What kind of steel shot particle size is used for casting shot blasting

　　1 Selection of steel shot

　　Since the properties of steel shot have a great relationship with cleaning efficiency, surface roughness and cleaning cost, the various indicators of three types of steel shot A, B and C are analyzed.The results show that: in terms of particle size distribution, C steel shot is larger, B steel shot is slightly larger, and only A steel shot particle size distribution meets the national standard. In terms of hardness distribution, the hardness required for cleaning castings is HRC40-50. The hardness of B and C steel shots exceeds HRC50. Only A steel shot meets the requirements, and the extreme difference is small, only HRC5.8.The density is directly related to the life of the steel shot. If the density of the steel shot is low, it means that there are more pores and cracks inside the steel shot, or the carbon content is high, and it is easy to break during use. A steel shot has few pores and microcracks, high density, and a service life of 2744.15 revolutions; B and C steel shots have lower density, more pores and microcracks, and correspondingly lower service life, only 2452.05 revolutions and 2218.42 revolutions. In addition, the forming process of the three steel shots is different, but they are all treated with secondary quenching and tempering. The three steel shots are tempered martensite structures, but the structure of C steel shot is coarse, which has a certain impact on the service life, while the microstructure of A and B steel shots is finer. After comprehensive analysis, A steel shot was selected for the test.

　　2. Test

　　The steel shot used in the test is S460 graded A steel shot, which is composed of 50% S460 + 30% S390 + 20% S330. 10t of material is added at one time, and the time for cleaning the workpiece is 45s, while the cleaning time of the original single-size S460 steel shot is 50s. Due to the good quality of steel shot, the wear of steel shot is a gradual process. In the future, it is only necessary to regularly and quantitatively replenish the steel shot of nominal size to keep the gradation unchanged. Figure 5 is one of the surface roughness measurement results. For good steel shot, the surface of the steel shot peels off layer by layer during the cleaning process, the steel shot gradually becomes smaller, and the steel shot particle size composition of the entire system will slowly become smaller. Therefore, it is best to replenish steel shot continuously to ensure that the number and particle size composition of steel shot remain unchanged, thereby ensuring the stability of cleaning quality. The a curve shown in Figure 6 is too much feeding at one time, and the e curve is too long in the feeding time interval. Both phenomena cause the cleaning efficiency to decrease and the surface roughness to change. In the process of shot blasting, the change of the replenishment method and particle size of good steel shot will have a greater impact on the cleaning efficiency. On the one hand, in the process of steel shot being accelerated on the blade, the large-particle steel shot leaves the blade first, and the small-particle steel shot leaves the blade later. The particle size composition of the steel shot remains unchanged, and the hot spot of the blasting head remains unchanged. If the particle size of the steel shot changes and the hot spot leaves the workpiece, the cleaning efficiency will decrease. On the other hand, the cleaning efficiency is not only related to the impact force, but also to the coverage rate. Large-particle steel shots have a large impact force and good effect, but the number of particles per kilogram is limited, the coverage rate is low, the cleaning efficiency is not high, and the surface is rough; small-particle steel shots have a small impact force, and the cleaning effect of each steel shot is poor, but the number of particles per kilogram of steel shots is large, so the coverage rate is high, and it can clean the fine details, reducing the surface roughness value. Graded steel shots contain 40%~50% of nominal-size steel shots and gradually smaller steel shots, taking into account the impact force and coverage rate during cleaning, so the cleaning efficiency is high and the surface roughness value is low. Figure 7 shows a schematic diagram of steel shots of different particle sizes cleaning castings. It can be seen that the cleaning efficiency of graded steel shots is high.

　　3. Conclusions

　　(1)The surface roughness of the casting cleaned by shot blasting is closely related to the particle size of the abrasive used and the projectile speed. The larger the particle size, the higher the surface roughness life value; the higher the projectile speed, the higher the surface roughness value; the surface roughness value obtained by graded steel shots is lower than that of single-particle steel shots. (2) The use of graded steel shot has a great influence on the cleaning efficiency to reduce the surface roughness of the casting, and can improve the cleaning efficiency of the casting. (3) The feeding method has a great influence on the cleaning efficiency and surface roughness. Timely replenishment of feeding and keeping the steel shot gradation unchanged can ensure that the cleaning efficiency does not decrease and the surface quality of the casting is stable. (4) Shot blasting time has a certain influence on the surface roughness of the casting. As time goes by, the surface roughness value of the casting tends to a certain value.