In modern industrial applications, the gear shaft serves as a critical component in reducers, functioning as a power transmission mechanism that converts the rotational speed of motors through gear-based speed converters to achieve desired output speeds and higher torque. This gear shaft is indispensable in various sectors due to its role in ensuring efficient power transmission under heavy loads, high speeds, and intense impact conditions. However, the gear shaft is prone to developing fatigue cracks during operation, which significantly influences the lifespan of gear reducers. To guarantee long-term safety and reliability, stringent requirements are imposed on the surface quality of the gear shaft, with magnetic particle indications being a key acceptance criterion.
Magnetic particle detection and penetrant testing are commonly employed methods for identifying surface defects. Penetrant testing is suitable for detecting open surface defects in non-porous materials, whereas magnetic particle testing is applied to inspect surface and near-surface flaws in ferromagnetic components. The principle of magnetic particle testing involves the magnetization of ferromagnetic materials; if local distortions occur in the magnetic flux lines at or near the surface, leakage magnetic fields form. When magnetic particles are applied, they accumulate at these leakage fields, revealing the position, shape, and size of defects. This process can be described by the magnetic flux density equation: $$ B = \mu H $$ where \( B \) is the magnetic flux density, \( \mu \) is the permeability of the material, and \( H \) is the magnetic field strength. The presence of defects alters \( \mu \), leading to detectable leakage fields.
In a specific case involving an 18CrNiMo7-6 gear shaft used in reducers, dense magnetic marks were observed along the axial direction on the shaft diameter surfaces after rough machining. These magnetic marks were detected using an MT-15000B magnetic particle testing machine with direct electrification and continuous wet methods. The gear shaft, manufactured from forged round steel with a diameter of 135 mm, exhibited these defects primarily at the stepped diameter regions, such as 60 mm, 70 mm, and 75 mm sections. The manufacturing process for the gear shaft includes steps like cutting, rough turning, ultrasonic testing, magnetic particle inspection, gear hobbing, carburizing, quenching, shot peening, precision turning, grinding, keyway milling, and final grinding, followed by magnetic particle testing before storage. This comprehensive process ensures the integrity of the gear shaft, but the emergence of magnetic marks highlights underlying material issues.
To investigate the root cause, a series of physical and chemical tests were conducted on the affected gear shaft. The chemical composition was analyzed using a direct reading spectrometer according to GB/T4336, and the results confirmed compliance with standard requirements, as summarized in Table 1. The composition plays a vital role in the performance of the gear shaft, as deviations could exacerbate defects like segregation and porosity.
| Element | Standard Requirement | Actual Measurement |
|---|---|---|
| C | 0.15-0.20 | 0.17 |
| Si | ≤0.40 | 0.33 |
| Mn | 0.50-0.90 | 0.79 |
| P | ≤0.020 | 0.015 |
| S | ≤0.020 | 0.007 |
| Ni | 1.40-1.70 | 1.52 |
| Cr | 1.50-1.80 | 1.70 |
| Mo | 0.25-0.35 | 0.29 |
| Cu | ≤0.30 | 0.04 |
Macrostructural examination was performed on transverse samples taken from different sections of the gear shaft, including the main body (125 mm diameter), one side diameter (90 mm), and the defective diameter (75 mm). The macrostructures were evaluated according to GB/T1979, and the results, detailed in Table 2, indicated that the main body met standard requirements with no significant issues. However, in the smaller diameter regions, porosity and segregation zones were exposed on the surface due to extensive machining, which correlated with the magnetic marks. The macrostructural integrity can be quantified using the porosity index formula: $$ P = \frac{V_p}{V_t} \times 100\% $$ where \( P \) is the porosity percentage, \( V_p \) is the volume of pores, and \( V_t \) is the total volume. Higher \( P \) values in the gear shaft diameters explain the magnetic indications.
