Powerful shot peening process in gear processing
The role of powerful shot peening
An important method to improve the bending fatigue strength and contact fatigue strength of gear teeth is an important way to improve gear anti-seizure ability and increase gear life.
Working principle
The powerful shot peening process mainly uses high-speed jetting of small steel shots to hit the surface of the workpiece to be sprayed at room temperature, causing elastoplastic deformation of the surface material of the workpiece and presenting a higher residual compressive stress, thereby improving the surface strength and fatigue strength of the workpiece. Shot peening makes the surface of the part elastically deform, but also produces a large number of twins and dislocations, so that the surface of the material is processed and strengthened. As shown in Figure 1:
Figure 1-a The surface of the part after shot blasting Figure 1-b The surface of the part without shot blasting
The impact of shot peening on surface morphology and performance is mainly manifested in changing the surface hardness, surface roughness, stress corrosion resistance and fatigue life of parts. The material surface of the part undergoes cyclic plastic deformation under the impact of the steel shot. According to the nature and state of the material, the surface of the material after shot peening will undergo the following changes: hardness change, organizational structure change, phase transition, surface residual stress field formation, surface roughness change, etc.
Method of measuring shot peening strength
When a piece of metal is hit by a stream of steel shot, it will bend. Saturation and shot peening strength are two important concepts in the shot peening process. The saturated state refers to the state when the spray continues under the same conditions without changing the mechanical characteristics of the sprayed area. The so-called shot peening strength is to make a metal sheet of a certain specification (ie a test piece) pre-made by hitting it to reach the strength of the saturated state within a specified time, and the arc height of the test piece is used to measure its blasting. Degree of strength.
At present, the most widely used American Society of Vehicle Engineering shot peening standard adopts the shot peening test method proposed by Alman-the arc height method. This method was proposed by JO Almen (Almen) of GM Company and was developed by SAEJ442a and The main point of the measurement method specified in the SAE443 standard is to use a spring steel test piece of a certain specification to reflect the shot peening effect by detecting the shape change after shot peening. When single-sided shot peening is performed on a thin plate test piece, the surface layer is subject to tensile deformation under the action of the shot, so the thin plate is spherically curved toward the shot surface. Usually, the arc height value of the spherical surface is measured at a certain span distance and used to measure the intensity of shot peening. The arc height value is determined by fixing the Almen test piece on a special fixture, after shot peening, and then removing the test piece, and then using the Almen gauge to measure the tensile deformation of the test piece produced by single-sided shot peening (That is, the arc height value). If the arc height measured by the test piece is 0.35mm, it is recorded as 0.35A.
Another inspection method of shot peening strength is residual stress inspection, that is, the residual stress inspection of the workpiece after strong shot peening. The specific inspection method is X-ray diffraction. The following method is recommended in the US SAE J784a standard: the incident and diffracted beams of X-rays must be parallel to the tooth root of the gear, the measurement position on the cylindrical spur gear and cylindrical helical gear should be in the center of the width of the tooth root, and the irradiation area must be concentrated on the tooth. The center of the root fillet cannot extend laterally beyond the specified measuring point of the depth of the tooth root fillet surface. The size of the irradiation area can be controlled by directing the beam and covering the tooth-root surface appropriately; on each selected gear to be inspected, At least two teeth should be selected for evaluation, and the interval between the two teeth is 180°. If the effective tooth profile of the tooth is protected and not ground, it can be considered that the gear root ground for measuring the residual stress under 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 strengthening 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 surface morphology.
Shot peening improves the surface properties of the material
In the process of strengthening shot peening, when the small spherical steel shot hits the surface of the sprayed workpiece at high speed, the surface material of the workpiece will be elastically and plastically deformed. The impact site will produce a crater due to plastic deformation. The impact will cause the surface material near the crater to develop a diameter. To extend. When more and more steel shots hit the surface of the workpiece to be sprayed, more and more parts of the surface of the workpiece absorb the kinetic energy of the high-speed moving steel shots and produce plastic rheology, which causes the radial extension of the surface material due to plastic changes. The area becomes larger and larger, and the plastically deformed surfaces are gradually connected into pieces so that a uniform plastic deformation layer is gradually formed on the surface of the workpiece. After the plastic deformation layer is formed, the continuous shot peening will make the plastic deformation layer gradually thinner due to the continuous extension. At the same time, the radial extension of the plastic deformation layer will be restricted by the adjacent area and cause the overlapping part to be destroyed. Continuous shot peening and peeling. Therefore, the time of shot peening must be strictly controlled.
