According to the above analysis, in order to thoroughly solve the undercut fracture accident of thin-walled gear and ensure the safety and reliability of the on-site operation of the mechanical transmission mechanism of air gate in the short, medium and long-term operation of the nuclear power plant in service, the optimization and improvement of thin-walled gear will be carried out from the aspects of material selection, process control and structural design improvement.

1. Material selection

According to the above inspection results, from the material point of view, too many MNS inclusions are the root cause of brittle fracture of gears, so it is necessary to control the content of S. On the one hand, the smelting analysis shall be increased to strictly control the content of each element within the scope required by the standards and specifications; on the other hand, the inclusion inspection shall be increased to strictly control the inclusion content, and the class a inclusion shall not exceed 0.5.

The gear material is usually made of low alloy steel or carbon structural steel with high strength and good toughness. The commonly used gear materials are 40Cr, 20CrMnTi and 20CrMnMo in GB / T 3077-2015. These brands of materials can be considered in material selection design.

2. Process control

In the manufacturing process control of thin-walled gear, the following improvement measures are recommended:

① In order to ensure the overall toughness of the surface and interior of the thin-walled gear, the material with good hardenability should be selected, and the forging processing and manufacturing should be considered;

② In the forming process of thin-walled gear, rolling or shaving should be preferred;

③ In order to ensure the overall strength of the thin-walled gear, the thin-walled gear should be tempered;

④ In order to improve the surface hardness and wear resistance of gears, carburizing treatment can be carried out on the surface of thin-walled gears.

3. Structural design optimization

In the aspect of structural design, the 12 tooth displacement gear is used in the mechanical transmission mechanism of the original airlock. This kind of displacement gear has a special structure. The root of the gear is a weak link, and the gear teeth are prone to undercut fracture. Therefore, the standard gear should be considered in the optimization of gear structure, and the standard gear with 18 teeth is recommended. In addition, in order to ensure the overall toughness of the surface and interior of the gear, the gear shaft and gear are forged as a whole, and the tooth is hobbed.

The change of the teeth number of thin-walled gears will cause the change of the transmission ratio of the whole mechanical transmission system of the airlock door. In order to ensure that the transmission ratio of the whole mechanical transmission system of the airlock door remains unchanged, other gears with transmission relationship with the thin-walled gear should be designed adaptively. In the case of a certain number of teeth, choose a reasonable module of gear to make each gear stress more reasonable, and ensure that the transmission torque load on the gear does not exceed the allowable limit value, so as to avoid overload failure of thin-walled gear teeth.