In the development of large diesel engine components, bearing caps play a critical role in securing and supporting the crankshaft, while enduring cyclic alternating loads during operation. As such, the quality requirements for these parts are extremely high. Recently, I was involved in a project focused on a large ductile iron bearing cap with substantial dimensions. Initial casting process designs, based on past experiences, led to defects such as shrinkage porosity and slag inclusions. This prompted a dedicated technical investigation to address these issues in ductile iron casting.
The bearing cap had an outline dimensions of 700 mm × 450 mm × 150 mm, with a rough casting weight of approximately 190 kg. The material used was QT400-15, a grade of ductile iron known for its toughness and ductility. The maximum thickness of the cap was 150 mm, and specific technical requirements mandated that no defects be present within 10 mm of the bolt holes, which were to be machined post-casting. The initial process design involved omitting the bolt holes during casting to avoid machining issues, incorporating insulating risers in thick sections paired with chills to mitigate shrinkage risks, and using two ceramic filters to enhance slag removal. However, this approach increased the thermal volume, failing to fully eliminate shrinkage porosity and resulting in a low casting yield of around 56%.
Defect analysis revealed surface inclusions and dispersed porosity in the bolt hole regions upon dissection. Scanning electron microscopy and energy-dispersive spectroscopy identified typical slag characteristics, with elements like oxygen, silicon, and calcium present. The root causes were attributed to inadequate filtration due to the short distance between the sprue and filters, as well as the vertical orientation of the ceramic filters, which reduced their efficiency during initial pouring stages. Additionally, the large thermal mass from the non-cast bolt holes limited the effectiveness of riser-based feeding, exacerbating shrinkage porosity in ductile iron casting.
To address these challenges, I implemented a comprehensive改进方案 focusing on the gating system, casting structure, and cooling mechanisms. The filtration system was reconfigured by relocating the sprue to increase the runner length and orienting the filters horizontally. This adjustment promotes bottom-up flow through the filters, enhancing slag capture and improving the overall quality of ductile iron casting. The casting structure was optimized by incorporating the bolt holes as cast features, significantly reducing the thermal volume. To prevent core breakage or deformation, steel reinforcements were embedded within the sand cores. For cooling optimization, custom-shaped chills were designed to cover the thick sections entirely, replacing the previous riser setup. This approach accelerates solidification and leverages the graphite expansion in ductile iron for self-feeding, in line with the theory of balanced solidification. The heat transfer during this process can be modeled using Fourier’s law: $$ q = -k \nabla T $$ where \( q \) is the heat flux, \( k \) is the thermal conductivity, and \( \nabla T \) is the temperature gradient. Furthermore, the solidification time can be estimated using Chvorinov’s rule: $$ t = B \left( \frac{V}{A} \right)^2 $$ where \( t \) is the solidification time, \( V \) is the volume, \( A \) is the surface area, and \( B \) is a mold constant. These principles are crucial in minimizing defects in ductile iron casting.
| Parameter | Initial Process | Optimized Process |
|---|---|---|
| Filter Orientation | Vertical | Horizontal |
| Bolt Hole Design | Non-cast | Cast with core reinforcement |
| Cooling Method | Riser + Chill | Custom-shaped chills only |
| Casting Yield | 56% | 82% |
| Defect Incidence | High (shrinkage, slag) | Negligible |
Validation through MAGMA software simulations demonstrated stable filling and a significant reduction in shrinkage propensity near the bolt holes. The porosity prediction model, based on the Niyama criterion, can be expressed as: $$ G / \sqrt{R} \leq C $$ where \( G \) is the temperature gradient, \( R \) is the cooling rate, and \( C \) is a critical value for defect formation. Physical inspections of cast samples confirmed the absence of slag inclusions, and dissections showed that any residual porosity was located more than 76 mm from the bolt holes, meeting the stringent requirements. Small-scale production of 20 units validated the consistency of these results, with no defects observed, affirming the effectiveness of the改进方案 in ductile iron casting.
| Element | Composition (wt%) |
|---|---|
| Carbon (C) | 3.6–3.8 |
| Silicon (Si) | 2.3–2.7 |
| Manganese (Mn) | ≤0.3 |
| Phosphorus (P) | ≤0.05 |
| Sulfur (S) | ≤0.02 |
| Magnesium (Mg) | 0.03–0.05 |
The改进方案 not only eliminated shrinkage and slag defects but also enhanced the casting yield from 56% to 82%, reducing machining costs and improving overall efficiency. Key factors in this success included the extended runner length and horizontal filter placement, which optimized slag removal, and the use of sand cores with chills to manage thermal masses. The mathematical modeling of solidification, such as the thermal modulus calculation: $$ M = \frac{V}{A} $$ where \( M \) is the thermal modulus, helped in designing effective cooling strategies for ductile iron casting. Additionally, the graphite expansion behavior in ductile iron can be described by the volume change equation: $$ \Delta V = V_0 \beta \Delta T $$ where \( \Delta V \) is the volume change, \( V_0 \) is the initial volume, \( \beta \) is the volumetric expansion coefficient, and \( \Delta T \) is the temperature change. This understanding is vital for leveraging self-feeding mechanisms in thick sections.
In conclusion, the optimized ductile iron casting process for large bearing caps demonstrates that strategic adjustments in filtration, casting geometry, and cooling can resolve common defects like shrinkage porosity and slag inclusion. The integration of horizontal filters, cast bolt holes with reinforced cores, and custom chills proved highly effective. This approach not only ensures high-quality castings but also boosts productivity and cost-effectiveness, making it a reliable method for similar applications in ductile iron casting. Future work could explore advanced simulation techniques or material modifications to further enhance performance in demanding environments.