Effect of Shot Peening on Residual Stress of Carburized Gear Surface
Regarding the reason for the formation of residual stress on the surface of the workpiece by shot peening, according to the viewpoint of Al-Obaid et al.: When the high-speed steel shot hits the surface of the sample, plastic deformation occurs at the impact site and a crater remains. When more and more steel shots When it hits the surface of the sample, a uniform plastic deformation layer will be formed on the surface of the sample. Because the volume expansion of the plastic deformation layer will be restricted from the unplastically deformed neighboring area, the entire plastic deformation layer is subjected to compressive stress.
Because the residual compressive stress and its distribution have a great influence on the fatigue life of the gear, the pros and cons of the shot peening process will directly affect the residual stress and its distribution. Therefore, accurate determination of the residual stress on the surface of the sprayed parts is an effective method for evaluating the pros and cons of the shot peening process.
The effect of shot peening on the surface roughness of parts
Strengthening shot peening will cause plastic deformation of the sprayed surface of the part, and change the surface roughness of the part. Surface roughness is a kind of microscopic geometric shape error, also called microscopic unevenness. The surface roughness is the same as the surface waviness and the shape error. It belongs to the geometric shape error of the part. The surface roughness has an important influence on the performance of the machine parts. The impact of shot peening on the surface roughness of the material is usually in the range of Ra0.6-20mm. 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 if the surface roughness before spraying is below 6.3mm, shot peening can increase or maintain the original surface roughness. If the original surface roughness is above 6.3mm, the surface roughness after shot peening will decrease. In production practice, in order to obtain a more ideal shot peening surface, we should start from the following aspects: provide a better original surface, the Ra value should be below 6.3mm; choose a reasonable steel shot diameter and shot pressure; After the diameter steel shot is shot-peened, it is covered once with a smaller steel shot at low pressure (the shot peening strength value cannot be changed) to achieve a better surface roughness.
The surface of the parts after shot peening should be lightly polished, and the amount of metal removal on the surface must be controlled during polishing. In this way, the strengthening effect of shot peening is not damaged, and the surface roughness can be improved. Of course, this is a multi-factor problem, no matter what method is adopted, the influence of other factors must be considered at the same time.
The influence of process parameters on shot peening effect
The main factors affecting the quality of shot peening are as follows: shot material, shot diameter, shot speed, shot flow rate, shot angle, shot distance, shot time, coverage rate, etc. The change of any one of these parameters will affect the effect of shot peening to varying degrees.
The influence of steel shot material, hardness, size and particle size on shot peening effect
Cast-iron shots and cast steel shots are usually used for shot peening of hardened gears. The disadvantage of cast iron shot is its low toughness. It is easy to be broken during shot peening and has a large amount of wear. The broken steel shot must be separated in time, otherwise, it will affect the surface quality of the shot. However, the advantages of cast iron shots are low price and high hardness, which can cause high residual compressive stress on the sprayed surface. Compared with cast iron shot, cast steel shot has the advantage that it is not easy to be broken and is beneficial to the geometry of the sprayed surface. However, the hardness of cast steel shot is lower than that of cast iron shot. Under other conditions, the residual compressive stress of the sprayed surface is lower than that of cast iron shot.
For the workpiece to be sprayed, the quality of the steel shot and the speed of the steel shot determine the stability of the shot peening effect. Among them, the quality of the steel shot has a great influence on the effect of shot peening. The general rule is: the diameter of the steel shot is small, the residual stress on the surface of the workpiece is higher, but the strengthening layer is shallow; the diameter of the steel shot is large, the residual stress on the surface of the workpiece is lower, but the strengthening layer Deeper; steel shot hardness is high, shot peening strength is also high; steel shot diameter increases, shot peening strength also increases; steel shot speed increases, shot peening strength, surface compressive stress and strengthening layer depth increase.
