1. There is a certain relationship between the yield strength and fatigue limit of yield strength materials. Generally speaking, the higher the yield strength of materials, the higher the fatigue strength. Therefore, in order to advance the fatigue strength of springs, try to advance the yield strength of spring materials, or use materials with high ratio of yield strength to tensile strength. For the same material, the fine grain arrangement has higher yield strength than the coarse grain arrangement.
2. The large surface stress usually occurs on the surface of the spring material, so the surface quality of the spring has a great impact on the fatigue strength. The cracks, defects, scars and other defects formed in the process of rolling, drawing and coiling of spring materials are often the cause of spring fatigue cracking. 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. With the increase of surface roughness, fatigue limit decreases. In the case of the same roughness, different steel grades and different coiling methods have different degrees of fatigue limit decline, for example, the decline of cold coil spring is smaller than that of hot coil spring. As the steel hot coil spring and its heat treatment heating, due to oxidation, the spring material surface becomes rough and decarburization occurs, which reduces the fatigue strength of the spring. The material surface shall be ground, pressed, shot blasted and rolled. Can advance the fatigue strength of the spring.
3. The larger the standard of standard effect materials, the higher the possibility of defects formed by various cold working and hot working processes, and the greater the possibility of surface defects, which will lead to the decline of fatigue function. Therefore, the standard effect should be considered when calculating the fatigue strength of the spring.
4. Metallurgical defects Metallurgical defects refer to non-metallic inclusions, bubbles, element segregation, etc. in materials. The inclusion existing on the surface is the source of stress concentration, which will lead to fatigue cracks between the inclusion and the matrix interface prematurely. The quality of steel can be greatly improved by means of vacuum smelting and vacuum pouring.
5. When the corrosive medium spring works in the corrosive medium, it becomes a source of fatigue due to pitting corrosion on the surface or corrosion of the surface grain boundary, and will gradually expand under the effect of variable stress, leading to cracking. For example, the fatigue limit of spring steel working in fresh water is only 10%~25% of that in air. The influence of corrosion on the fatigue strength of spring is not only related to the number of times the spring is subjected to variable load, but also related to the operating life. Therefore, the service life should be taken into account when planning and accounting the spring affected by corrosion. In order to ensure the fatigue strength of springs operating under corrosive conditions, materials with high corrosion resistance function can be used, such as stainless steel, non-ferrous metals, or protective coatings can be added on the surface, such as plating, oxidation, plastic spraying, painting, etc. Practice shows that cadmium plating can greatly advance the fatigue limit of the spring.
6. The fatigue strength of temperature carbon steel decreases from room temperature to 120 ℃, increases again from 120 ℃ to 350 ℃, and decreases again when the temperature is higher than 350 ℃. There is no fatigue limit at high temperature. Heat resistant steel shall be considered for springs operating under high temperature. Under the condition lower than room temperature, the fatigue limit of steel is increased.