In our pursuit of expanding into overseas markets and producing high-value-added products, we have developed various steering gear castings, with weights ranging from 15 kg to 250 kg. These nodular cast iron components are critical for heavy machinery, and their internal soundness is paramount due to the high mechanical loads they endure. However, achieving defect-free nodular cast iron castings, especially those with complex geometries and thick sections, remains a significant challenge in the foundry industry. Internal shrinkage porosity and cavities are common defects that compromise the integrity of nodular cast iron parts. This article, based on our practical experience, explores the differential effectiveness of top feeding versus side feeding methods in mitigating these defects, with a focus on a specific large steering gear casting made of nodular cast iron.
The steering gear casting in question is used in large excavators, requiring high internal density and no visible shrinkage defects—meeting a quality standard of Grade 2 or above. The material is QT500-7 nodular cast iron, with overall dimensions of 545 mm × 470.6 mm × 471.9 mm and a weight exceeding 81.5 kg. The casting features a basic wall thickness of over 30 mm and multiple isolated hot spots, with the largest hot spot having an equivalent diameter of approximately 60 mm. Such characteristics make feeding difficult, as the solidification process must be carefully controlled to ensure soundness in nodular cast iron.
Shrinkage defects in nodular cast iron castings arise from various factors, including casting design, melting practices, mold and core materials, and process design. For nodular cast iron, the graphite expansion during eutectic solidification can partially compensate for shrinkage, but inadequate feeding still leads to porosity. The primary reasons can be summarized as follows:
- Casting Structure: Thick and uneven sections create thermal gradients that hinder directional solidification. Isolated hot spots cause localized overheating, disrupting the feeding sequence. Since the design is dictated by functional requirements, altering the structure is often not feasible.
- Melting and Mold/Core Materials: Poor heat dissipation from molds and cores slows solidification. High gas content in the molten nodular cast iron, due to improper melting or handling, leads to gas evolution that blocks feeding. Inadequate inoculation can result in carbide precipitation, increasing shrinkage volume.
- Process Design: Improper gating and riser systems violate solidification principles. Riser size, location, and neck design are critical; suboptimal choices reduce feeding efficiency or cause reverse feeding.
Given the constraints in modifying casting design and the stability of our production line—which uses automatic molding and cupola melting—process design adjustments became the focal point for solving shrinkage issues in nodular cast iron. Our initial approach for the large steering gear involved using an exothermic riser sleeve (80/110 mm) placed laterally in a side core to feed the largest hot spot. The riser neck was formed by the core, with dimensions of 45 mm × 25 mm. This side-feeding method aimed to leverage the exothermic riser’s high feeding efficiency (up to 50% volume yield compared to 10% for conventional risers). However, upon evaluation through sectioning (as per a predefined inspection plan), severe shrinkage defects were found in the hot spot area: a shrinkage cavity measuring 37.0 mm × 20.0 mm with a maximum pore of 8.0 mm × 3.0 mm. This indicated insufficient feeding in the nodular cast iron casting.
To enhance feeding, we incorporated a chill—a 75 mm × 50 mm × 15 mm iron plate embedded in the main core—to promote directional solidification. This reduced the defect size to 13.0 mm × 5.0 mm with a 1.0 mm × 3.0 mm cavity, but it still fell short of the Grade 2 requirement. The persistence of defects suggested that the side-feeding riser, despite its exothermic properties, was not optimally positioned for efficient feeding in this nodular cast iron component.
We then reconsidered the feeding strategy. Theoretical analysis indicates that feeding effectiveness in nodular cast iron depends on the riser’s location relative to the hot spot. The feeding distance can be modeled using Chvorinov’s rule for solidification time, and the pressure head for feeding is influenced by the riser height and neck geometry. For a riser placed on top of the hot spot, the feeding channel is shorter and more direct, potentially improving feeding efficiency. The solidification time for a section is given by:
$$ t = k \left( \frac{V}{A} \right)^2 $$
where \( t \) is the solidification time, \( k \) is the mold constant, \( V \) is the volume, and \( A \) is the surface area. For feeding to be effective, the riser must solidify after the casting section. The feeding requirement can be expressed as:
$$ V_{riser} \geq \frac{\beta V_{casting}}{\eta} $$
where \( \beta \) is the shrinkage coefficient for nodular cast iron (typically 4-6% for liquid shrinkage and 2-3% for eutectic expansion), \( V_{casting} \) is the volume of the section to be fed, and \( \eta \) is the feeding efficiency of the riser. For exothermic risers, \( \eta \) can be as high as 0.5. However, the riser neck design critically affects \( \eta \); a circular neck minimizes early freezing compared to a rectangular one.
