In the automotive industry, the drive axle assembly plays a critical role in power transmission, and the spiral bevel gear set is a core component. The gear shaft, particularly its threaded section, is essential for securing connections and transmitting torque. However, during the assembly of a rear axle drive system, we encountered multiple instances of thread fracture in the gear shaft, which severely impacted the reliability and service life of the entire assembly. The fracture occurred near the transition radius (R-area) of the gear shaft, presenting a flat fracture surface with metallic luster. Through detailed analysis, we identified that the root cause was related to inadequate heat treatment processes and surface machining defects, leading to stress concentration and premature failure. This article comprehensively discusses our investigation, including macro and micro examinations, metallographic analysis, hardness testing, energy-dispersive spectroscopy, and low-magnification studies, all conducted from our perspective as engineers involved in failure analysis. We also propose effective improvement measures to prevent recurrence, emphasizing the importance of optimizing heat treatment and machining parameters for gear shaft integrity.
The gear shaft in question is manufactured from 20CrMnTiH steel, a material commonly used for its high strength and hardenability. The thread fracture was observed during the assembly process, where the fastener torque induced stress concentrations at the R-area. Initial visual inspection revealed that the fracture originated from one side of the surface and propagated radially, with a distinct blackened region at the crack initiation site. The fracture surface exhibited two clear stages: an initial extension zone with slight oxidation and a secondary extension zone that appeared fresher. This prompted us to delve deeper into the underlying causes, focusing on the gear shaft’s material properties, heat treatment history, and surface conditions.
To understand the fracture mechanism, we performed a macro-morphological analysis of the failed gear shaft. The fracture was located adjacent to the R-area, characterized by a flat and glossy surface. The crack initiation point showed a radial pattern, indicating a brittle fracture mode. The presence of a blackened area at the source suggested oxidative effects, possibly due to pre-existing microcracks that were exposed during heat treatment. This observation led us to hypothesize that surface imperfections, combined with thermal stresses, contributed to the failure. The gear shaft’s design, including the transition R-area, was scrutinized for potential stress risers, as any abrupt change in cross-section can amplify stress under load.
Further examination using scanning electron microscopy (SEM) revealed critical details at the micro-scale. At the crack initiation sites, we observed mixed modes of tearing and dimples, along with localized oxidation that obscured morphological features. This indicated the presence of original microcracks, which likely served as stress concentrators. The subsurface regions exhibited intergranular brittle fracture, consistent with high-stress conditions. In the primary extension zone, the fracture morphology was predominantly intergranular, with slight oxidation, while the secondary zone showed cleavage and dimple patterns, suggesting a shift in failure mode as the crack propagated into the core material. These findings reinforced our suspicion that the gear shaft’s surface integrity was compromised during manufacturing.
We conducted a thorough metallographic analysis to assess the material’s microstructure. Samples were sectioned longitudinally along the gear shaft and prepared for examination. The non-metallic inclusions were evaluated according to standard methods, and the results are summarized in the table below. This analysis confirmed that the material itself met specifications, with no significant abnormalities in inclusion levels.
| Inclusion Type | Grade |
|---|---|
| Sulfides | 0.5 (Fine Series) |
| Alumina | 0.5 (Fine Series) |
| Silicates | 0.5 (Fine Series) |
| Globular Oxides | 1 (Fine Series) |
At the crack initiation site, we identified surface folding defects, with depths of approximately 20 μm. These folds exhibited a “pouch-like” morphology and branching, indicating machining-induced imperfections. The surface layer showed an oxide layer with a depth rating of 4 (12–20 μm), suggesting that the microcracks were present prior to heat treatment and were exacerbated during processing. The microstructural analysis of the thread area revealed a carburized layer composed of acicular martensite, carbides, and retained austenite, which is undesirable for thread regions due to its brittleness. In contrast, the core microstructure consisted of finer constituents, as expected for 20CrMnTiH steel. The presence of acicular martensite in the thread surface indicated inadequate heat treatment control, leading to excessive hardness and susceptibility to cracking.
Hardness testing was performed on the thread surface to evaluate uniformity and compliance with technical requirements. We measured Rockwell hardness (HRC) at various points, including the thread crest and root. The results, tabulated below, showed significant inconsistency, with values ranging from 31.4 HRC to 52.0 HRC. This non-uniformity, coupled with areas exceeding the specified range of 32–45 HRC, highlighted the inefficiencies in the heat treatment process for the gear shaft.
| Location | Hardness (HRC) |
|---|---|
| Thread Crest | 52.0, 45.0, 48.9 |
| Thread Root | 50.5, 31.4, 50.7 |
Energy-dispersive spectroscopy (EDS) was employed to analyze the chemical composition at the crack initiation and nearby regions. The spectra primarily indicated the presence of oxygen and iron, with trace amounts of chromium and manganese, consistent with surface oxidation of the original microcracks. As we moved away from the crack source, the oxygen content decreased, confirming that oxidation was most severe at the initiation point. No anomalous elements were detected, ruling out material contamination as a factor in the gear shaft failure.
Chemical composition analysis of the gear shaft core material was conducted to verify conformity with 20CrMnTiH standards. The results, presented in the table below, confirmed that the elemental composition was within specified limits, further supporting that the failure was not due to material deficiencies but rather processing issues.
| Element | Measured Value (%) | Standard Range (%) |
|---|---|---|
| C | 0.19 | 0.17–0.23 |
| Si | 0.26 | 0.17–0.37 |
| Mn | 1.01 | 0.80–1.10 |
| P | 0.016 | ≤0.035 |
| S | 0.0035 | ≤0.035 |
| Cr | 1.16 | 1.00–1.30 |
| Ti | 0.056 | 0.04–0.10 |
Low-magnification metallographic examination of transverse and longitudinal sections from unfractured regions of the gear shaft revealed no significant defects such as segregation, porosity, shrinkage, slag inclusions, bubbles, or cracks. This indicated that the base material was sound, and the issues were localized to the surface and heat treatment zones.
