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Brief Introduction of Gear Strengthening Shot Peening

Time: 2021-06-04 Hits: 24

   Gear-strengthening shot peening mainly uses the process of continuously hitting the tooth surface with high-speed shot pellets, just like countless small hammers hammering the tooth surface, causing extremely strong plastic deformation of the tooth surface, and cold work hardening of a certain thickness of the tooth surface. The hardened layer is the strengthening layer of the tooth surface, which strengthens the tooth root surface and generates residual compressive stress on the tooth root surface, achieving a surface cold treatment strengthening technology that improves the fatigue strength of the gear and increases the service life.

1. Purpose and key parts
   Through gear strengthening shot peening, the stress state of the tooth root surface is improved to achieve the goal of anti-fatigue and long life of the gear. Due to the transmission torque, the tooth root is subject to great alternating bending stress, especially the tooth root must have sufficient strength. Therefore, the focus of gear strengthening shot peening is to strengthen the tooth root surface.

2. Effect of gear strengthening shot peening
    (1) Enhanced shot peening can significantly improve the fatigue strength of gears

   After the gear is strengthened by shot peening, due to the transformation of retained austenite to martensite and the combined effect of high residual compressive stress, surface work hardening and microstructure refinement and the hardness of the tooth surface is significantly improved.

   After the gear is heat-treated, the tooth surface is continuously impacted by the high-speed jet of projectiles, which can transform the retained austenite into martensite. The martensite needles are obviously smaller and denser than the unshot peened gear, which can refine the martensite. The role of body substructure is conducive to the increase of residual compressive stress.

  After the gear is strengthened and shot peened, the maximum residual compressive stress value is about 0.05~0.10 (mm) under the tooth surface, and the residual compressive stress can be as high as -800~-1200MPa. When there are micro cracks in the tooth root, the residual compressive stress can inhibit the propagation of the crack. When the depth of the residual compressive stress layer is about 5 times the depth of the crack, the effect of the crack can be eliminated. The residual compressive stress has a great influence on the bending fatigue strength of the gear, and the residual compressive stress of the tooth surface can reduce the peak value of the bending tensile stress.

   (2) Shot peening can significantly increase the service life of gears

   According to relevant data, strengthening shot peening of gears subjected to cyclic bending and alternating loads can effectively increase their fatigue limit. Strengthening shot peening can particularly effectively increase the fatigue strength of stress concentration parts. It can eliminate the machining process (Such as gear tooth surface grinding and shaft gear shaping treatment). At the same time, it can improve the stress concentration caused by continuous cutter marks or grooves, holes and transition fillets and other structural factors. After strengthening shot peening, the dispersion of the gear life can be significantly reduced, and the gear life can be prolonged several times, tens of times, or even hundreds of times.

3. Precautions in gear strengthening shot peening
   (1) The non-destructive testing (magnetic particle testing and color testing) of the gear should be carried out before the gear strengthening shot peening, because the surface plastic flow during strengthening shot peening will cover up the fine cracks.

   (2) Higher temperature (above 232°C) will release the residual compressive stress and weaken the shot peening effect. According to relevant data, Professor He Jiawen of Xi’an Jiaotong University has conducted a special study on this compressive stress. The pressure gradually attenuates with the rise of temperature, generally at around 400℃º. Therefore, all heat treatment processes must be completed before strengthening shot peening.

   (3) Generally speaking, all mechanical processing should also be completed before strengthening shot peening. Before gear strengthening shot peening, all processing items should be checked to see if they meet the requirements of the drawings, including dimensions, form and position tolerances, surface roughness and other requirements (including internal and external fillet and polishing requirements, etc.).

   (4) Before gear strengthening shot peening, dust, oil, corrosion, etc. on the surface of the part should be removed; unless otherwise specified, the protective layer of the part should also be removed.

   (5) At present, the surface residual compressive stress of heavy duty gears with hard tooth surfaces is required in foreign countries to be -800~-1200MPa, which is difficult to achieve by carburizing and quenching alone. This requirement can be met by secondary shot peening. The value of residual compressive stress formed by strengthening shot peening is measured by X-ray diffractometer.

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