Spring is a common part in our life. Its work is mainly reflected in the stainless spring corroded by corrosive media such as electronics, air, steam and water and chemical aggressive media such as acid and alkali salt. It can be divided into various types of springs according to the stress properties of spring materials. Let's talk about the main factors of spring fatigue fracture.
1. Yield strength there is a certain relationship between the clothing strength and fatigue of materials. Generally speaking, the higher the yield strength of materials, the higher the fatigue strength. Therefore, in order to improve the fatigue strength of the spring, we should try to improve the yield strength of the spring material or use the material with high ratio of yield strength to resistance strength. For the same material, fine grain structure has higher yield strength than coarse and fine grain structure.
2. The surface state mostly occurs on the surface of the spring material, so the surface quality of the spring has a great impact on the fatigue strength of the spring. The cracks, defects, scars and other defects caused by the spring material in the process of rolling, drawing and rolling are often the reasons for the fatigue fracture of the spring. The smaller the surface roughness of spring material, the smaller the stress concentration and the higher the fatigue strength. Effect of material surface roughness on fatigue. With the increase of surface roughness, fatigue decreases. In the case of the same roughness, the fatigue degree of different steel grades and different paper winding methods is also different. For example, the reduction degree of cold coil spring is smaller than that of hot coil spring. Because when the steel hot coil spring and its heat treatment are heated, the surface of the spring material becomes thicker and decarburized due to oxidation, which reduces the fatigue strength of the spring. The fatigue strength of spring can be improved by grinding, forced pressing, shot blasting and rolling on the surface of spring material.
3. The larger the size of the size effect material, the higher the possibility of defects caused by various cold working and hot working processes, and the greater the possibility of surface defects. These reasons will lead to 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 non-metallic inclusions, bubbles, element segregation, etc. the impurities on the surface are stress concentration, which will lead to excessive fatigue lines between the inclusion and the matrix interface. The quality of steel can be greatly improved by adopting measures such as vacuum smelting and vacuum pouring.
5. When the corrosion medium spring works in the corrosion medium, it becomes a fatigue source due to pitting corrosion on the surface or corrosion of the surface grain boundary, which will gradually expand under the action of variable stress and lead to fracture. The fatigue of spring steel working in fresh water is only 10% - 25% of that in air. The influence of corrosion on the spring strength is related not only to the number of times the spring is subjected to variable load, but also to the working life. Therefore, the working life should be taken into account when designing and calculating the spring affected by corrosion. For springs working under corrosive conditions, materials with high corrosion resistance can be used to ensure their fatigue strength. Such as stainless steel, non-ferrous metal, or protective layer on the surface, such as coating, oxidation, plastic spraying, painting, etc. The practice shows that chromium plating can greatly improve the fatigue of spring.
6. The fatigue strength of temperature carbon steel decreases from room temperature to 120 ℃, increases from 120 ℃ to 350 ℃, and decreases after the temperature is higher than 350 ℃. There is no fatigue at high temperature. Heat resistant steel shall be considered for springs working under high temperature conditions. Under the condition of lower than room temperature, the fatigue of steel increases.
1. Yield strength there is a certain relationship between the clothing strength and fatigue of materials. Generally speaking, the higher the yield strength of materials, the higher the fatigue strength. Therefore, in order to improve the fatigue strength of the spring, we should try to improve the yield strength of the spring material or use the material with high ratio of yield strength to resistance strength. For the same material, fine grain structure has higher yield strength than coarse and fine grain structure.
2. The surface state mostly occurs on the surface of the spring material, so the surface quality of the spring has a great impact on the fatigue strength of the spring. The cracks, defects, scars and other defects caused by the spring material in the process of rolling, drawing and rolling are often the reasons for the fatigue fracture of the spring. The smaller the surface roughness of spring material, the smaller the stress concentration and the higher the fatigue strength. Effect of material surface roughness on fatigue. With the increase of surface roughness, fatigue decreases. In the case of the same roughness, the fatigue degree of different steel grades and different paper winding methods is also different. For example, the reduction degree of cold coil spring is smaller than that of hot coil spring. Because when the steel hot coil spring and its heat treatment are heated, the surface of the spring material becomes thicker and decarburized due to oxidation, which reduces the fatigue strength of the spring. The fatigue strength of spring can be improved by grinding, forced pressing, shot blasting and rolling on the surface of spring material.
3. The larger the size of the size effect material, the higher the possibility of defects caused by various cold working and hot working processes, and the greater the possibility of surface defects. These reasons will lead to 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 non-metallic inclusions, bubbles, element segregation, etc. the impurities on the surface are stress concentration, which will lead to excessive fatigue lines between the inclusion and the matrix interface. The quality of steel can be greatly improved by adopting measures such as vacuum smelting and vacuum pouring.
5. When the corrosion medium spring works in the corrosion medium, it becomes a fatigue source due to pitting corrosion on the surface or corrosion of the surface grain boundary, which will gradually expand under the action of variable stress and lead to fracture. The fatigue of spring steel working in fresh water is only 10% - 25% of that in air. The influence of corrosion on the spring strength is related not only to the number of times the spring is subjected to variable load, but also to the working life. Therefore, the working life should be taken into account when designing and calculating the spring affected by corrosion. For springs working under corrosive conditions, materials with high corrosion resistance can be used to ensure their fatigue strength. Such as stainless steel, non-ferrous metal, or protective layer on the surface, such as coating, oxidation, plastic spraying, painting, etc. The practice shows that chromium plating can greatly improve the fatigue of spring.
6. The fatigue strength of temperature carbon steel decreases from room temperature to 120 ℃, increases from 120 ℃ to 350 ℃, and decreases after the temperature is higher than 350 ℃. There is no fatigue at high temperature. Heat resistant steel shall be considered for springs working under high temperature conditions. Under the condition of lower than room temperature, the fatigue of steel increases.
