Because springs fatigue after a long time of use, meaning that common deformations can cause you to lose your original elastic properties. Spring fatigue refers to the frequent deformation of the metal, the crack will increase, the crack gap will increase, and eventually lead to fatigue. Spring makes our daily life is becoming more and more common, it's job is mainly manifested in the electronic, corrosive medium such as air, steam, water, and acid, alkali, salt and other chemical corrosion medium of the corrosion of the stainless steel spring, depending on the spring material mechanical properties can be divided into various types of spring, and used in our daily life, there are mainly six of the cause of spring fatigue.

1. There is a certain relationship between yield strength and yield strength. The higher the yield strength of the material, the higher the fatigue strength. The yield strength of fine grain structure is higher than that of coarse grain structure for the same material.

2, the maximum stress of the surface state occurs in the surface of the spring material, so the surface quality of the spring has a great impact on the fatigue strength. Cracks, defects and scars caused by spring materials in the rolling, drawing and winding process are often the causes of spring fatigue fracture.

The smaller the surface roughness is, the smaller the stress concentration is and the higher the fatigue strength is. Influence of material surface roughness on fatigue limit. With the increase of surface roughness, the fatigue limit decreases. Under the condition of the same roughness, different kinds of steel and different winding methods have different reduction degrees of fatigue limit. For example, the reduction degree of cold coil spring is smaller than that of hot coil spring. Because the steel hot coil spring and its heat treatment heating, due to the oxidation of the spring material surface rough and decarbonization phenomenon, so as to reduce the fatigue strength of the spring.

3. The larger the material size is, the greater the possibility of defects arising from various cold and hot processing processes and the greater the possibility of surface defects, resulting in the decline of fatigue performance. Therefore, the influence of size effect should be considered when calculating the fatigue strength of spring.

4. Metallurgical defects refer to segregation of non-metallic inclusions, bubbles, elements, etc. The inclusions existing on the surface are the source of stress concentration, which will lead to fatigue cracks on the inclusions and the interface between matrix and matrix. The quality of steel can be greatly improved through vacuum melting, vacuum pouring and other measures.

5. When the spring of corrosive medium works in corrosive medium, it becomes the fatigue source due to surface pitting or surface grain boundary corrosion, which gradually expands and leads to fracture under the action of variable stress. For example, the fatigue limit of spring steel working in fresh water is only 10% or less than 25% of the fatigue limit in air. The influence of corrosion on the fatigue strength of spring is not only related to The Times of spring under variable load, but also to the service life of spring. Therefore, in the design and calculation of springs affected by corrosion, the service life of the spring should be taken into account.

In order to ensure the fatigue strength of the spring under corrosion conditions, corrosion resistant materials such as stainless steel, non-ferrous metal or surface protective layer can be used, such as coating, oxidation layer, plastic spraying layer, paint layer, etc. The practice shows that cadmium plating can greatly improve the fatigue limit of spring.

6 temperature the fatigue strength of carbon steel decreases from room temperature to 120℃, from 120℃ to 350℃, and decreases after the temperature is higher than 350℃. There is no fatigue limit at high temperature. The spring working at high temperature should consider heat-resistant steel.