1. There is a certain relationship between the yield strength and fatigue limit of a material. Generally speaking, the higher the yield strength of a material, the higher its fatigue strength. Therefore, in order to improve the fatigue strength of a spring, measures should be taken to increase the yield strength of the spring material, or use materials with a high ratio of yield strength to tensile strength. For the same material, fine grain structure has higher yield strength than coarse grain structure.

2. The maximum stress in the surface state often occurs in the surface layer of the spring material, so the surface quality of the spring has a significant impact on its fatigue strength. Defects such as cracks, flaws, and scars caused by rolling, drawing, and coiling processes in the spring material are often the reasons for fatigue fracture of the spring.
The lower the surface roughness of a material, the less stress concentration there is, and the higher the fatigue strength. The surface roughness of a material affects its fatigue limit. As the surface roughness increases, the fatigue limit decreases. Under the same roughness condition, different steel grades and different rolling methods result in varying degrees of fatigue limit reduction. For example, the reduction in fatigue limit for cold-rolled springs is less than that for hot-rolled springs. This is because when steel hot-rolled springs are heated during heat treatment, oxidation causes the surface of the spring material to become rough and produces decarburization, thereby reducing the fatigue strength of the spring.
Performing processes such as grinding, high-pressure pressing, shot blasting, and rolling on the surface of the material can all enhance the fatigue strength of the spring.

3. Size effect: The larger the size of the material, the higher the likelihood of defects caused by various cold and hot working processes, and the greater the possibility of surface defects. These factors all contribute to a decrease in fatigue performance. Therefore, the influence of size effect should be considered when calculating the fatigue strength of springs.

4. Metallurgical defects refer to non-metallic inclusions, bubbles, elemental segregation, etc. in materials. Inclusions present on the surface are sources of stress concentration, which can lead to premature fatigue cracks between the inclusion and matrix interfaces. Measures such as vacuum smelting and vacuum casting can greatly improve the quality of steel.

5. Corrosive Medium: When a spring operates in a corrosive medium, pitting corrosion occurs on its surface or the surface grain boundaries are corroded, becoming fatigue sources. Under variable stress, these sources gradually expand and lead to fracture. For example, the fatigue limit of spring steel operating in fresh water is only 10% to 25% of that in air. The impact of corrosion on the fatigue strength of a spring is not only related to the number of variable load cycles the spring experiences, but also to its working life. Therefore, when designing and calculating springs affected by corrosion, the working life should be taken into account.
For springs operating under corrosive conditions, to ensure their fatigue strength, materials with high corrosion resistance, such as stainless steel and non-ferrous metals, can be used, or protective layers can be applied to the surface, such as plating, oxidation, powder coating, painting, etc. Practice has shown that cadmium plating can greatly enhance the fatigue limit of springs.

6. The fatigue strength of carbon steel decreases from room temperature to 120℃, then increases from 120℃ to 350℃, and decreases again when the temperature exceeds 350℃. 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.