Analysis of Causes of Motor Shaft Fracture of Air Compressor in a Power Plant

Zhang Liubin, Qi Jinfeng, Wang Xiaochen (Henan Electric Power Company Electric Power Research Institute, Zhengzhou 450053, Henan Province, under the impact of loading and unloading, internal cracks expand and fracture occurs.

2 Air compressor motor shaft, a fracture accident, 2 air compressor is the M 250 air compressor produced by Ingersoll Rand, the motor shaft material is 42 steel.

1 On-site inspection and macro analysis On-site inspection found that the motor shaft break is located in the drive side bearing sleeve, about 200mm away from the connecting gear. It is understood that during the operation of the shaft, there was a collapse accident and caused the shaft to wear out. The power plant's external committee carried out spraying treatment. It was found that the entire surface of the shaft near the fracture was sprayed and the thickness was about 1 mm. The motor shaft fracture had obvious radiation pattern, and there were many metallurgical defects on the section, and the crack source was torn and broken by the metallurgical defect.

At the scene, 2 air compressor motor current records were taken. It was found that the air compressor load was continuously adjusted before the shaft broke, and the motor current fluctuated between 1432A every 1H. 12 hours before the incident, the air compressor was always running with load, and the current was maintained at around 30A. In February 2012, around 17æ›°2120, the current rose to 32A, and the motor shaft broke.

2 Fracture test analysis The X-MetB000TX hand-held spectrometer was used to test the motor shaft. It was found that the motor shaft base and the sprayed layer contained two elements, Fe and Mn, and no other alloying elements were found. Both materials were carbon steel.

2.1 Section Microstructure Morphology Detection Under the stereo microscope, the fracture morphology was observed. There were many metallurgical defects inside the fracture, and the crack cracked along the metallurgical defect.

The sample was lighted and etched with 4% nitric acid solution. The sample section showed four different microstructures (see). The cross-section of the sample is different, and the morphology of the regional structure is not the same. Among them, the microstructure of the coating is in the position of the metallographic phase. The position of the metallographic structure of the coating is similar to that of the metallographic structure. At the location of the sprayed layer, the microstructure of the structure is wavy, and it is impossible to determine the normal tissue morphology of the non-carbon steel.

It can be seen that the shape of the near-spray layer is composed of two kinds of microstructures, and the species of needle-shaped martensite is pro-eutectoid ferrite + pearlite. The reason for the occurrence of acicular martensite is that the temperature at the time of spraying exceeds the phase change point of the shaft material Ac1, and no tempering occurs after air cooling.

It can be seen that the microstructure at the position near the substrate is pro-eutectoid ferrite + pearlite.

It can be seen that the microstructure of the substrate at the position of the substrate is pearlite + Wei's body structure, which is a typical superheated microstructure.

It can also be seen from the figure that the crack originates from the internal metallurgical defect and expands from the inside to the outside, and the crack opening is large.

The metallographic structure of the matrix position. 2 Hardness test Using the Leeb hardness tester, the hardness test of the motor shaft base and spray coating is carried out. The test results show that the hardness of the base is about 156 HB, and the hardness of the sprayed layer is about 356HB, which is very different.

3 Reason analysis (1) It can be seen from the fracture morphology, macroscopic and microscopic examination that there are a large number of metallurgical defects and macroscopic cracks inside the motor shaft base. The crack originates from the metallurgical defects inside the matrix and expands from the inside to the outside.

(2) The common shaft body should be in a quenched and tempered structure, that is, quenching + high temperature tempering structure. The metallographic structure of the broken shaft matrix is ​​a coarse pearlite structure and a Wei's body structure, which belongs to the normalizing structure. When the normalizing heat is heated, the superheating time is longer, resulting in coarse tissue. The division of the Wei's structure and the coarse structure to the matrix reduces the strength and impact toughness of the motor shaft and increases the brittleness.

(3) It can be seen from the hardness test results that the hardness difference between the sprayed layer and the substrate is about 200HB, which results in a large surface stress at the joint surface; the heating temperature is too high when the motor shaft is sprayed and repaired, resulting in hardened martensite in the local region of the shaft body. Organization, these factors promote the cracking and expansion of defects under the action of external loads.

(4) It can be seen from the motor current recording curve that the air compressor is frequently loaded and unloaded during normal operation, and the impact force is continuously formed on the drive side of the motor shaft. The internal defects of the matrix are continuously expanded under the impact force, and finally the brittle fracture of the motor shaft is caused.

4 Conclusion The manufacturing quality of the motor shaft body is seriously unqualified, and there are macroscopic cracks and a large number of metallurgical defects inside the matrix. Under the impact force generated by the loading and unloading during the normal operation of the air compressor, the internal crack expands and breaks.

In the manufacturing process of the motor shaft, the manufacturer shall adopt qualified forgings, strictly formulate the heat treatment process, and conduct product quality inspection. When the power plant purchases the equipment, the relevant drawings and quality certification documents should be obtained.

It is recommended to perform random sampling on the remaining motor shafts to avoid such problems.

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