In the field of turbine and fan manufacturing, bearing boxes play a critical role in supporting and lubricating bearings, ensuring smooth operation under demanding conditions. As an engineer specializing in foundry technology, I have developed a robust casting method for a complex multi-tubing bearing box made of ductile iron castings. This article details the entire process, from design to production, focusing on overcoming challenges such as irregular oil tube integration, thermal deformation, and quality assurance. The methodology emphasizes simulation-backed design, precise工艺控制, and innovative solutions for镶铸 components, ensuring high-integrity ductile iron castings. Throughout this discussion, I will incorporate tables and formulas to summarize key aspects, reinforcing the technical depth required for such advanced ductile iron castings.
The bearing box in question is a front bearing box for steam turbines, fabricated from QT400-18 ductile iron. Its intricate design includes multiple embedded oil tubes, which are essential for lubrication but pose significant铸造 challenges. The success of this project hinges on a meticulous approach that addresses every facet of the casting process, particularly for ductile iron castings where shrinkage and porosity risks are prevalent. By leveraging凝固 simulation and practical工艺 adjustments, I achieved a first-time success, demonstrating the viability of this method for similar ductile iron castings.
Features and Structure of the Bearing Box Casting
The bearing box is a sizable component with overall dimensions of 2,613 mm in length, 1,056 mm in width, and 740 mm in height. It weighs approximately 3,187 kg, featuring substantial wall thickness variations ranging from 40 mm to 220 mm. Such disparities necessitate careful thermal management during solidification to prevent defects in these ductile iron castings. The box incorporates four oil tubes: two straight tubes and two that are three-dimensionally curved. These tubes are镶铸 using steel pipes (ASTM A106 GR.B) to form integral conduits within the ductile iron matrix. The specifications of these tubes are summarized in Table 1, highlighting their critical role in the functionality of ductile iron castings.
| Tube Type | Inner Diameter (mm) | Length (mm) | Wall Thickness (mm) |
|---|---|---|---|
| Straight Oil Tube | 90 | 740 | 11 |
| Curved Oil Tube | 26 | 450 | 11 |
The curved tubes, with their complex spatial geometry, require precise shape control and positioning during casting. Additionally, fusion rings with a thickness of 4 mm are welded onto the tubes to enhance bonding with the ductile iron castings. This integration is vital for preventing leakage and ensuring structural integrity in such ductile iron castings.
Technical Quality Requirements
To meet the high standards for turbine components, the bearing box must satisfy stringent mechanical and non-destructive testing (NDT) criteria. These requirements are essential for ensuring the reliability and longevity of ductile iron castings in critical applications. The mechanical properties, as outlined in Table 2, must be achieved consistently across the casting.
| Property | Requirement |
|---|---|
| Yield Strength (N/mm²) | ≥ 300 |
| Tensile Strength (N/mm²) | ≥ 420 |
| Elongation (%) | ≥ 11 |
| Hardness (HB) | 155–200 |
For NDT, 100% magnetic particle inspection (MT) and 100% ultrasonic testing (UT) are mandated. UT standards require Level 2 for critical areas like flange faces and Level 3 for other regions, ensuring no internal flaws compromise these ductile iron castings. The oil tubes themselves must be free of cracks, porosity, and internal obstructions, with strict tolerances on roundness (e.g., less than 2 mm deviation in curved sections). Pre-casting treatments, such as degreasing, pickling, and shot blasting, are applied to the tubes, followed by镀锡 to improve fusion with the ductile iron castings. The镀锡 specifications are detailed in Table 3, which is crucial for achieving a metallurgical bond in ductile iron castings.
| Coating Aspect | Requirement |
|---|---|
| Coating Hardness (HB) | 5 |
| Appearance | Bright White |
| Thickness (μm) | 25–30 |
Casting Method and Process Design
As the lead engineer on this project, I identified several key challenges specific to these ductile iron castings. First, the three-dimensionally curved oil tubes presented difficulties in dimensional verification, shape control, and precise positioning within the mold. Second, thermal expansion of the tubes during pouring could cause deformation or mold damage, leading to defects like sand inclusion. Third, the box includes long, thin cored holes (e.g., 60 mm diameter over 560 mm height) that require accurate core alignment to avoid machining issues. To address these, I developed a comprehensive工艺方案 centered on robust core design, tube integration strategies, and optimized gating and risering systems for ductile iron castings.
Manufacture of镶铸 Oil Tubes
The镶铸 tubes are fabricated from steel according to material specifications. For the curved tubes, I extended one end by 60 mm to facilitate支撑 and positioning in the mold. Shape accuracy is verified using custom metal gauges, ensuring conformity to design图纸 before casting. To prevent fusion defects, the tubes undergo surface preparation: removal of oxides and contaminants, followed by electroplating with tin. The镀锡 layer, as per Table 3, promotes wetting and bonding with the molten ductile iron, a critical step for defect-free ductile iron castings. This process enhances the interface integrity, which is paramount for the performance of such ductile iron castings.
Core and Mold Design
I opted for a parting line along the top平面 of the bearing box, with the entire casting located in the drag half. This simplifies molding and facilitates tube placement. The main cavity is formed by a large core seated in the drag, while separate cores are used for the gating system and intricate features. To prevent sand-related issues like veining or burning, all cores are made from chromite sand, which offers high refractoriness—a key consideration for ductile iron castings with thick sections.
For the long cored holes, precise定位 is achieved through core prints in the drag and a dedicated checking fixture during assembly. This ensures alignment within machining allowances, critical for avoiding scrap in these ductile iron castings. The core design incorporates vents, such as hollow nylon ropes embedded along the length, to exhaust gases during pouring, preventing blows or porosity in ductile iron castings.
