Microstructural and finite element simulation analysis of the crack formed during heat treatment in the carburized 22CrMoH steel gear shaft

This study analyzed the crack formed during heat treatment in the carburized 22CrMoH gear shaft. Qualitative analysis of composition, observation of microstructure and morphology analysis of crack section were carried out by Optical Microscope (OM) and Scanning Electron Microscope (SEM) with an Energy Dispersive Spectrometer (EDS). The results showed that a large size abnormal inclusion with a length of 2.4 mm and a width of 0.2 mm existed in the crack initiation region 10 mm from the surface of the gear shaft. The inclusion was composed of Ca, Al, Mg and Si oxides. The finite element simulation results showed that a significant stress concentration existed near the inclusion, and the maximum value exceeded the yield strength of the 22CrMoH gear shaft during quenching process and led to the generation of cracks. Long strip MnS inclusions were observed in the crack propagation direction and promoted the crack propagation. In addition, the finite element method was used to simulate the stress concentration of inclusions with different length and positions in the gear shaft during quenching. The results showed that the stress concentration increased with the decrease of the distance between the inclusion and the surface or the increase of the inclusion size. In order to avoid the recurrence of cracks, the steel plant was advised to strictly control the size and positions of inclusions. After improvement, cracks did not appear again.