In this investigation, we analyze the failure of a straight bevel gear used in the peeling mechanism of a corn harvester. The straight bevel gear is critical for transmitting torque and altering motion direction during the peeling process. However, frequent tooth fractures were observed during initial harvesting operations, leading to operational failures. This analysis aims to identify the root causes using various testing methodologies, including hardness measurements, metallographic examination, and chemical composition analysis. The focus is on the straight bevel gear’s material properties and heat treatment processes, as improper parameters can lead to catastrophic failures. Throughout this report, the term “straight bevel gear” will be emphasized to highlight its significance in agricultural machinery.
The straight bevel gear is manufactured from 20CrMnTi steel, as per design specifications, which requires carburizing and quenching heat treatments. Key technical requirements include a carburized layer depth of 0.8 to 1.2 mm, surface hardness of 58 to 62 HRC, and core hardness of 33 to 48 HRC. However, no specific microstructural requirements were outlined in the design documents. To understand the failure, we conducted a series of tests on both fractured and unused straight bevel gear samples, labeled as Sample A and Sample B, respectively. Both samples were produced in the same batch with identical processing and heat treatment conditions, allowing for comparative analysis.
Initial visual inspection of the fractured straight bevel gear (Sample A) revealed that the tooth broke near the root area, exhibiting characteristics of brittle fracture. No significant stress concentrators, such as machining marks or surface irregularities, were observed on either Sample A or Sample B. This suggests that the failure might be related to material or heat treatment issues rather than external factors. The straight bevel gear’s role in withstanding impact loads during peeling necessitates a balance of surface hardness and core toughness, which could be compromised by improper carburizing.
Chemical composition analysis was performed on the raw material used for the straight bevel gear to ensure it met standard specifications. The results, obtained using optical emission spectrometry, are summarized in Table 1. The composition aligns with GB/T 3077-1999 standards for 20CrMnTi steel, indicating that the material itself is not the primary cause of failure. This reinforces the need to examine the heat treatment processes applied to the straight bevel gear.
| Element | C | S | Mn | Si | P | Cr | Ti |
|---|---|---|---|---|---|---|---|
| Measured Value | 0.20 | 0.021 | 0.97 | 0.24 | 0.02 | 1.15 | 0.06 |
| Standard Range (20CrMnTi) | 0.17–0.23 | ≤0.035 | 0.80–1.10 | 0.17–0.37 | ≤0.035 | 1.00–1.30 | 0.04–0.10 |
Hardness testing was conducted on the straight bevel gear samples to evaluate both surface and core hardness. Samples were sectioned using wire cutting, cleaned, and tested at specific locations: the surface near the pitch circle and the core at the intersection of the tooth centerline and root circle. Rockwell hardness (HRC) measurements, as shown in Tables 2 and 3, indicate that surface hardness meets the design requirements. However, core hardness values exceed the specified range, suggesting potential over-carburization of the straight bevel gear. This deviation could lead to reduced toughness and increased brittleness.
| Measurement Point | 1 | 2 | 3 | Average |
|---|---|---|---|---|
| Surface Hardness | 60 | 61 | 60 | 60.3 |
| Core Hardness | 57 | 57 | 56 | 56.7 |
| Measurement Point | 1 | 2 | 3 | Average |
|---|---|---|---|---|
| Surface Hardness | 60 | 60 | 60 | 60.0 |
| Core Hardness | 57 | 57 | 58 | 57.3 |
Microstructural analysis was performed on transverse sections of the straight bevel gear teeth after polishing and etching with 4% nital solution. Observations at 400x magnification revealed abnormal structures. The surface microstructure of both samples consisted of acicular martensite and significant retained austenite, with no visible carbides, as depicted in the micrographs. The core microstructure showed tempered martensite and retained austenite, which is atypical for 20CrMnTi steel after carburizing and quenching. Normally, the core should contain tempered martensite and ferrite (if fully hardened) or ferrite and troostite (if not fully hardened). The presence of retained austenite in the core indicates that carburization may have penetrated the entire tooth cross-section, altering the material properties of the straight bevel gear.
