Practice has proved that the powerful shot peening process is an important method to improve the bending fatigue strength and contact fatigue strength of the gear teeth, and it is an important way to improve the gear anti-biting ability and improve the gear life. The powerful shot peening process was first produced in the 1920s, and is mainly used in the military field. With the promotion of the application range, the ability of the powerful shot peening technology to improve the fatigue strength and life of gears has been confirmed by many companies. working principle The powerful shot peening process mainly uses high-speed jetted fine steel shots to impact the surface of the sprayed workpiece at room temperature, so that the workpiece surface material produces elastic-plastic deformation and presents a high residual compressive stress, thereby increasing the surface strength and fatigue strength of the workpiece. Shot blasting causes the surface of the part to be elastically deformed. At the same time, a large amount of twins and dislocations are also generated, and the surface of the material is processed and strengthened. As shown in Figure 1: Fig. 1-a Surface of the shot-blasted part Fig. 1-b Surface of the shot-blasted part The effect of shot peening on surface morphology and performance is mainly reflected in the change of surface hardness, surface roughness, resistance to stress corrosion and the fatigue life of the part. The material surface of the part undergoes cyclic plastic deformation under the impact of the steel pill bundle. Depending on the nature of the material and the state of the material, the surface layer of the material after shot peening will undergo the following changes: hardness changes, changes in the structure of the structure, phase transitions, formation of residual stress field in the surface layer, changes in surface roughness, and the like. Shot peening intensity measurement method When a piece of metal receives a shot from the steel shot, it bends. Saturation and shot peening are two important concepts in the peening process. Saturation refers to the state when the spray is continued under the same conditions without changing the mechanical properties of the sprayed area. The so-called shot peening intensity is to measure the strength of the spray by hitting a piece of metal that has been prefabricated into a certain size (that is, the test piece), and at the specified time to reach the saturation level. Strength level. At present, the most widely used American Society of Automotive Engineering shot peening standards adopts the peening test method—arc height method proposed by Alman. This method was proposed by the United States GM company's JO Almen (Almen), and was adopted by SAEJ442a and The main point of SAE 443 standard measurement method is to reflect the shot peening effect by measuring the shape change after shot peening with a certain standard spring steel test piece. In the single-face shot peening of the thin plate test piece, since the surface layer is involved in the tensile deformation under the action of the projectile, the thin plate is spherically curved toward the shot surface. The spherical arc height is usually measured over a span of distance and used to measure shot peening intensity. The height of the arc is determined by fixing the Almen test piece on a special fixture. After shot peening, the specimen is removed. Then, the Almen gauge is used to measure the participating tensile deformation variables produced by the single side shot peening. (ie arc height value). If the arc height measured with the test piece is 0.35mm, it is recorded as 0.35A. Another test method for shot peening strength is residual stress testing, which is the detection of the residual stress after a strong shot peening. The specific test method is X-ray diffraction. The following methods are recommended in the US SAE J784a standard: the incident and diffracted X-ray beams must be parallel to the tooth root of the gear, the measurement position on the cylindrical spur gear and the cylindrical helical gear should be at the center of the width of the tooth root, and the irradiated area must be concentrated on the tooth The center of the fillet, which cannot extend laterally beyond the measuring point of the specified depth of the tooth fillet surface, can be controlled by direct light beam and proper covering of the root surface; on each selected gear selected , At least two teeth are to be evaluated, and the two teeth are separated by 180?. If the effective tooth profile of the tooth is protected from grinding, it can be considered that the gear grinding of the tooth root for the measurement of the residual stress at the surface is not damaged and can be used for production. The effect of shot peening on improving the fatigue resistance of parts The essence of material surface hardening by means of surface cold deformation is that cold deformation causes changes in the surface structure of the material, the introduction of residual compressive stress, and changes in the surface topography. 