Reasonable selection and control of shot peening parameters can achieve good shot peening effects. Under normal circumstances, the diameter of the steel shot is affected by the parts being sprayed. Generally, the diameter of the steel shot should not be greater than half of the fillet diameter of the gear transition area. Steel shots that are too large cannot be sprayed on the round corners of the gear. When the surface roughness is required, smaller steel shots should be used as much as possible. In order to meet the coverage requirements, the shot peening time will increase rapidly as the size of the steel shot increases, and small steel shots can quickly meet the coverage requirements. Therefore, the diameter of the steel shot should not be too large. According to the actual situation, our company chooses steel shots with diameters of φ0.6mm and φ0.8mm, and the effect obtained is ideal.
At the same time, the material of steel shot is also very important. National standards have already given strict specifications on the metallographic structure, chemical composition, minimum density, and hardness deviation range of steel shot. The quality of steel shots of qualified materials should be strictly controlled to ensure uniform spherical shape and size, and sufficient steel shots. The decrease in the amount of steel shot will reduce the corresponding shot peening strength. Therefore, steel shots must be checked at certain intervals, unqualified steel shots must be removed in time, and a certain amount of steel shots must be replaced and increased. Otherwise, the edges and corners of the deformed steel shot are likely to cause micro-cracks on the surface of the sprayed parts and cause fatigue sources. Generally, the number of qualified steel shots should be no less than 80%. The content of qualified steel shots is generally controlled by screens of different specifications
The hardness of the steel shot should consider the hardness of the workpiece material. When the hardness of the steel shot is very close to the hardness of the gear material, the maximum compressive stress and compression depth will not be affected by the hardness of the steel shot. Therefore, when selecting a steel shot, the hardness of the steel shot should be greater than or equal to the hardness of the gear shot peened surface. For carburized gears, it is best to use steel shots with a hardness of 55-65HRC to obtain a satisfactory compressive stress effect.
The influence of steel shot flow rate, speed, and injection angle on shot peening effect
The throwing head is directly driven by a variable frequency motor, and the speed of the throwing head can be changed by changing the frequency of the motor. Under the action of centrifugal force, the steel shot overflows from the hole on the impeller shaft to the blade and then is thrown at a fixed angle by the high-speed rotating blade. The speed of the impeller determines the initial speed of the steel shot. The maximum speed of the motor is 3000r/min.
As the blasting head rotates, steel shots will be continuously thrown out, so the flow of steel shot entering the impeller shaft of the blasting head must be able to ensure that the blasting head has a sufficient supply of steel shots, which requires frequent supplementation of the shot blasting machine's steel shot recovery system, More importantly, the stock of steel shots in the medium is adjusted by adjusting the opening size of the shot control valve to adjust the flow of steel shots passing through the shot control valve into the throwing head. The input volume of the shot blasting machine's steel shot is fixed once it is adjusted. In normal use, the change of the steel shot flow rate is achieved by adjusting the rotation speed of the blasting head, that is, it increases when the input volume of the steel shot remains unchanged. If the impeller rotates, the steel shot flow rate per unit time is larger, and vice versa. On the shot blasting machine, each blasting head has an ammeter connected to it to display the flow rate of the steel shot. When the shot peening quality does not meet the technical requirements, the motor frequency needs to be adjusted. The adjustment is to determine the degree of adjustment through the readings displayed on the ammeter. The reading range of the ammeter is 0-30A.
in conclusion
In the shot peening process, the material surface is subjected to the violent impact of the steel shot to produce a deformation hardened layer, which will cause two effects:
First, the structure causes sub-crystal refinement, dislocation density increases, and lattice distortion increases;
The second is to introduce high macroscopic residual compressive stress.
In addition, the surface roughness is increased due to the impact of the steel shot, which will make the sharp tool marks produced during cutting tend to be smooth. These changes will significantly improve the fatigue resistance and stress corrosion resistance of the material, thereby significantly improving the life of the gear.
