1. There is a certain relationship between the yield strength of yield strength materials and the fatigue limit. Generally speaking, the higher the yield strength of materials, the higher the fatigue strength. Therefore, in order to improve the fatigue strength of springs, it is necessary to try to improve the yield strength of spring materials, or use materials with a high ratio of yield strength and tensile strength. For the same material, fine grain texture has higher yield strength than coarse grain texture.

2. The maximum stress of the surface state mostly occurs in the surface layer of the spring material, so the surface quality of the spring has a great influence on the fatigue strength. Spring material in rolling, drawing and rolling process caused by cracks, defects and scars and other defects are often the cause of spring fatigue fracture.

The smaller the surface roughness is, the smaller the stress concentration is, and the higher the fatigue strength is. Effect of material surface roughness on fatigue limit. As the surface roughness increases, the fatigue limit decreases. In the case of the same roughness, different types of steel and different rolling methods have different degrees of fatigue limit reduction. For example, the reduction degree of cold rolling spring is smaller than that of hot rolling spring. Because the steel hot coil spring and its heat treatment heating, because the oxidation makes the surface of the spring material become rough and produce decarbonization phenomenon, so as to reduce the fatigue strength of the spring.

The surface of the material is ground, pressed, shot blasting and rolled. Can improve the fatigue strength of the spring.

3. The larger the size of the size effect material, the higher the possibility of defects caused by various cold processing and hot processing processes, and the greater the possibility of surface defects, which will lead to the decline in fatigue performance. Therefore, the influence of size effect should be considered when calculating the fatigue strength of spring.

4. Metallurgical defects metallurgical defects refer to nonmetallic inclusions, air bubbles, segregation of elements, and so on in materials. The inclusion on the surface is the stress concentration source, which will lead to premature fatigue crack between the inclusion and the matrix interface. The quality of steel can be greatly improved by means of measures such as vacuum smelting and vacuum pouring.

5. When working in the corrosive medium, the spring of corrosive medium becomes the fatigue source due to the pitting corrosion on the surface or the corrosion of the grain boundary on the surface, which will gradually expand and lead to fracture under the action of variable stress. For example, spring steel working in fresh water has a fatigue limit of only 10% to 25% in air. The influence of corrosion on the fatigue strength of the spring is not only related to the number of times the spring is subjected to variable load, but also to its working life. Therefore, when designing and calculating the spring affected by corrosion, the working life should be taken into account.

Spring working under corrosion conditions, in order to ensure its fatigue strength, can be used to resist corrosion of high performance materials, such as stainless steel, non-iron metals, or surface with a protective layer, such as coating, oxidation, spray, paint, etc. Practice shows that cadmium plating can greatly improve the fatigue limit of springs.

6. Temperature the fatigue strength of carbon steel decreases from room temperature to 120℃, increases from 120℃ to 350℃, and decreases from above 350℃. There is no fatigue limit at high temperature. Heat resistant steel should be considered for springs operating at high temperatures. The fatigue limit of steel is increased at below room temperature.