1. Shot peening improves the surface properties of the material During the shot peening process, when the tiny spherical steel shot hits the surface of the sprayed workpiece at a high speed, the surface material of the workpiece is elastically and plastically deformed, and a pressure pit is generated due to plastic deformation at the impacted position. The impact causes the surface material near the pressure pit to have a diameter. Extend. As more and more steel shots impact the surface of the workpiece, more and more parts of the workpiece surface undergo plastic flow due to the kinetic energy absorbed by the high-speed motion steel pellets, causing the surface material to undergo radial extension due to plastic changes. As the area becomes larger and larger, plastically deformed surfaces are gradually connected into pieces, a uniform layer of plastic deformation is gradually formed on the surface of the workpiece. After the plastic deformation layer is formed, the shot peening will cause the thickness of the plastic deformation layer to gradually thin due to continuous extension, and at the same time, the radial extension of the plastic deformation layer will be damaged due to the limitation of the adjacent area, and finally the plastic deformation layer will be damaged. Continuous shot peening and spalling. Therefore, the time for shot peening must be strictly controlled. 2. Effect of shot peening on residual stress of surface layer of carburized gear Regarding shot peening, the cause of residual stress on the surface of the workpiece is based on the view of Al-Obaid et al.: When the high-speed steel shot hits the surface of the specimen, plastic deformation occurs at the impact and a residual pressure crater remains, as more and more steel shots are formed. When impacted on the surface of the specimen, a uniform layer of plastic deformation occurs on the surface of the specimen. Because the volume expansion of the plastic deformation layer is limited by the non-plastic deformation of the immediate vicinity, the entire plastic deformation layer is subjected to a compressive stress. Because the residual compressive stress and its distribution have a greater impact on the fatigue life of the gear, the quality of the shot peening process will directly affect the size and distribution of the residual stress. Therefore, the accurate determination of the surface residual stress of sprayed parts is an effective means to evaluate the advantages and disadvantages of the shot peening process. 3, the impact of shot peening on the surface roughness of parts Intensified shot peening will cause plastic deformation of the part by the spray surface, causing the surface roughness of the part to change. Surface roughness is a microscopic geometric shape error, also known as microscopic roughness. The surface roughness is the same as the surface waviness and shape error, which is a geometric error of the part. The surface roughness has an important influence on the performance of the machine parts. The effect of shot peening on the surface roughness of the material is usually within the range of Ra 0.6 to 20 mm. Without changing the process parameters, the higher the original surface roughness of the material, the greater the Ra value after shot peening. Production practice has proved that under normal circumstances, the surface roughness before spraying is less than 6.3mm. Shot peening can improve or maintain the original surface roughness. If the original surface roughness is 6.3mm or more, the surface roughness after shot peening is reduced. In the production practice, in order to obtain a better shot peening surface, we should proceed from the following aspects: provide a better original surface, Ra value should be below 6.3mm; choose a reasonable steel shot diameter and shot pressure; in large After shot peening of the diameter steel shots, the smaller steel shots were used for low pressure (no change in shot-penetration value) to cover one time and good surface roughness could be achieved. The surface of the parts after shot peening should be lightly polished, and the surface metal removal amount should be controlled when grinding. In this way, the surface roughness can be improved without impairing the shot-peening effect. Of course, this is a multi-factor problem. Regardless of the method used, the influence of other factors must be considered at the same time. Effect of process parameters on shot peening The impact on the shot peening quality mainly includes the following aspects: steel shot material, steel shot diameter, steel shot speed, steel shot flow rate, spray angle, shot distance, shot time, and coverage rate. Changes in any one of these parameters will affect the effect of shot peening to varying degrees. 1. Effect of the material, hardness, size and granularity of steel shots on shot peening Cast iron pellets and cast steel pellets are commonly used for shot peening of hardened gears. The disadvantage of cast iron pellets is low toughness, which is easy to crush in the shot peening process and consumes large amounts. The broken steel pellets must be separated in time, otherwise the quality of the sprayed surface will be affected. However, the advantages of cast iron pellets are that they are inexpensive and have high hardness, which can result in high residual compressive stress on the sprayed surface. Compared with cast iron pellets, the advantages of cast steel pellets are that they are not easily broken, which is beneficial to the geometry of the sprayed surface. However, the hardness of cast steel pellets is lower than that of cast iron pellets. Under the same conditions, the residual compressive stress on the sprayed surface is lower than that of cast iron pellets. For sprayed workpieces, the pellet quality and pellet speed determine the stability of the shot peening effect. Among them, the shot quality has a great influence on