum pouring and other measures can greatly improve the quality of steel.

5. Corrosive medium

When the spring works in a corrosive medium, it becomes a source of fatigue due to pitting 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 has a fatigue limit of only 10% to 25% of that 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 variable loads, but also to the working life. Therefore, when designing and calculating a spring affected by corrosion, the working life should be taken into consideration.

Several factors affecting spring fatigue strength
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. Practice shows that cadmium plating can greatly increase the fatigue limit of the spring.

6. Temperature

The fatigue strength of 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. For springs that work under high temperature conditions, heat-resistant steel should be considered. Below room temperature, the fatigue limit of steel increases.