Oil Tube Casting Integration
Integrating the oil tubes into the mold required innovative solutions to manage thermal effects. For the curved tubes, one end is positioned by a core, while the extended end is fixed during mold assembly. To accommodate thermal expansion, I designed expansion gaps at both tube ends. For instance, foam blocks are placed at tube terminals during coring and closing to absorb expansion without stressing the mold. This prevents sand collapse and tube distortion, common pitfalls in ductile iron castings with embedded metals.
For the straight tubes, I created full-length tube cores that encapsulate the tubes. These cores have prints at both ends, with one end fitting into the drag and the other into the main cavity core. Expansion allowances are calculated based on the thermal expansion coefficient of steel and the solidification characteristics of ductile iron castings. The tube interiors are coated with refractory paint to prevent sand sintering and ease cleaning—a vital step for maintaining oil flow paths in ductile iron castings. The expansion gap size can be derived from the formula for linear thermal expansion:
$$\Delta L = L_0 \cdot \alpha \cdot \Delta T$$
where $\Delta L$ is the expansion length, $L_0$ is the initial tube length, $\alpha$ is the coefficient of thermal expansion for steel (approximately $12 \times 10^{-6} \, \text{K}^{-1}$), and $\Delta T$ is the temperature change from room temperature to the铁液 temperature (around $1,200^\circ\text{C}$). For a tube length of 740 mm, this yields:
$$\Delta L = 740 \times 12 \times 10^{-6} \times (1200 – 25) \approx 10.4 \, \text{mm}$$
Thus, gaps of at least 11 mm are designed at each end to accommodate expansion without constraint, ensuring dimensional stability in these ductile iron castings.
Gating and Riser System Design
The gating system is designed as an open type, with ingates placed along the flange bottom to ensure平稳 filling. The cross-gate is positioned below the casting to act as a slag trap, with a choke坝 to maintain a full sprue. Ingate velocities are kept below 1 m/s to minimize turbulence, which is crucial for preventing oxide formation and inclusions in ductile iron castings.
To address shrinkage risks inherent in ductile iron castings, I employed a combination of chills and risers. The design is guided by modulus calculations, where the modulus $M$ is defined as the volume-to-surface area ratio:
$$M = \frac{V}{A}$$
This modulus helps identify hot spots that may lead to porosity. Using simulation software, I computed moduli across the casting and placed chills to locally increase cooling rates, promoting directional solidification. Riser sizing follows the principle that risers must solidify after the casting to feed shrinkage. The required riser volume $V_r$ can be estimated based on the casting volume $V_c$ and the solidification shrinkage $\varepsilon$ of ductile iron (typically 4–6%):
$$V_r = V_c \cdot \varepsilon \cdot f_s$$
where $f_s$ is a safety factor accounting for feeding efficiency. For this bearing box, two insulated top risers are used, with dimensions optimized via simulation to ensure soundness in these ductile iron castings. The gating system parameters are summarized in Table 4, highlighting key design choices for ductile iron castings.
| Parameter | Value/Description |
|---|---|
| Gating Type | Open System with Choke |
| Ingate Velocity | < 1 m/s |
| Number of Risers | 2 (Insulated) |
| Riser Material | Exothermic Sleeves |
| Chill Material | Cast Iron |
Solidification Simulation and Validation
I utilized MAGMA software to simulate the solidification process, verifying the effectiveness of the gating and risering design. The simulation outputs thermal moduli and temperature gradients, confirming that risers remain液态 longer than adjacent casting sections. This is expressed through the solidification time $t$, given by Chvorinov’s rule:
$$t = k \cdot M^2$$
where $k$ is a mold constant dependent on material and process conditions. For ductile iron castings in chromite sand, $k$ is relatively high, necessitating careful riser design. The simulation confirmed that hot spots were adequately fed, with no isolated liquid pools that could cause shrinkage defects in these ductile iron castings.
Production Results and Quality Assessment
Following the above method, the bearing box was successfully cast. Post-casting, it underwent full dimensional inspection, NDT, and mechanical testing. All properties met or exceeded the requirements in Table 2, with no defects detected in MT or UT. The镶铸 oil tubes showed excellent fusion, no变形, and smooth internal surfaces, confirming the effectiveness of the expansion control and coating techniques. The curved tubes maintained roundness within 2 mm, and the cored holes were accurately positioned, allowing for straightforward machining. This outcome underscores the reliability of the proposed method for producing high-quality ductile iron castings with complex embedded features.
The image above illustrates a typical ductile iron casting similar to the bearing box, highlighting the surface quality and intricate geometry achievable with this工艺. For our specific bearing box, the as-cast appearance was equally impressive, with no visible flaws and precise oil tube integration, demonstrating the robustness of the process for ductile iron castings.
Conclusion
In this article, I have detailed a comprehensive casting method for a multi-tubing bearing box made of ductile iron. The approach addresses critical challenges through meticulous design: from镶铸 tube preparation and expansion management to optimized gating and risering backed by solidification simulation. The successful first-time production validates the method’s efficacy, offering a reliable framework for similar ductile iron castings in heavy machinery applications. Key takeaways include the importance of thermal expansion allowances, the use of high-refractoriness sands for cores, and the integration of simulation tools to predict and mitigate defects. By adhering to these principles, foundries can consistently produce high-integrity ductile iron castings that meet stringent industrial standards, paving the way for advanced applications in turbines and beyond.
The versatility of this method extends to other complex ductile iron castings, where embedded components or irregular geometries are involved. Future work could explore automated tube positioning or advanced coating materials to further enhance bonding in ductile iron castings. Nonetheless, the current results affirm that with careful工艺 design, even the most demanding ductile iron castings can be manufactured successfully, ensuring performance and durability in critical engineering systems.