| Parameter | Standard Requirement | Gear Shaft Body (125 mm) | Side Diameter (90 mm) | Defective Diameter (75 mm) |
|---|---|---|---|---|
| General Porosity | ≤2.0 | 1.5 | 1.8 | 2.2 |
| Center Porosity | ≤2.0 | 1.5 | 1.7 | 2.1 |
| Ingot Pattern Segregation | ≤2.0 | 1.5 | 1.6 | 2.0 |
Metallographic analysis of longitudinal samples from the gear shaft body and the defective diameter region, etched with 4% nitric alcohol, revealed uniform microstructure in the body with a banding structure of grade 1.5, whereas the defective diameter showed severe segregation with a banding structure of grade 2.5. No high-level non-metallic inclusions were detected, emphasizing that the magnetic marks stemmed from intrinsic material imperfections rather than external contaminants. The banding structure can be modeled using the segregation coefficient: $$ k = \frac{C_s}{C_l} $$ where \( k \) is the partition coefficient, \( C_s \) is the solute concentration in the solid, and \( C_l \) is the solute concentration in the liquid during solidification. Values of \( k \) deviating from 1 indicate pronounced segregation, which adversely affects the gear shaft performance.
Further magnetic particle testing on longitudinally sectioned samples, using a GDC-4000 multifunctional fluorescent magnetic particle tester, displayed bright white bands in the central regions corresponding to porosity zones observed in macrostructural tests. This alignment confirms that the magnetic marks are directly linked to the material’s internal defects. The magnetic particle accumulation can be described by the force equation: $$ F_m = \nabla (B \cdot H) $$ where \( F_m \) is the magnetic force driving particle accumulation at defect sites. In the gear shaft, this force highlights areas with compromised structural integrity.
An investigation into the production process of the forged round steel revealed that it was manufactured from a 650 mm continuous cast round billet using a 30 MN fast forging press and a 16 MN precision forging machine in a single heat, with a forging ratio of 12 and sufficient deformation, followed by annealing at 650°C. No abnormalities were noted during production, and all initial tests met standards. However, the inherent segregation and porosity in the original billet were exacerbated during machining of the gear shaft, particularly in smaller diameters where material removal was extensive. The relationship between machining depth and defect exposure can be expressed as: $$ D_e = D_i – 2 \times M_d $$ where \( D_e \) is the effective diameter after machining, \( D_i \) is the initial diameter, and \( M_d \) is the machining depth. As \( M_d \) increases for smaller diameters, the core defects of the gear shaft become surface-visible, leading to magnetic marks.
Statistical analysis of the gear shaft defects showed that magnetic marks predominantly occurred at the stepped diameters, with severity increasing as the diameter decreased due to greater machining exposure of the core material. This underscores the importance of optimizing the gear shaft design and processing to minimize defect revelation. The stress concentration factor \( K_t \) at these steps can be calculated using: $$ K_t = 1 + 2 \sqrt{\frac{a}{\rho}} $$ where \( a \) is the defect depth and \( \rho \) is the radius of curvature. Higher \( K_t \) values in machined regions of the gear shaft amplify the magnetic indications.
In conclusion, the magnetic marks on the gear shaft surface are primarily caused by porosity and segregation in the forged round steel, which become exposed during machining of smaller diameters. This exposure results in magnetic particle indications during inspection. To mitigate these issues, several improvements are recommended: reducing the content of segregation-prone elements like phosphorus and sulfur, optimizing continuous casting parameters such as pouring temperature and cooling rate to promote equiaxed crystal growth, extending forging heating and holding times for better homogenization, and adopting near-net shape forging to minimize machining and preserve the sound surface layer of the gear shaft. Implementing these measures will enhance the reliability and longevity of the gear shaft in reducer applications, ensuring consistent performance under demanding conditions.
The comprehensive analysis demonstrates that the gear shaft’s magnetic marks are not due to external factors but inherent material characteristics. By addressing these through refined manufacturing processes, the quality of the gear shaft can be significantly improved, reducing the incidence of such defects in future productions. This approach aligns with industry standards and emphasizes the critical role of material science in advancing gear shaft technology for high-performance applications.