Based on our analysis, we concluded that the thread fracture in the gear shaft was primarily caused by a combination of factors: unsuitable microstructure due to improper heat treatment, non-uniform hardness, and poor surface roughness from machining. The original heat treatment process involved overall carburizing and quenching followed by high-frequency induction annealing, which resulted in a brittle martensitic structure on the thread surface. Additionally, machining imperfections, such as tool-induced microcracks at the R-area, acted as stress concentrators. Under assembly preload forces, these microcracks propagated, leading to fracture. The stress concentration at the R-area can be described by the theoretical stress concentration factor formula: $$ K_t = 1 + 2\sqrt{\frac{a}{\rho}} $$ where \( a \) is the crack length and \( \rho \) is the radius of curvature. For the gear shaft, a small R-radius and surface defects significantly increased \( K_t \), promoting crack initiation.
To address these issues, we implemented several improvements focused on the gear shaft. First, we modified the heat treatment process: instead of overall carburizing and quenching, we applied an anti-carburizing coating to the thread and R-area prior to heat treatment. After carburizing and quenching, we performed high-frequency induction annealing specifically on the threaded section. This approach ensured a more ductile microstructure, such as pearlite, carbides, and少量 bainite, while maintaining adequate strength. The revised process parameters are summarized in the table below, which outlines the optimized heat treatment stages for the gear shaft.
| Process Stage | Temperature (°C) | Carbon Potential (%) | Media |
|---|---|---|---|
| Pre-oxidation | 430 | — | Air |
| Heating | 910 | — | Methanol |
| Strong Carburizing 1 | 920 | 1.2 | Propane |
| Strong Carburizing 2 | 920 | 1.1 | Propane |
| Diffusion | 910 | 0.85 | Nitrogen |
| Cooling | 850 | 0.78 | — |
| Quenching | 860 | 0.9 | Fast Quench Oil |
| High-Frequency Annealing | As per Table 2 | — | Induction Heating |
The high-frequency induction annealing parameters were carefully controlled to achieve uniform heating and cooling. The table below details the steps involved in this process for the gear shaft thread, ensuring precise temperature and time management to avoid residual stresses.
| Step | Instruction | Coordinates (mm) | Time (min) | Speed (mm/min) | Power (V/A) |
|---|---|---|---|---|---|
| NO010 | G1 | X-35.1 | — | F1200 | — |
| NO020 | M03 | — | — | — | — |
| NO030 | M24 | X-19.4 | — | — | — |
| NO040 | G4 | — | X0.5 | — | — |
| NO050 | M32 | — | — | — | 200 V, 45 A |
| NO060 | G4 | — | X17 | — | — |
| NO130 | G1 | X-63.5 | — | F80 | — |
| NO150 | M23 | — | — | — | 210 V, 48 A |
| NO180 | G4 | — | X20 | — | — |
| NO190 | M33 | — | — | — | — |
| NO200 | M08 | — | — | — | — |
| NO210 | G1 | — | — | F1200 | — |
Additionally, we increased the R-radius from 3 mm to 4 mm to reduce stress concentration. The modified geometry decreases the stress concentration factor, as shown by the formula: $$ \sigma_{\text{max}} = K_t \cdot \sigma_{\text{nom}} $$ where \( \sigma_{\text{max}} \) is the maximum stress, \( K_t \) is the stress concentration factor, and \( \sigma_{\text{nom}} \) is the nominal stress. By increasing the radius, \( K_t \) is reduced, thereby lowering the peak stress in the gear shaft. We also optimized the thread length to prevent rolling onto the R-area during machining, minimizing the risk of microcrack formation. After implementing these changes, we produced over 10,000 units of the improved gear shaft, with no instances of thread fracture reported during assembly or service, demonstrating the effectiveness of our approach.
To quantify the improvement, we re-evaluated the hardness of the gear shaft thread after process modification. The results, presented in the table below, show consistent values within the required range of 32–45 HRC, confirming better control over the heat treatment process.
| Location | Hardness (HRC) |
|---|---|
| Thread Crest | 42.3, 41.0, 38.9 |
| Thread Root | 40.5, 33.4, 40.7 |
Furthermore, we considered the fatigue life aspect of the gear shaft. The crack propagation rate can be modeled using Paris’ law: $$ \frac{da}{dN} = C (\Delta K)^m $$ where \( da/dN \) is the crack growth per cycle, \( \Delta K \) is the stress intensity factor range, and \( C \) and \( m \) are material constants. By reducing surface defects and optimizing the microstructure, we effectively decreased \( C \) and \( m \), extending the fatigue life of the gear shaft. Our improvements also involved enhancing surface finish through better tool maintenance and reducing the number of parts processed per tool to ensure higher roughness quality.
In summary, the fracture of the gear shaft thread was a multifactorial issue rooted in heat treatment-induced brittleness, hardness inconsistencies, and machining flaws. Our comprehensive analysis, incorporating metallography, hardness testing, and spectroscopy, enabled us to identify these contributors. The implemented measures—such as anti-carburizing coatings, optimized high-frequency annealing, and geometric modifications—have proven successful in eliminating thread fractures. This case underscores the importance of integrated process control in manufacturing critical components like the gear shaft, where even minor deviations can lead to catastrophic failures. Future work will focus on continuous monitoring and refinement of these parameters to ensure long-term reliability.