Failure mechanism analysis of broken teeth of thin-walled gear used in mechanical transmission mechanism of airlock in nuclear power plant

It is found that the broken tooth thin-walled gear is a 2 ″ straight cylindrical gear with 12 teeth, which belongs to a special structure displacement gear. In order to determine the root cause of the undercut fracture of the thin-walled gear teeth and provide theoretical guidance and support for the next optimization and improvement of the design of the thin-walled gear, the fracture of the thin-walled gear teeth was dissected, and the failure mechanism was analyzed from the macro fracture state, chemical composition, inclusions, metallographic structure, SEM fracture morphology and hardness.

1.Macro fracture inspection for gear tooth break

It can be seen from the macro fracture of the broken teeth of the thin-walled gear that the fracture is from the whole undercut of the root of the thin-walled gear, the fracture is straight, and there is no feature of fatigue fracture. It is judged that the gear belongs to the cutting of the teeth caused by a serious overload. This kind of fracture is usually due to the fact that the strength of the gear is not enough. When it is impacted instantaneously, the gear is subjected to severe overload load, resulting in the instantaneous brittle fracture.

2.Chemical composition of materials

The chemical composition of gear material was determined according to GB / T 4336-2016 and GB / T 20123-2006. It can be seen from the results that the sulfur content of the measured value is higher than that of the standard component. Sulfur can improve the cutting performance, so a proper amount of sulfur will be added to the free cutting steel, but the high content of sulfur element will affect the mechanical properties. Therefore, sulfur is usually regarded as a harmful element and needs to be controlled. As the main load transmission part in the mechanical transmission mechanism of the airlock door, the thin-walled gear needs to bear a certain torque, which has a high demand for mechanical properties. However, the sulfur content in the thin-walled gear material is obviously high, which has a great impact on the performance of the gear.

3.Inclusion inspection

A large number of inclusions were found in the raw materials of thin-walled gears. The inclusions have a wide range of shape ratio (length / width) and extend in the same direction.

According to GB / T 10561-2005, it can be judged as a type a sulfide inclusion. The inclusions are gray. Combined with the chemical composition analysis, it is known that these inclusions are MNS. MNS is hard and brittle, which has a very adverse effect on the strength of the material.

4.Metallographic microstructure inspection

The metallographic microstructure was analyzed according to GB / T 13298-2015. The metallographic inspection surface is at the root of the gear and perpendicular to the axial direction of the gear. The metallographic microstructure inspection results are shown in the figure.

According to the results of metallographic examination, the structure of thin-walled gear raw material is pearlite + ferrite. Ferrite is distributed in the original austenite grain boundary as a network, forming a network structure. A large number of inclusions can also be seen in the photos.

When the hypoeutectoid steel is cooled slowly after overheating, the ferrite with network distribution is precipitated along the grain boundary. The higher the processing temperature is, the slower the cooling speed is, and the more obvious the grid is. The grain size of this structure is coarse and its plasticity and impact toughness are poor. It can be seen from the metallographic micrograph that the thin-walled gear is in non quenched and tempered state. According to the morphology of inclusions and fracture of gear teeth, it can be concluded that the thin-walled gear belongs to hot rolling state.

5.SEM fracture morphology analysis

The fracture surface of thin-walled gear teeth was analyzed by SEM. It can be seen from the full picture analysis of SEM (the number in the figure is the number of scanning electron microscope microscopic observation position) that the fracture of thin-walled gear teeth is relatively rough, with slight plastic deformation characteristics.

SEM analysis of the fracture surface showed that the fracture surface of the gear teeth were dimple morphology, and dimple morphology was characterized by small hole or strip shape. The conclusion proves that the fracture mode of gear is overload shear fracture.

6.Hardness test

Rockwell hardness test shall be carried out according to GB / T 230.1-2009 to test the hardness of gear surface and gear center respectively. The hardness testing surface of the gear surface is on the tooth surface of the unbroken tooth and parallel to the gear. The hardness testing surface of the gear center is located at the root of the gear and perpendicular to the gear axis.

According to the hardness test results, the average hardness of gear surface is 98.6 HRB (Rockwell hardness), and the average hardness of gear center is 97.6 HRB. The results show that the hardness of the gear surface is similar to that of the center part, which indicates that the gear has not been hardened or hardened.

According to GB / T 33362-2016 for hardness conversion, 97 HRB is approximately equal to 212 HB (Brinell hardness). GB / t3077-2015 stipulates that the hardness of 40Mn2 in annealing or high temperature tempering state shall not be greater than 217 Hb. The results show that the hardness of the thin-walled gear is too high, which further confirms the conclusion that the MNS hard inclusion in the material is too high.