the shot peening effect. The general rule is: the diameter of the shot is small, the residual stress on the workpiece surface is high, but the strengthening layer is shallow; the diameter of the steel shot is large, and the residual stress on the workpiece surface is low, but the strengthening layer Deeper; steel shots of high hardness, shot peening intensity is also high; steel shot diameter increases, shot peening strength also increased; steel shot speed increases, shot peening strength, surface compressive stress and reinforced layer depth are increased. Reasonable choice of control shot peening parameters can get a good shot peening effect. Normally, the diameter of the steel shot is affected by the sprayed part. The diameter of the steel shot should generally not be greater than half the diameter of the fillet in the transition zone of the gear. Excessive shots cannot be sprayed on the geared corners. When the surface roughness is required, smaller steel shots should be used as far as possible. The shot peening time required to reach the coverage requirement will increase rapidly with the size of steel shots, and small steel shots can reach the coverage requirements quickly. Therefore, the diameter of steel pellets should not be selected too large. According to the actual situation, our company chooses steel pellets with diameters of φ0.6mm and φ0.8mm, and the obtained results are ideal. At the same time, the material of the steel shot is also very important. The national standard has given strict specifications on the metallographic structure, chemical composition, minimum density, and hardness deviation range of the steel shot. Qualified material steel pellets should be strictly controlled quality, to ensure uniform spherical shape size, to ensure adequate number of shots. The reduction in the amount of steel pellets will also reduce the corresponding shot peening strength. Therefore, it is necessary to check the steel shots at certain intervals to remove unqualified steel shots in time and exchange and increase a certain amount of steel shots. Otherwise, the corners of deformed steel pellets can easily cause micro-cracks on the surface of sprayed parts and cause fatigue sources. Generally, the number of qualified steel shots should be no less than 80%. The content of qualified steel pellets is generally controlled by different specifications of screens (as shown in Figure 2). Figure 2 Screens for testing The hardness of the steel shot should be chosen taking into account the hardness of the workpiece material. When the hardness of the steel shot is very close to that of the gear material, the maximum compressive stress and the depth of compression will not be affected by the hardness of the shot. Therefore, when selecting steel shots, the hardness of steel shots should be greater than or equal to the hardness of gear shot peening surfaces. For carburizing gears, steel balls with a hardness of 55-65 HRC are preferred for satisfactory compressive stress. 2. Effect of shot, flow rate and spray angle on shot peening The throwing head is directly driven by the variable frequency motor, and the rotational speed of the throwing head can be changed by changing the frequency of the motor. Under the action of centrifugal force, the steel ball overflows from the hole in the impeller shaft to the blade (as shown in Fig. 3), and is then thrown by a high-speed rotating blade at a fixed angle. The speed of the impeller determines the initial velocity of the steel ball. The maximum speed of the motor is 3000r/min. As the throwing head rotates, the steel pellets will be continuously thrown out. Therefore, the steel pellet flow entering the throwing impeller shaft must ensure that the throwing head has sufficient pellet supply, which requires constant replenishment of the shot blasting pellet recovery system. In the stock of steel shots, it is more important to adjust the opening of shot control valve to adjust the flow rate of shots entering the head through the shot control valve. The input of shot blasting pellets is fixed after being adjusted. In normal use, changing the pellet flow rate is achieved by adjusting the rotational speed of the throwing head, ie, increasing the steel pellet input amount. With impeller speed, the flow rate of steel shots per unit time is large, and vice versa. On the shot peening machine, each head has an ampere meter connected to it to show the flow of steel shots. When the shot peening quality fails to meet the technical requirements, the frequency of the motor needs to be adjusted. The adjustment is determined by the reading of the ampere meter to determine the degree of adjustment. Ampere meter reading range is 0 ~ 30A. in conclusion During the shot peening process, the surface of the material is subjected to severe impact of steel shots to produce a deformation hardening layer, which will lead to two effects: First, the subgrain is refined in the organization, the dislocation density increases, and the lattice distortion increases; secondly, the introduction of high The macro residual compressive stress. In addition, due to the increase in surface roughness due to the impact of steel shots, sharp cutting marks produced during cutting tend to be rounded. These changes will significantly increase the material's fatigue resistance and stress corrosion resistance, thereby significantly increasing the life of the gear. Laser Welding Of Spiral Finned Tubes
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