Several factors affecting the spring fatigue strength
1. The yield strength of material has a certain relationship between the yield strength and fatigue limit, in general, the higher the yield strength of the material, the higher fatigue strength is, therefore, we should try to improve in order to improve the fatigue strength of spring spring the yield strength of material, or use the high yield strength and tensile strength ratio of the material. For the same material, fine grained organizations than grain degree has higher yield strength.
2. The surface state of maximum stress occurs in the surface layer of spring material, so the quality of the surface of the spring has much effect on the fatigue strength. Spring material caused in the process of rolling, drawing, and rolling defects such as cracks, defects and scars are often the cause of spring fatigue fracture.
The smaller the surface roughness, the smaller the stress concentration and fatigue strength is also higher. The influence of surface roughness on the fatigue limit. With the increase of surface roughness, fatigue limit. In the case of the same roughness, different steel grade and different extent of the coiling method of the fatigue limit is different also, such as cold heat coil spring small extent of the coil spring. When heated, because steel hot coil spring and its heat treatment due to oxidation make the spring material surface roughness and decarburization phenomenon, thus reduced the fatigue strength of the spring.
On the material surface grinding, crush, blasting and rolling, etc. All can improve the fatigue strength of the spring.
3. The greater the size effect of material size, due to various defects caused by the cold and hot working technology, the higher the possibility to produce the greater the likelihood of surface defects, these reasons will lead to fatigue performance degradation. Therefore when calculating the fatigue strength of the spring to consider the influence of size effect.
4. The metallurgical defects of non-metallic inclusions in metallurgical defects refers to materials, bubble, element segregation, and so on. Exist on the surface of the inclusions is a source of stress concentration, can lead to inclusion and matrix interface between prematurely to produce fatigue crack. Adopt measures, such as vacuum smelting, vacuum casting can greatly improve the quality of steel.
5. The corrosive medium spring when you are working in a corrosive medium, because of the surface pitting or surface grain boundary corrosion and become a source of fatigue, under the effect of varying stress will gradually expand and cause fracture. Such as the spring steel work in fresh water, the fatigue limit is only 10% ~ 25% in the air. Corrosion influence on spring fatigue strength, not only related to the spring by the effect of variable load frequency, and associated with the working life. So design calculation affected by corrosion of the spring, it should be working life into consideration.
Work under corrosion condition of spring, in order to ensure its fatigue strength, high corrosion resistance of materials can be used, such as stainless steel, nonferrous metals, or surface coating, such as plating, oxidation, coating, paint, etc. Practice showed that cadmium plating can greatly improve the fatigue limit of spring.
Fatigue strength of carbon steel, 6. Temperature from room temperature to 120 ℃, from 120 ℃ to 350 ℃ rise again and again down after temperature higher than 350 ℃, there is no fatigue limit at high temperature. Work under high temperature conditions of spring, want to consider using heat-resistant steel. Under room temperature conditions, the fatigue limit of steel increases.