Shot peening is a process that uses high-speed jetting of sand shot and iron shot to impact the surface of the workpiece to improve some mechanical properties of the part and change the surface state. It can be used to improve the mechanical strength of parts, wear resistance, fatigue resistance and corrosion resistance, etc. It can also be used for surface matting, descaling and optimizing the residual stress of casting, forging and welding parts.
The shot peening process is the process of spraying a large number of projectiles onto the surface of the part, just like countless small hammers hammering the surface. Therefore, the surface of the metal part produces extremely strong plastic deformation, which makes the surface of the part produce a certain thickness of cold work hardening layer, which is called Surface strengthening layer, this strengthening layer will significantly improve the fatigue strength of the part.
Before understanding shot peening technology, it is necessary for us to explain the three confusing concepts of shot blasting, sand blasting, and shot peening.
These three concepts are actually four words: spray, throw, shot, sand. Among them, blasting is the process method, and shot sand is the material used. Spraying is to use high-pressure air to blow shot and sand to the surface of the workpiece, and throwing is to use high-speed rotating blades to project the shot to the surface of the workpiece. The shot is made of steel shots, and the sand is made of quartz sand.
Characteristics of parts after shot peening
The distribution law of the stress along the depth direction after spraying is expressed by the distribution curve of the shot peening residual stress. The surface residual compressive stress, the depth of the compressive stress layer, the maximum residual compressive stress and the position of the maximum residual compressive stress are four characteristic quantities.
Among them, the surface compressive stress and the thickness of the compressive stress layer have a more obvious impact on the surface strengthening characteristics of the part. In addition to the properties of the sprayed material itself, the size of the surface residual compressive stress and the depth of the compressive stress layer mainly depend on the shot peening intensity and surface coverage.
Generally speaking, appropriately increasing the shot peening intensity and shot peening coverage will help increase the shot peening effect, but it will also cause an increase in surface roughness. For shot peening coverage, when the coverage is insufficient, the residual compressive stress of the surface layer is relatively large, but stress relaxation is prone to occur. Therefore, it is necessary to reasonably select the shot peening intensity and shot peening specimen in combination with the material characteristics and strengthening requirements, so that the shot peening process can maximize the strengthening effect.
Changes in the material structure of the sprayed surface
Sprayed surfaces become rough. The metal on the sprayed surface is squeezed out, forming tiny metal peaks, thus affecting the surface roughness. With the increase of shot peening intensity, the decrease of surface hardness and the prolongation of shot peening time, the surface roughness will also increase.
Three factors affecting shot peening
There are three basic parameters for evaluating the quality of reinforced pellets: strength, coverage, and surface roughness.
1. Shot peening intensity
The process parameters that affect the shot peening intensity mainly include: projectile diameter, elastic flow velocity, projectile flow rate, shot peening time, etc. The larger the diameter of the projectile, the faster the speed, the greater the momentum of the collision between the projectile and the workpiece, and the greater the intensity of the shot peening. The residual compressive stress formed by shot peening can reach 60% of the tensile strength of the part material, the depth of the residual compressive stress layer can usually reach 0.25mm, and the maximum limit value is about 1mm. The shot peening intensity needs a certain shot peening time to guarantee. After a certain period of time, the shot peening intensity reaches saturation, and then the shot peening time is extended, and the intensity will no longer increase significantly. In the Almen test of shot peening strength, the shot peening strength is characterized by the crown height of the deformation of the test piece.
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2. Shot peening coverage
Some people often think this way about the shot peening coverage rate: My nozzle sprays the workpiece 2 times with 1 up and 1 down, can it meet the 200% coverage rate? It sounds reasonable at first glance, but it is not the case.
The measurement of coverage is as follows: first coat a layer of colored glaze or fluorescent glaze on the surface of the workpiece, then blast the workpiece according to the process parameters, take out the workpiece after spraying the surface once, and observe the residual under a microscope (magnifying glass). The proportion of the coating on the surface, if there is 20% remaining, the coverage rate is 80%. When there are only 2% residues, that is, the coverage rate is 98%, it can be regarded as completely cleared, that is, the coverage rate is 100%, and there is a time at this time. If 400% coverage is achieved, that is 4 times the time.
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3. Surface roughness
Due to the injection of steel shot, the roughness of the workpiece surface has a certain change. The factors that affect the surface roughness are the strength and hardness of the part material, the diameter of the projectile, the angle and speed of the spray, and the original surface roughness of the part.
Under the same other conditions, the higher the strength and surface hardness value of the part material, the more difficult the plastic deformation, the shallower the crater, and the smaller the surface roughness value; the smaller the diameter of the projectile, the slower the speed, the shallower the crater, The surface roughness value becomes smaller; the larger the spray angle, the smaller the normal component of the projectile velocity, the smaller the impact force, the shallower the crater, the greater the tangential velocity of the projectile, and the greater the abrasive effect of the projectile on the surface. The smaller the roughness value; the original surface roughness of the part is also one of the influencing factors. The rougher the original surface, the smaller the decrease in the surface roughness value after shot peening; on the contrary, the smoother the surface, the rougher the surface after shot peening.
When the parts are subjected to high-intensity shot peening, deep craters not only increase the surface roughness value, but also form a large stress concentration, which seriously weakens the effect of shot peening.
