Powerful Shot Peening Technology in Gear Machining

　　An important method to improve the bending fatigue strength and contact fatigue strength of gear processing teeth is an important way to improve gear seizure resistance and increase gear life.

　　working principle

　　The powerful shot peening process mainly uses small steel shots sprayed at high speed to hit the surface of the sprayed workpiece at room temperature, causing the surface material of the workpiece to produce elastoplastic deformation and present a higher residual compressive stress, thereby improving the surface strength and fatigue strength of the workpiece.Shot peening on the one hand causes elastic deformation of the surface of the part, 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:

　　  Shot peened surface of parts    

 The impact of shot peening on the surface morphology and performance of parts without shot peening is mainly manifested in changing the surface hardness, surface roughness, stress corrosion resistance and fatigue life of the parts.The material surface of the part undergoes cyclic plastic deformation under the impact of the steel shot bundle.According to the nature and state of the material, the surface of the material will undergo the following changes after shot peening: hardness change, organizational structure change, phase transition, surface residual stress field formation, surface roughness change, etc.

　　Shot peening strength measurement method

　　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) by striking it, and make it reach the strength of the saturated state within a specified time, and use the arc height of the test piece 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 in the United States, and was developed by SAEJ442a and SAEJ443a. The main point of the measurement method specified in the SAE0.35 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 the 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, then remove the test piece, and then use 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 with the test piece is 0.35mm, it is recorded as XNUMXA.

　　Another inspection method of shot peening strength is residual stress inspection, which is to inspect the residual stress of the workpiece after strong shot peening. The specific inspection method is X-ray diffraction.The following methods are 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 measurement point of the depth of the root fillet surface. The size of the irradiation area can be controlled by directing the beam and appropriately covering the root surface; 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 tooth root ground gear used for subsurface residual stress measurement 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 lies in the changes in the surface structure of the material caused by cold deformation, the introduction of residual compressive stress, and the 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 produce elastic and plastic deformation. The impact site will produce a crater due to plastic deformation, and the impact will cause the surface material near the crater to develop 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 surface material to extend radially due to plastic changes. The area becomes larger and larger, and the plastically deformed surface is 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 gradually reduce the thickness of the plastic deformation layer 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 off.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. Since 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 a compressive stress.

　　Since 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 size and distribution of the residual stress.Therefore, the accurate measurement 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

　　Intensified 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 known as microscopic unevenness.The surface roughness is the same as the surface waviness and the shape error, which 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 in general, 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 the 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 peening pressure; After the diameter steel shot is shot peened, it is covered once with a smaller steel shot at a 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 impaired, 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.