Cast Steel Shot, Magnetic Polishing Needles, Stainless Steel Shot
Shot peening is a shot peening technique that is different from shot peening. The purpose of shot blasting is to remove oil, scale, rust and machining burrs on the surface of the workpiece. The shot peening of the gear surface mainly relies on the impact of high-speed projectiles on the surface of the part, causing it to undergo elastic-plastic deformation, thereby producing favorable changes such as residual compressive stress, work hardening and microstructure refinement, so as to improve the bending fatigue strength of the gear. And contact fatigue strength, it is to improve the anti-occlusion ability of the gear, reduce the noise of the gear and improve the service life of the gear by more than 30%.
1. Mechanism and function of shot peening 1. Shot peening treatment can improve the stress distribution on the surface of parts The residual stress after shot peening comes from the inhomogeneous plastic deformation of the surface layer and the phase transformation of the metal structure, among which the inhomogeneous plastic deformation is the main one. After shot peening, plastic deformation in the form of a large number of pits occurs on the metal surface, the surface dislocation density greatly increases, and subgrain boundaries and grain refinement also appear. As shown in Figure 1. Part of the retained austenite on the surface of the gear after shot peening will turn into martensite, and compressive stress will be generated due to volume expansion during phase transformation, so that the retained austenite field on the surface will change towards a larger compressive stress direction, so Improve the fatigue strength of the gear. Through shot peening, it can eliminate heat treatment stress, remove surface oxide skin, improve the notch sensitivity of parts, and convert the residual stress that is prone to failure of parts into compressive stress, effectively restricting the generation and expansion of crack sources. Improve the fatigue life of parts. 2. Shot peening can form a high compressive stress layer on the surface of the workpiece Because shot peening increases the surface compressive stress and significantly improves its fatigue performance, it is more effective for workpieces that withstand high-frequency fatigue loads. The residual compressive stress formed by shot peening can offset part of the applied load. During shot peening, small-sized spherical steel shots hit the surface of the workpiece to form compressive stress. The impact of each shot will cause a certain plastic deformation of the metal, and finally the surface cannot be completely restored and a permanent compressive stress state is formed. As a surface strengthening process, shot peening can form residual compressive stress on the surface equivalent to 55%~60% of the ultimate tensile strength of the material, and the surface of the workpiece is just the place where cracks are easy to initiate. For carburized and quenched gears, the formed compressive stress can reach 1177~1725MPa, which can greatly improve the fatigue performance. The depth of the compressive stress layer is a function of the peening intensity (or peening energy) and increases with shot size or shot velocity. 2. Shot peening process parameters The shot peening process has high requirements on the shape, size and hardness of the projectile. The shot peening process is controlled by shot peening strength and surface coverage, and the surface strengthening effect is detected by residual stress and fatigue tests. Shot peening process parameters include projectile material, projectile diameter, projectile velocity, projectile flow rate, injection angle, injection distance, shot peening time and coverage, etc. The change of any one of these parameters will affect the shot peening intensity to varying degrees, that is, the effect of strengthening . (1) Arc height test piece The standard ALMEN arc height test piece is a special gauge for comprehensive evaluation of shot peening process parameters. It is made of No. 70 spring steel and has three specifications, code-named N, C, and A, respectively, and is used in three occasions with different shot peening intensity requirements. (2) Arc height curve The arc height curve is the change of the shot peening arc height value of the same test piece with the shot peening time (or the number of shot peening times) under the condition of other process parameters being fixed, which marks the curve of the arc height value-time relative relationship. (3) Shot peening intensity Shot peening intensity is usually determined by the arc height value method, the main point of which is to use a certain spring steel test piece to reflect the shot peening effect by detecting the shape change after the shot peening intensity. The specific operation is to use the Almen test piece (arc height test piece , the general hardness is 44~50HRC), fixed on the fixture, after shot peening, the test piece is removed, and then the height of the bending arc is measured with a checking tool (such as an Almen measuring instrument). Another inspection method for shot peening strength is residual stress detection, that is, the detection of residual stress on workpieces after enhanced shot peening, and the specific inspection method is X-ray diffraction method. (4) Surface coverage The coverage rate refers to the ratio of the indentation area of the projectile to the surface area of the sprayed workpiece after the surface of the workpiece is shot peened. Usually expressed as a percentage. The main point of the measurement is to enlarge the Almen test piece about 50 times after shot peening, and measure the indentation area of the projectile. Because it is difficult to guarantee 100% coverage, 98% coverage is actually defined as full coverage. For product patterns requiring 300% coverage, this is typically achieved with three times the peening time required to achieve 98% effective coverage. (5) Projectile quality The quality of the projectile has a great influence on the strengthening effect. The general rule is: the diameter of the projectile is small, the residual stress on the surface of the workpiece is high, but the strengthening layer is shallow; the diameter of the projectile is large, the residual stress on the surface of the workpiece is low, but the strengthening layer is deep; the hardness of the projectile is high , the shot peening intensity is also high; the shot diameter increases, the shot peening intensity also increases; the shot velocity increases, the shot peening intensity, the surface compressive stress and the depth of the strengthening layer increase. (6) Shot peening time Under the condition that other shot peening process parameters remain unchanged, the best strengthening effect can be obtained only when the shot peening reaches the "saturation" time or twice the "saturation" time. Usually, insufficient strengthening time is more unfavorable than excessive strengthening time. Therefore, when it is found that the strengthening time is lower than the specified time, supplementary strengthening can be performed on the workpiece again. 