Based on this, we modified the process to use top feeding. The same 80/110 mm exothermic riser was repositioned above the hot spot, embedded in the upper and lower main cores, with a circular riser neck of 35 mm diameter and a length of 15 mm. This change aimed to shorten the feeding path and utilize a more favorable neck shape. After implementation, the casting was re-evaluated. The results showed complete elimination of shrinkage defects, meeting the Grade 2 standard. This demonstrated the superior effectiveness of top feeding over side feeding for this nodular cast iron casting.
To quantify the differences, we summarize the experimental data in the following tables. Table 1 compares the defect characteristics under different feeding methods, while Table 2 provides key parameters of the riser designs.
| Feeding Method | Additional Measures | Shrinkage Area (mm²) | Maximum Cavity Size (mm) | Quality Grade Achieved |
|---|---|---|---|---|
| Side Feeding (Exothermic Riser) | None | 740 (37.0 × 20.0) | 8.0 × 3.0 | Below Grade 2 |
| Side Feeding (Exothermic Riser) | Chill Added | 65 (13.0 × 5.0) | 1.0 × 3.0 | Below Grade 2 |
| Top Feeding (Exothermic Riser) | None | 0 | 0 | Grade 2 or Above |
| Parameter | Side Feeding Design | Top Feeding Design |
|---|---|---|
| Riser Type | Exothermic Sleeve (80/110 mm) | Exothermic Sleeve (80/110 mm) |
| Riser Location | Lateral, in Side Core | Vertical, Above Hot Spot |
| Riser Neck Shape | Rectangular (45 mm × 25 mm) | Circular (35 mm diameter) |
| Riser Neck Length | ~25 mm (core-formed) | 15 mm (core-formed) |
| Feeding Distance | Longer, indirect path | Shorter, direct path |
| Theoretical Feeding Efficiency (η) | Estimated 0.3-0.4 | Estimated 0.4-0.5 |
The improvement with top feeding can be analyzed through feeding pressure and solidification dynamics. The pressure available for feeding from a riser is given by:
$$ P = \rho g h $$
where \( \rho \) is the density of molten nodular cast iron (approximately 7000 kg/m³), \( g \) is gravity (9.81 m/s²), and \( h \) is the height of the liquid metal column above the feeding point. For top feeding, \( h \) is maximized due to the riser’s vertical alignment, whereas for side feeding, \( h \) is reduced. Additionally, the feeding channel resistance is lower with a circular neck, as the freezing time can be approximated using the modulus method. The modulus \( M \) is defined as:
$$ M = \frac{V}{A} $$
For a cylindrical neck, \( M = \frac{d}{4} \) for a long cylinder, where \( d \) is the diameter. For the circular neck (35 mm diameter), \( M \approx 8.75 \) mm, while for the rectangular neck (45 mm × 25 mm), the equivalent modulus is lower, leading to earlier solidification. This ensures that the top-feeding riser remains open longer, enhancing feeding for nodular cast iron.
Furthermore, the solidification sequence in nodular cast iron is influenced by graphite expansion. The expansion pressure can partially compensate for shrinkage, but it requires adequate feeding channels. The net shrinkage volume \( V_{net} \) in nodular cast iron can be expressed as:
$$ V_{net} = V_{liquid\_shrinkage} + V_{eutectic\_shrinkage} – V_{graphite\_expansion} $$
For typical nodular cast iron, values range from 1% to 3% net shrinkage. Efficient feeding must supply this volume. The riser feeding volume \( V_{feeding} \) is:
$$ V_{feeding} = \eta V_{riser} $$
With top feeding, \( \eta \) is higher due to better thermal conditions, making it more effective for nodular cast iron castings with thick sections and isolated hot spots.
Our validation involved multiple castings to ensure consistency. The top-feeding method not only eliminated defects but also simplified the process by removing the need for chills, reducing manufacturing steps. This success has established a robust platform for developing similar nodular cast iron components with high integrity requirements. In summary, while side feeding is commonly used for its simplicity, top feeding offers superior feeding effectiveness for demanding nodular cast iron castings, as evidenced by our experimental results.
In conclusion, the choice of feeding method significantly impacts the internal quality of nodular cast iron castings. Through practical verification, we have demonstrated that top feeding with exothermic risers provides better feeding efficiency than side feeding for large, complex nodular cast iron components with isolated hot spots. This approach leverages shorter feeding paths, favorable neck geometries, and enhanced pressure heads, ensuring sound castings that meet high-density standards. Future work could involve modeling these effects using simulation software to optimize riser designs further for nodular cast iron. This experience underscores the importance of iterative process design in overcoming shrinkage challenges in nodular cast iron foundry practice.