Several factors affecting the fatigue strength of spring
Several factors affecting the fatigue strength of spring
1. Yield strength there is a corresponding correlation between the yield strength and fatigue limit of the material. Generally, the higher the yield strength of the material, the higher the fatigue strength. Therefore, in order to improve the fatigue strength of the spring, try to improve the yield strength of the spring material, or select the material 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 surface condition is large, and the stress mostly occurs on 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 of spring materials in the process of rolling, drawing and rolling are usually the factors causing spring fatigue cracking. The smaller the surface quality of the material, the smaller the stress concentration and the higher the fatigue strength. The effect of surface quality of material on fatigue limit. With the addition of surface quality, the fatigue limit decreases. In the case of the same roughness, the fatigue limit reduction degree of different steel grades and different rolling methods is also different. For example, the reduction degree of cold coil spring is smaller than that of hot coil spring. Because when steel hot coil spring and heat treatment are heated, the surface of spring material becomes rough and decarburized due to oxidation, which reduces the fatigue strength of spring. Finish grinding, forced pressing, shot blasting and rolling on the material surface. Can improve the fatigue strength of the spring.
3. The greater the standard of standard effect material, because the higher the probability of defects caused by various cold working and hot working processes, the greater the probability of surface defects. These factors will reduce the fatigue function. Therefore, the influence of standard effect should be considered when calculating the fatigue strength of spring.
4. Metallurgical defects metallurgical defects refer to non-metallic inclusions, bubbles, element segregation, etc. The inclusions on the surface are the source of stress concentration, which will lead to fatigue cracks at the interface between inclusions and matrix. The selection of vacuum smelting, vacuum pouring and other methods can greatly improve the quality of steel.
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, and will gradually expand under the action of variable stress, resulting in 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 spring fatigue strength is not only related to the number of times the spring is subjected to variable load, but also related to the service life. Therefore, the working life should be taken into account when planning and accounting the spring affected by corrosion. For springs operating under corrosive conditions, in order to ensure their fatigue strength, materials with high anti-corrosion function can be selected, such as stainless steel, non-ferrous metals, or protective layers can be added on the surface, such as plating, oxidation, plastic spraying, painting, etc. Practice shows that cadmium plating can greatly improve the fatigue limit of spring.
6. The fatigue strength of 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 limit at high temperature. For springs operating under high temperature conditions, heat-resistant steel shall be considered. Under the condition of lower than room temperature, the fatigue limit of steel is added.
1. Yield strength there is a corresponding correlation between the yield strength and fatigue limit of the material. Generally, the higher the yield strength of the material, the higher the fatigue strength. Therefore, in order to improve the fatigue strength of the spring, try to improve the yield strength of the spring material, or select the material 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 surface condition is large, and the stress mostly occurs on 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 of spring materials in the process of rolling, drawing and rolling are usually the factors causing spring fatigue cracking. The smaller the surface quality of the material, the smaller the stress concentration and the higher the fatigue strength. The effect of surface quality of material on fatigue limit. With the addition of surface quality, the fatigue limit decreases. In the case of the same roughness, the fatigue limit reduction degree of different steel grades and different rolling methods is also different. For example, the reduction degree of cold coil spring is smaller than that of hot coil spring. Because when steel hot coil spring and heat treatment are heated, the surface of spring material becomes rough and decarburized due to oxidation, which reduces the fatigue strength of spring. Finish grinding, forced pressing, shot blasting and rolling on the material surface. Can improve the fatigue strength of the spring.
3. The greater the standard of standard effect material, because the higher the probability of defects caused by various cold working and hot working processes, the greater the probability of surface defects. These factors will reduce the fatigue function. Therefore, the influence of standard effect should be considered when calculating the fatigue strength of spring.
4. Metallurgical defects metallurgical defects refer to non-metallic inclusions, bubbles, element segregation, etc. The inclusions on the surface are the source of stress concentration, which will lead to fatigue cracks at the interface between inclusions and matrix. The selection of vacuum smelting, vacuum pouring and other methods can greatly improve the quality of steel.
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, and will gradually expand under the action of variable stress, resulting in 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 spring fatigue strength is not only related to the number of times the spring is subjected to variable load, but also related to the service life. Therefore, the working life should be taken into account when planning and accounting the spring affected by corrosion. For springs operating under corrosive conditions, in order to ensure their fatigue strength, materials with high anti-corrosion function can be selected, such as stainless steel, non-ferrous metals, or protective layers can be added on the surface, such as plating, oxidation, plastic spraying, painting, etc. Practice shows that cadmium plating can greatly improve the fatigue limit of spring.
6. The fatigue strength of 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 limit at high temperature. For springs operating under high temperature conditions, heat-resistant steel shall be considered. Under the condition of lower than room temperature, the fatigue limit of steel is added.
