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Brief Introduction of Three Heat Treatment Processes for Gear Root Surface Hardening

Time: 2021-06-21 Hits: 28

   The strength and load-carrying capacity of gears increase with the tooth surface hardness. Thus, heat treatment technology of gear root surface hardening has been widely used in the production of domestic and external gears. The heat treatment methods used for surface hardening of gears and tooth roots mainly include the following three processes.

1. Carburizing and quenching
   After carburizing and quenching, the gear surface has high hardness, wear resistance and contact fatigue resistance. Meanwhile, the core has high strength, sufficient impact toughness, and good comprehensive mechanical properties. Therefore, the carburizing and quenching process has become the leading technology for machining hardened gears and more and more used.

   The comprehensive mechanical properties of carburized and quenched gears are higher than those of surface induction hardened gears. However, the carburizing and quenching process is complex and the heat treatment deformation is large. Generally, the carburized and quenched gears must be ground to eliminate heat treatment deformation to ensure the gear precision it deserves.

   Since the hardened tooth surface gear after carburizing and quenching is not allowed to grind the round tooth groove part of the tooth root, the hardness of the tooth root surface after carburizing and quenching is increased by carburizing and the hardness of the tooth The residual compressive stress formed on the root surface is preserved, thereby effectively improving the bending fatigue strength of the gear.

2. Glow ion nitriding
    Since the glow ion nitriding is carried out at low temperature, no phase change occurs, especially the heat treatment deformation is small. It has the advantages of fast nitriding speed, short nitriding time, energy saving, high nitriding quality, and strong adaptability to materials. Moreover, the process has a good operating environment and is basically pollution-free.

   Therefore, it has developed rapidly, applied to the surface treatment of hardened gears. Gears after glow ion nitriding generally do not need to be ground. Due to the limitation of the depth of the nitriding layer, the application of heavy-duty gears with large modulus hard tooth surfaces is still limited.

   After the glow ion nitriding, the teeth will not be ground, so the hardness of the tooth root circular tooth groove after the glow ion nitriding surface and the residual compressive stress on the tooth root surface after strengthening shot peening can be preserved, thus effectively improving gear bending fatigue strength.

3. Induction heating surface hardening
   Due to the high heating speed of induction hardening, the surface oxidation and decarburization of the gear can be avoided. Since the gear core is still in a low temperature state and has high strength, the heat treatment deformation is greatly reduced. The quenching quality is high. Due to the fast heating speed, the austenite grains are not easy to grow. After quenching, the surface layer can obtain acicular martensite, and the surface hardness is 2~3HRC higher than ordinary quenching. A series of advantages such as easy control of quenching heating temperature and hardening depth. Therefore, the technology of induction heating surface hardening is developing rapidly.

    Since the hard tooth surface gear after intermediate frequency quenching is not allowed to grind the tooth root groove part during the grinding of the teeth, the hardness of the tooth root surface after the intermediate frequency quenching is increased by the hardening of the surface and the surface of the tooth root formed after strengthening shot peening. The residual compressive stress can be preserved, thereby effectively improving the gear bending fatigue strength.

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