Abstract: After more than 30 hours of use, the shaft of a gear reducer bent and fractured during cold straightening. Through macro- and microscopic analysis of the fracture surface, metallographic examination, and hardness testing, it was determined that the shaft underwent early fatigue fracture under symmetrical rotating bending loads under stress concentration conditions. The cause of the fatigue fracture was an unreasonable heat treatment process, resulting in the material's mechanical properties failing to meet design requirements, leading to reduced fatigue resistance. Poor fillet machining further contributed to stress concentration during operation, accelerating the fatigue fracture. Keywords: Gear reducer; Shaft; Fatigue fracture; Relief groove A gear reducer purchased from abroad by a coal mine bent after more than 30 hours of use, rendering it unusable. During cold straightening, the bent shaft suddenly fractured. Technical data on the reducer shaft indicated that it was made of 17CrNiMo6 steel, and after overall tempering, the surface underwent medium-frequency treatment, achieving a Rockwell hardness of 59–62 HRC on the shaft surface and at the root of the relief groove. 1. Physical and Chemical Testing 1.1 Macroscopic Analysis of Shaft Fracture The fracture is located at the root of the relief groove on the surface of the reducer shaft, as shown in Figure 1. [align=center] Figure 1 Shaft fracture location (mm)[/align] [align=center] Figure 2 Macroscopic fracture morphology[/align] The macroscopic fracture surface is shown in Figure 2. The fracture surface has obvious conchoidal patterns, which is typical of fatigue fracture. The fracture surface consists of three regions: the fatigue crack initiation zone, the crack propagation zone, and the instantaneous fracture zone. Upon close observation of the crack initiation zone, its surface is relatively flat, and its size is within 5 mm from the surface (Figure 2A). The conchoidal lines in the crack propagation zone are relatively flat. The instantaneous fracture zone is opposite the crack initiation zone, and is elliptical in shape. The fracture morphology is fibrous, indicating that the reducer shaft is mainly subjected to rotational bending stress. The instantaneous fracture zone is small and relatively round, accounting for about 1/6 of the entire fracture area, indicating that the overall stress on the shaft is relatively small, which is typical of high-cycle fatigue fracture. The morphology of the fatigue zone and the shell marks indicates that the fatigue crack propagation was faster on both sides, suggesting stress concentration at the root of the relief groove. 1.2 Microscopic analysis of the fracture surface The fracture surface of the sample was observed using an AMRAY21000B scanning electron microscope. The fracture originated at the root of the relief groove on the shaft surface, where there were machining marks (see Figure 3). Fatigue striations were visible in the crack propagation zone (see Figure 4). The instantaneous fracture zone consisted of small dimples. [align=center] Figure 3 Morphology of the fracture origin[/align] [align=center] Figure 4 Fatigue striations in the crack propagation zone[/align] 1.3 Chemical composition analysis The sample was taken from near the fracture surface. The analysis results (mass fraction) are listed in Table 1. The chemical composition meets the technical requirements. Table 1 Chemical composition of the failed shaft 1.4 Rockwell hardness test Samples were taken near the fracture surface. The cross-section was ground flat, and the hardness was measured point by point from the edge to the center. The results were all in the range of 36-37 HRC. The hardness was measured along the longitudinal surface of the shaft, and the results were in the range of 38-39 HRC. The hardness results show that the surface hardness and core hardness of the shaft are similar and both are lower than the design requirements. 1.5 Metallographic inspection Samples were taken near the crack source for metallographic analysis. The non-metallic inclusions were A2, B1, and D1e (evaluated according to GB10561-1989); the grain size was grade 7.5 (evaluated according to GB6394-1986); the microstructure of the fatigue source area, surface and core was tempered sorbite. Through metallographic analysis, it is believed that the shaft was put into use directly without any surface treatment in the quenched and tempered heat treatment state. 2 Analysis and discussion (1) The Rockwell hardness test results of the longitudinal surface and transverse end face of the reducer shaft show that the hardness value of the failed shaft is 36-39 HRC, which is far lower than the technical requirement of 59-62 HRC, obviously not in line with the design requirements. (2) The microstructure of the shaft from the surface to the core is tempered sorbite, indicating that the shaft was used in the quenched and tempered heat treatment state, which is consistent with the measured Rockwell hardness of the shaft. The working condition of the shaft requires that its surface hardness is high and wear-resistant, and the core hardness is relatively low and toughness is good. Under normal circumstances, the shaft surface is generally treated with high frequency or medium frequency before use [1]. However, the tempering and use condition of the failed shaft does not match the theoretical requirement of high frequency or medium frequency surface treatment. Due to the unreasonable process, the fatigue resistance of the shaft is reduced. (3) From the location of the reducer shaft fracture, the fatigue originates from the stress concentration point of the shaft's relief groove. From the microscopic fracture surface, there are three obvious regions: crack initiation zone, propagation zone and instantaneous fracture zone, which are typical fatigue fractures. The fracture surface ripples are relatively flat, the crack propagation speed on both sides of the crack propagation front line is relatively large, and the instantaneous fracture zone is opposite to the crack initiation zone. It can be seen that the failed shaft is mainly subjected to rotational bending stress. From the fact that the instantaneous fracture zone is relatively small and round, the overall stress of the failed shaft is relatively small [2]. Based on the above fracture analysis results and fracture morphology, it is believed that the shaft fracture is a fatigue fracture caused by rotational bending under medium nominal stress concentration conditions. Under rotational bending stress, the shaft's low surface hardness, coupled with stress concentration in the relief groove, causes premature fatigue cracks to form at the relief groove under normal operating stress. With cyclic loading, these fatigue cracks propagate into the shaft matrix, reducing its effective load-bearing capacity and causing bending. During cold straightening, applying a downward force to the shaft's bulge direction leads to fracture. 3. Conclusion The reducer shaft fracture is due to an improper heat treatment process resulting in material mechanical properties lower than design requirements, and stress concentration at the bottom of the relief groove. This reduces the shaft's fatigue strength, leading to fatigue cracks and bending deformation, ultimately resulting in fracture during straightening.