Abstract: Through analysis of the macroscopic and microscopic morphology and metallographic structure of the fracture surface, it was found that defects inherent in the forging itself led to cracks in the spiral bevel gear shaft during quenching. The cracks then propagated radially and axially until they penetrated longitudinally. Keywords: Spiral bevel gear shaft; crack; banded structure; defect 0 Introduction A batch of five spiral bevel gear shafts made of 20Cr2Ni4 material all developed longitudinal penetrating cracks at different time intervals after the carburizing and quenching + low-temperature tempering heat treatment process. After the first three spiral bevel gear shafts cracked, the heat treatment process of the remaining two shafts was tracked. During the tracking, it was found that 15 minutes after the low-temperature tempering, a small axial crack first appeared in the middle of the shaft body (the shaft body was copper-plated for waterproofing). The crack then propagated axially towards both ends until it penetrated the entire shaft, with a crack depth of 23% of the diameter. The crack initiation area and sampling location are shown in Figure 1. [align=center]Figure 1 Cracked spiral bevel gear shaft[/align] 1. Macroscopic Fracture Analysis The macroscopic fracture morphology of the spiral bevel gear shaft is shown in Figure 2. Firstly, the fracture surface is relatively clean, without oil or other contaminants, indicating that the macroscopic cracking occurred after tempering. The fracture surface is clearly divided into three parts: Part 2 is the near-surface area of ​​the workpiece; the fracture surface is gray, relatively flat, and has a rapidly expanding herringbone pattern, with a shear lip on the outer surface, which is the final crack zone; Part 3 is a bright gray area, which is also the crack propagation zone; Part 1 is grayish-brown, relatively rough, with uneven small facets, and the pits are grayish-brown and glossy. Based on the morphology, color, and propagation pattern of the macroscopic fracture surface, Part 1 is preliminarily identified as the crack initiation zone. [align=center]Figure 2 Macroscopic fracture morphology of the spiral bevel shaft[/align] 2 Microscopic analysis Axial specimens were ground near the fracture surface. It was observed that cracks in zones 2 and 3 on one side of the fracture were relatively flat, while the crack in zone 1 was serrated, with angular pores and branch cracks beside the crack, as shown in Figure 3. [align=center]Figure 3 Condition near crack in zone 1 (100×)[/align] Figure 4 shows the axial microstructure of zone 1, revealing obvious banded microstructure. This is due to regional segregation of the material composition. The banded microstructure significantly reduces the transverse plasticity and toughness of the steel. [align=center]Figure 4 Axial banded microstructure in zone 1 (400×)[/align] 3 Scanning electron microscopy fracture analysis Scanning electron microscopy observation of the fracture surface in zone 1 revealed numerous irregularly shaped areas, as shown in Figure 5. It can be seen that the surface of this defect is smooth, very thin and brittle, and poorly bonded to the matrix. It also has microcracks and pores. According to the energy dispersive spectroscopy micro-area analysis, the main component is Fe, with a content of 99.44%, and other elements are residual. It is believed that this is a defect inherent in the forging itself. The presence of this thin film defect will affect the intergranular bonding force. At the same time, some intergranular fracture characteristics were found in this area, as shown in Figure 6. Therefore, it can be further confirmed that area 1 is the crack initiation area. [align=center] Figure 5 Area 1 SEM 79.4× Figure 6 Area 1 SEM 500×[/align] Fracture morphology of area 2: Quasi-cleavage + some dimples, with a single brittle inclusion. Fracture morphology of area 3: Quasi-cleavage + dimples, with loose and non-dense defects found on it. 4 Conclusion (1) Through the above series of experimental observations, it can be determined that the crack first occurs in area 1, and then propagates radially and axially. Areas 2 and 3 are crack propagation areas. (2) The presence of non-dense defects such as angular pores and porosity in the steel, as well as axially non-uniform structures, all have an adverse effect on mechanical properties. (3) Through comprehensive analysis of the fracture surface by scanning electron microscopy, it was determined that there is an irregularly shaped thin film defect in the forging (zone 1). Its surface is smooth and has microcracks and pores. It is precisely because of the presence of this defect that the intergranular bonding force in this area is greatly reduced, and the microcracks and pores on it become dangerous stress sources. In the subsequent heat treatment process, especially during quenching, the workpiece will be subjected to large thermal stress, which will inevitably lead to the formation of microcracks at the defect, which become the crack initiation area. Some of these cracks will extend along the grain, and under the continuous action of stress, the cracks will continue to extend radially and axially until they form macroscopic longitudinal through cracks.