To quantify the carburized layer depth, Vickers hardness testing was conducted across the tooth cross-section, following standards such as GB/T 9450-2005. Hardness values were measured at various distances from the surface, and the results for Samples A and B are presented in Tables 4 and 5. The hardness distribution curves, derived from these data, show that hardness remains elevated even at the core, exceeding 550 HV, which is the threshold for defining effective case depth. This confirms that the carburized layer extends through the entire tooth, violating the design specifications for the straight bevel gear. The hardness profile can be modeled using an exponential decay function, such as:
$$ HV(x) = HV_{\text{surface}} \cdot e^{-kx} + HV_{\text{core}} $$
where \( HV(x) \) is the hardness at distance \( x \) from the surface, \( HV_{\text{surface}} \) is the surface hardness, \( HV_{\text{core}} \) is the core hardness, and \( k \) is a decay constant. For the straight bevel gear, the high hardness values throughout indicate minimal decay, supporting the over-carburization hypothesis.
| Distance from Surface (mm) | 0.2 | 0.4 | 0.6 | 0.8 | 1.0 | 1.2 | 1.4 | 1.6 | 1.8 | 2.0 | 2.5 | 3.0 | 4.5 | 6.0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Hardness (HV) | 700 | 698 | 696 | 696 | 694 | 692 | 692 | 690 | 690 | 684 | 682 | 676 | 647 | 635 |
| Distance from Surface (mm) | 0.3 | 0.5 | 0.7 | 0.9 | 1.1 | 1.3 | 1.5 | 1.7 | 1.9 | 2.1 | 2.5 | 3.0 | 4.5 | 6.0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Hardness (HV) | 696 | 696 | 694 | 694 | 694 | 693 | 692 | 691 | 690 | 685 | 683 | 678 | 644 | 639 |
The discussion centers on how over-carburization and improper quenching contribute to the failure of the straight bevel gear. Carburizing is intended to create a hard, wear-resistant surface while maintaining a tough core. However, excessive carburizing depth, as observed, results in full-section hardening. After quenching and tempering, this leads to high hardness but low toughness throughout the straight bevel gear tooth. The core hardness values of 56-57 HRC far exceed the design limit of 33-48 HRC, making the gear susceptible to brittle fracture under impact loads. Additionally, the coarse martensitic structure and high retained austenite content at the surface, due to elevated quenching temperatures above the Accm point, further increase brittleness. The straight bevel gear experiences significant shock during corn peeling, and the compromised material properties cannot withstand these stresses, leading to tooth breakage.
To mathematically represent the stress on the straight bevel gear tooth, we can use the bending stress formula for gear teeth:
$$ \sigma_b = \frac{F_t \cdot K_a \cdot K_v \cdot K_m}{b \cdot m \cdot Y} $$
where \( \sigma_b \) is the bending stress, \( F_t \) is the tangential force, \( K_a \) is the application factor, \( K_v \) is the dynamic factor, \( K_m \) is the load distribution factor, \( b \) is the face width, \( m \) is the module, and \( Y \) is the Lewis form factor. For the straight bevel gear, if the material toughness is reduced, even normal operational loads can cause \( \sigma_b \) to exceed the fracture strength, resulting in failure.
Further analysis involves the carbon diffusion during carburizing, which can be described by Fick’s second law:
$$ \frac{\partial C}{\partial t} = D \frac{\partial^2 C}{\partial x^2} $$
where \( C \) is the carbon concentration, \( t \) is time, \( D \) is the diffusion coefficient, and \( x \) is the distance from the surface. Prolonged carburizing time or high temperature increases \( D \), leading to deeper carbon penetration in the straight bevel gear. In this case, the process parameters were not controlled, causing carbon to reach the core and homogenize the hardness profile.
In conclusion, the primary cause of straight bevel gear tooth fracture is the excessive carburized layer depth, which results in full hardening and reduced toughness. The straight bevel gear’s microstructure, characterized by coarse martensite and retained austenite, further exacerbates brittleness. To prevent such failures, it is essential to strictly control carburizing parameters, such as time and temperature, to achieve the specified case depth. Additionally, quenching should be performed at optimal temperatures to avoid grain growth and excessive retained austenite. Regular quality checks, including hardness and microstructural examinations, should be implemented for every batch of straight bevel gears. Collaborating with heat treatment suppliers to refine processes based on these findings will enhance the reliability of straight bevel gears in agricultural applications.
This comprehensive analysis underscores the importance of precise heat treatment in manufacturing durable straight bevel gears. Future work could involve finite element analysis to simulate stress distributions and optimize the gear design for better performance. By addressing these issues, the service life of straight bevel gears in corn harvesters can be significantly improved, ensuring efficient and uninterrupted harvesting operations.