In this study, we investigate the fracture failure of a gear shaft in a rolling mill gear distribution box. The gear shaft, a critical component in the transmission system, experienced brittle fracture shortly after commissioning, leading to significant downtime. We conducted a comprehensive analysis of the fracture morphology, chemical composition, mechanical properties, and metallographic structure to identify the root causes. Based on our findings, we propose improvements in manufacturing processes to enhance the durability and performance of future gear shafts.
The gear distribution box is positioned between the mill reducer and the drive shaft, distributing torque to the upper and lower drive shafts. The failed gear shaft was part of this system, and its fracture occurred at the transition fillet of the herringbone gear and run-out groove. This location was not the design’s maximum stress area, indicating underlying material or processing issues. Our analysis reveals that the gear shaft failure was primarily due to material imperfections, insufficient forging ratio, and suboptimal heat treatment, resulting in mechanical properties below design requirements.
We begin by examining the technical specifications of the gear shaft. The gear shaft was designed with specific parameters to handle high loads in the rolling mill. Key technical details are summarized in Table 1. The material specified was 17CrNiMo6, with carburizing and quenching to achieve a hardened layer depth of at least 4.8 mm and surface hardness of 58-62 HRC. The precision level corresponded to AGMA 11, equivalent to ISO grade 7. Understanding these requirements is essential for evaluating the gear shaft’s performance and failure.
| Parameter | Specification |
|---|---|
| Number of Teeth (z) | 28 |
| Normal Module (m, mm) | 30 |
| Pitch Diameter (d, mm) | 840 |
| Material | 17CrNiMo6 |
| Heat Treatment | Carburizing and Quenching |
| Hardened Layer Depth (mm) | ≥4.8 |
| Surface Hardness (HRC) | 58-62 |
| Precision | AGMA 11 (ISO 7) |
Macroscopic examination of the fracture surface showed characteristics of brittle fracture. The fracture originated from the interior of the gear shaft, with coarse crack propagation tear ridges radiating outward. No fatigue features were observed at the outer edges, suggesting that the failure was sudden and not due to cyclic loading. This brittle behavior indicates inadequate toughness, which we further explored through material testing.
Chemical composition analysis was performed on the fractured gear shaft. The results, compared to the JB/T 6395-2010 standard for 17CrNiMo6, are presented in Table 2. The composition met the standard requirements, with elements like carbon, manganese, chromium, nickel, and molybdenum within specified limits. However, the presence of harmful elements such as sulfur and phosphorus was minimal, but non-metallic inclusions and segregation issues were detected later, highlighting that chemical conformity alone does not guarantee performance.
| Element | Design Range | Measured Value |
|---|---|---|
| C | 0.15-0.20% | 0.18% |
| Mn | 0.40-0.60% | 0.45% |
| Si | ≤0.40% | 0.25% |
| Cr | 1.50-1.80% | 1.53% |
| Ni | 1.40-1.70% | 1.53% |
| Mo | 0.20-0.40% | 0.26% |
| P | ≤0.03% | 0.006% |
| S | ≤0.03% | 0.006% |
Mechanical properties were evaluated through tensile and impact tests at different locations of the gear shaft: near the outer surface and at the R/2 position (mid-radius). The results, detailed in Table 3, show that the yield strength (Rp0.2) and tensile strength (Rm) were below design values. For instance, the longitudinal yield strength near the outer surface was 740 MPa, compared to the required ≥940 MPa, and at R/2, it dropped to 397 MPa. The impact energy averaged 38 J, below the design minimum of 41 J. These deficiencies indicate that the gear shaft lacked the necessary strength and toughness to withstand operational stresses.
| Property | Design Value | Longitudinal (Near Surface) | Longitudinal (R/2) | Tangential (Average) |
|---|---|---|---|---|
| Yield Strength Rp0.2 (MPa) | ≥940 | 740 (739-741) | 397 (395-400) | 528 (462-626) |
| Tensile Strength Rm (MPa) | ≥1080 | 937 (934-941) | 720 (717-724) | 742 (702-810) |
| Elongation A (%) | 8 | 17 | 19 | 17.6 |
| Reduction in Area Z (%) | – | 56 | 47 | 44 |
| Impact Energy Akv (J) | ≥41 | 38 | 57 | – |
Low-magnification organizational analysis revealed defects such as center looseness, shrinkage cavities, cracks, and inclusions from the surface to the core. The longitudinal structure lacked distinct forging flow lines, indicating a casting-like morphology and an insufficient forging ratio. This suggests that the forging process did not adequately refine the microstructure, leading to poor density and segregation.
Metallographic examination further confirmed these issues. The microstructure at the fracture surface included sorbite and a small amount of ferrite near the outer surface, while the R/2 and core regions showed ferrite, sorbite, and pearlite with segregation. Non-metallic inclusions, such as sulfides and oxides, were present throughout, with higher concentrations in the core. Energy dispersive spectroscopy identified oxide spots (e.g., iron oxide) in the core, which exhibited brittle characteristics. The hardened layer of the gear teeth consisted of acicular martensite and retained austenite, but the core microstructure included tempered sorbite and free ferrite, indicating incomplete quenching during carburizing.
