Cause analysis of internal crack in flaw detection of 42CrMo4 gear hub blank

The gear hub is assembled in the drive gearbox of railway vehicles. The material used is 42CrMo4 and the specification of raw material is large. The use mode and product characteristics of railway locomotive gear hub determine the strict quality requirements and service life requirements of gear hub, and then the quality requirements of 42CrMo4 steel for processing gear hub blank are more strict, especially the internal quality must be subject to nondestructive testing. During the nondestructive testing of a batch of 42CrMo4 steel in the gear hub blank processing process, it was found that there were quality defects beyond the standard requirements. According to the flaw detection waveform, it should be a strip defect, and the defect position is basically within the center Φ 150 mm area. In order to determine the nature and causes of defects and prevent the same quality problems in subsequent production, the blank 42CrMo4 material was dissected and analyzed, and the causes of defects were summarized, which provided control and improvement measures for production.

From the above test and analysis results, it can be seen that the microstructure of gear hub blank is flake pearlite + a small amount of massive ferrite, which is a normal structure after forging, and the number of non-metallic inclusions is very small and the level is very low. The normal fracture surface is porcelain like, and the defect separation surface is longitudinal strip structure and a small amount of crystal structure. There are no friction marks on both sides, which indicates that the steel defects are not produced during forging.

The results show that the shape of internal crack is wide in the middle and narrow at both ends. The cracks are distributed in a curved discontinuous curve without obvious transition shape. The defects at both ends are characterized by stress crack. At the same time, there are a small amount of gray inclusions at both ends of the crack, which indicates that the defects will age and stress release with time, and the defects will grow into larger defects along the inclusions.

To sum up, the internal crack defect is the internal micro crack of the ingot, which is not fully welded under the subsequent forging, and then aging occurs and further expands into a larger defect. In the forging process, it can’t be welded completely, and it can be further expanded into larger defects through aging.

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