Hongxin steel shot finishing the new technology of shot peening and strengthening, no more nonsense, read the summary, understand you become an expert!

　　1. Hard shot peening technology of gear

　　(1) Hard peening technology

　　Hard shot peening is different from conventional shot peening, but 700hv high hardness steel shot is used for high-strength shot peening, which makes the A-type test piece produce more than 0.6mm arc height, and form a larger residual compressive stress, so as to obtain high fatigue strength. It is suitable for high quality carburizing steel (for example, Dsg1 steel, Si content is reduced below 0.15%, which greatly reduces internal oxidation and non martensite; it can reduce the content of brittle element P in grain boundary < 0.015%, and increase the content of Mo to 0.4%, which greatly improves the toughness of carburizing layer). It has good effect in eliminating internal oxidation and other carburizing defects and ensuring the toughness of carburizing layer.

　　(2) Two shot peening (double shot peening) technology

　　For gears with carburizing and quenching hardness above 600hv, it is difficult to achieve high pressure stress through normal shot peening. Therefore, the second shot peening is used to improve the fatigue strength, that is, the 700hv high hardness shot is used for high-strength shot peening (such as 0.6mm steel shot) first, and the A-type test piece produces an arc height of more than 0.6mm to obtain a certain depth of surface strengthening layer, and then the small small low-strength shot peening (such as 0.08mm steel shot) is used for one low-strength shot peening, which can form residual compressive stress on the workpiece surface and the secondary surface. The purpose of the second shot peening is to reduce the surface work hardening, improve the surface roughness, improve the surface compressive stress, that is to further improve the fatigue performance of gears. This technology belongs to cold working treatment.

　　2. Application of new hard shot peening process

　　Example 1: for spur gears, materials Dsg1 (chemical composition (mass fraction,%) are 0.20c, < 0.15si, 0.70mn, < 0.015p, 0.015s, 1.00cr and 0.40mo) and SCM420 are carburized, Quenched and tempered. Finally, centrifugal shot peening machine and ￠ 0.8mm cast steel shot (chemical composition is 0.85c-1si-1mn) are used for shot peening, and the shot peening intensity is 0.45mma and 0.7mma arc height respectively. The former belongs to conventional shot peening, while the latter belongs to hard shot peening. Gear fatigue test is carried out on the current consuming gear fatigue test machine.

　　(1) Shot peening parameters and test results

　　The shot peening parameters of test gear are shown in Table 2. It can be seen from table 2 that the root fatigue strength of hard shot peened gear is higher than that of conventional shot peening. The surface hardness and residual compressive stress of hard shot peened gear increase, while the residual austenite and internal oxidation degree decrease.

　　(2) Comparison of hard shot peening, small shot peening and secondary shot peening

　　After shot peening, the maximum residual stress of the workpiece is about 0.05mm below the surface, but the surface is lower than this value. In order to solve this defect, small shot (diameter < 0.1mm) is used for low-strength shot peening. The three shot peening process parameters are shown in Table 3.

　　The highest surface hardness was obtained by hard shot peening, followed by secondary shot peening and small shot peening. Small shot peening makes the surface of gear get very high pressure stress, up to 1.2gpa. And the second shot peening can get the highest fatigue strength. Compared with the carburized and quenched gear, the fatigue strength is increased to 1.5 times. The gears in carburized state have cut marks, which disappear after shot peening, and the surface roughness of gears is improved. As the machining tool mark may become the stress concentration part, the improvement of machining surface during shot peening is also a reason to improve the fatigue strength.

　　1. Projectile and its quality requirements

　　After chemical heat treatment, the surface hardness of gears has reached 58 ~ 63hrc. When shot peening is carried out, shot peening with hardness higher than or equal to the surface hardness should be used to ensure the residual compressive stress value and the depth of surface strengthening layer. In addition, the selection of shot size should ensure that the root of gear is strengthened. In the process of shot peening, because the chosen shot diameter is too large, the root of small radius can not be jet strengthened. To do this, make sure that the diameter of the projectile is less than half the radius of the dedendum.

　　Because the quality of steel shot has a great influence on the strengthening effect, the national standard has given strict specifications for the metallographic structure, chemical composition, minimum density and hardness deviation of steel shot.

　　In the process of using, the bullets are damaged continuously, so it is necessary to supplement new bullets in time. The qualified number of bullets should be kept over 80%. Therefore, different specifications of screens can be selected to control, so as to ensure the effect of shot peening. Because the broken projectile has sharp angle, it is easy to produce many tiny sharp cuts on the workpiece surface, thus reducing the strengthening effect. Therefore, the broken projectile should be screened out in the production process.

　　Selection principle of shot size: ① when there is a requirement for surface roughness, smaller steel shot shall be used as far as possible. ② When the workpiece has internal and external fillets and grooves, the diameter and size of the projectile shall be smaller than the radius of the internal and external fillets and the width of the groove. ③ Ensure that the spherical shape and size of the projectile are uniform.

　　2. Steel wire shot cutting

　　The new generation of surface cleaning and strengthening high-quality shot material produced by pre passivation steel wire shot cutting technology not only has uniform hardness, uniform size and long service life, but also has low dust production rate in production, which can be called green and environmental protection shot, and can be applied to gear shot peening strengthening treatment, etc.

　　3. Strong toughness projectile and its chemical formula

　　(1) Shot for shot peening

　　Generally, cast steel type or tangent type is adopted. Generally, cast steel shot is produced by water atomization and tempered to 392 ~ 513hv. For hard shot peening, the hardness of shot is required to reach about 700hv.

　　(2) New steel shot

　　The dendrite structure of medium carbon cast steel was improved by centrifugal atomization. At the same time, the carbon content is required to be no less than 0.5%, and the content of Mn, s and P are also reduced. The chemical composition is shown in Table 4. During shot peening, the flow rate of shot is 0.75kg/s, the spray speed is 106m / s, and the arc height after shot peening is 1.0mma. As the second choice of high-strength and toughness projectile, it is high-strength tangent projectile pretreated with high carbon, with the size range of ￠ 0.8mm. In order to obtain high hardness of about 700hv, its carbon content increases to 0.8%, and its chemical composition is shown in Table 5.