The strength analysis from a design perspective indicated that the maximum stress should have occurred at the input side shoulder of the upper gear shaft, not at the fracture location. This discrepancy underscores that material flaws, rather than design oversights, caused the failure. The gear shaft’s internal cracks propagated under stress, culminating in brittle fracture.
Based on our failure analysis, we attribute the gear shaft failure to several factors: brittle fracture initiated from internal defects, inadequate mechanical properties due to material impurities, low forging ratio, and improper heat treatment. The presence of non-metallic inclusions and segregation reduced the gear shaft’s integrity, while insufficient quenching led to soft zones and reduced hardness.
To address these issues, we propose improvements in the manufacturing process for future gear shafts. Referring to standards like GB/T 3480.5-2021, JB/T 13027-2017, and GB/T 37400.8-2019, we recommend enhancing material purity, optimizing forging, and refining heat treatment. Key suggestions are summarized in Table 4.
| Aspect | Improvement Measure | Rationale |
|---|---|---|
| Material Quality | Control harmful elements (S, P ≤ 0.015%), gases (H ≤ 2 ppm, O ≤ 25 ppm, N ≤ 100 ppm), and non-metallic inclusions per ME grade of GB/T 3480.5-2021. Use “steel refining + vacuum casting”. | Reduce inclusions and segregation to improve toughness and strength of the gear shaft. |
| Forging Process | Ensure forging ratio ≥ 4, incorporate high-temperature diffusion, and use normalizing after forging. | Enhance microstructure density and eliminate casting-like features in the gear shaft. |
| Heat Treatment | Apply normalizing + high-temperature tempering for preheating. Control quenching temperature and cooling rate post-carburizing. Aim for core hardness ≥ 35 HRC. | Achieve uniform hardness and microstructures like low-carbon martensite in the gear shaft core. |
| Testing | Sample from R/3 position per GB/T 37400.8-2019 for mechanical properties. | Verify gear shaft performance in critical regions. |
For material quality, controlling non-metallic inclusions is crucial. The allowable inclusion levels for the gear shaft should adhere to ME grade specifications, as shown in Table 5. This ensures a cleaner material with fewer stress concentrators.
| Inclusion Type | Thin Series | Thick Series |
|---|---|---|
| A (Sulfide) | 2.5 | 1.5 |
| B (Alumina) | 2.0 | 1.0 |
| C (Silicate) | 0.5 | 0.5 |
| D (Globular) | 1.0 | 1.0 |
| Ds (Single Type) | 2.0 | – |
In terms of forging, a higher forging ratio improves the grain flow and density. The forging process for the gear shaft should include high-temperature diffusion to mitigate chemical segregation. The relationship between forging ratio and mechanical properties can be expressed using the formula for true strain: $$ \epsilon = \ln\left(\frac{A_0}{A_f}\right) $$ where \( A_0 \) is the initial cross-sectional area and \( A_f \) is the final area. For a forging ratio of 4, \( \epsilon = \ln(4) \approx 1.386 \), which promotes recrystallization and homogeneity in the gear shaft.
Heat treatment modifications are essential for achieving the desired microstructure. The preheating treatment should involve normalizing followed by high-temperature tempering to refine the grain structure. During carburizing, the gear shaft should be slowly cooled to room temperature and then reheated for quenching to prevent distortion and cracks. The core hardness after quenching should meet: $$ \text{Core Hardness} \geq 35 \, \text{HRC} $$ This ensures sufficient strength to resist crack propagation. The quenching process can be modeled using the cooling rate equation: $$ \frac{dT}{dt} = -k(T – T_{\text{medium}}) $$ where \( k \) is the heat transfer coefficient, and \( T_{\text{medium}} \) is the quenching medium temperature. Controlling this rate avoids excessive thermal stresses in the gear shaft.
Additionally, mechanical testing should be conducted on samples taken from the R/3 position (one-third of the radius from the surface) to represent the critical stress areas. This aligns with GB/T 37400.8-2019 and ensures that the gear shaft meets performance standards throughout its cross-section.
In conclusion, the failure of the gear shaft was primarily due to material and processing deficiencies, including internal segregation, inclusions, low forging ratio, and improper heat treatment. These led to brittle fracture and inadequate mechanical properties. By implementing the proposed improvements—such as enhancing material purity, increasing forging ratio, and optimizing heat treatment—future gear shafts can achieve higher reliability and performance. This analysis provides a foundation for refining manufacturing processes and preventing similar failures in heavy-duty applications.
Further research could focus on advanced non-destructive testing methods to detect internal flaws in gear shafts during production. For instance, ultrasonic testing or computed tomography could identify inclusions and cracks early. Moreover, finite element analysis (FEA) could simulate stress distributions in the gear shaft under load, using equations like the von Mises stress criterion: $$ \sigma_v = \sqrt{\frac{(\sigma_1 – \sigma_2)^2 + (\sigma_2 – \sigma_3)^2 + (\sigma_3 – \sigma_1)^2}{2}} $$ where \( \sigma_1, \sigma_2, \sigma_3 \) are principal stresses. This would help validate design improvements and ensure the gear shaft operates within safe limits.
Overall, a holistic approach combining material science, mechanical engineering, and quality control is essential for producing robust gear shafts. By addressing the root causes identified in this study, manufacturers can enhance the longevity and efficiency of rolling mill components, reducing downtime and maintenance costs.