3. Application of shot peening technology 1. Improve the surface hardness of carburized gears Example 1: The shot peening equipment of the FAW Heat Treatment Branch Factory adopts the TR5SVR-1 stress shot peening equipment made in Germany. The shot peening process is as follows: steel shot with a diameter of ￠0.8mm is used, the shot peening time is 9 minutes, and the shot peening speed is 2800r/min. The gear material is 22CrMoH steel, which has been carburized, quenched and tempered. After the shot peening treatment, the surface structure of the gear has been refined, and the retained austenite content of the surface layer is about 10% lower than that of the workpiece without shot peening treatment. Within the range of 0.15mm from the surface, the change amount is relatively Obviously; the surface hardness of the gear after enhanced shot peening treatment is increased by 0.5~2HRC. 2. Increase the residual compressive stress on the gear surface and improve the surface microstructure Example 2: The second-speed driven gear of Santana car transmission has been carburized, quenched and tempered. The impeller type shot peening machine is used for shot peening, the shot peening speed is 2900r/min, and the shot hardness is 57HRC. The results of the shot peening test are as follows. (1) The microhardness of the surface layer of shot peened parts is significantly improved, which is the result of the combined effect of high residual compressive stress, work hardening and microstructure change (refinement), and the significant reduction of retained austenite also contributes to the increase in hardness. contributed. (2) Due to the shot peening, the retained austenite in the surface layer of the workpiece is changed to martensite, which can be used to improve the surface hardness. The martensitic needles are obviously smaller and denser than those without shot peening, which plays a role in refining the martensite substructure. It is conducive to the improvement of residual stress, thereby improving the fatigue performance of gears. 3. Improve the fatigue life of gears Example 3: FAW used the enhanced shot peening process to conduct a fatigue life test on the first gear of the "Jiefang" brand automobile transmission, which significantly improved the fatigue life of the gear. In order to improve the fatigue life of the "Jiefang" brand active helical gear, a large arc hob is used to cut the teeth. Increasing the gear fillet can increase the life of the active helical gear from 208,300 times to 695,400 times. Increase its fatigue life to 3.477 million times. The gears are shot peened after carbonitriding to improve the contact fatigue strength. For example, SCM420H steel gears are carbonitrided through ammonia gas, etc. With the increase of nitrogen content, ΔHV (hardness drop) increases, that is, the tempering resistance performance is improved, and the tempering temperature can reach 300°C. The problem of contact fatigue damage of conventional carburized gear tooth surface of automatic transmission AIT for automobiles is solved. 4. Projectile requirements After the heat treatment of the gears, most of the surface hardness has reached 58~63HRC. When performing shot peening, the shot peening treatment should be higher than or equal to the surface hardness to ensure the residual compressive stress on the surface and the surface strengthening layer. depth. In addition, the selection of the projectile size should ensure that the root of the gear is strengthened. In the process of shot peening, the tooth root with a small radius often cannot be shot peened because the diameter of the projectile selected is too large. To do this, ensure that the projectile diameter is less than half the root radius. Since the quality of the steel shot has a great influence on the strengthening effect, the national standard has given strict specifications on the metallographic structure, chemical composition, minimum density and hardness deviation of the steel shot. During the use of projectiles, it is necessary to replenish new projectiles in time, and the qualified quantity of projectiles should be kept above 80%. Therefore, different specifications of screens can be selected for control to ensure the effect of shot peening. Because the broken projectile has sharp corners, it is easy to produce many tiny sharp cuts on the surface of the workpiece, thereby reducing the strengthening effect. Therefore, broken projectiles should be screened out during the production process. Shot size selection principles: ① When there is a requirement for surface roughness, smaller steel shots should be used as much as possible. ② When the workpiece has inner and outer fillets and grooves, the diameter of the projectile should be smaller than the radius of the inner and outer fillets and the width of the groove. ③Ensure that the spherical shape and size of the projectile are uniform. 5. The steel shot adopts 1. BPS reinforced steel shot is a product developed by Aobote. It adopts high-quality strips, heat-treated prestressed wire drawing, CNC cutting, and passivated round shot technology to produce a new generation of surface cleaning and strengthening high-quality shot materials. HRC: 40-50 respectively /50-55/55-63 steel shot not only has uniform hardness, uniform size and long service life, but also does not generate dust and is not broken during production. It can be called green and environmentally friendly steel shot. It is mainly used for spraying gears and spring frames. Pill strengthening treatment, etc.
