In the manufacturing of high-speed punch presses, the disc component plays a critical role in ensuring uniform punching force and extended service life. This ductile iron casting, with a mass of 38,000 kg and overall dimensions including a height of 1,700 mm and an outer diameter of 4,800 mm, features a complex structure with slots for weight reduction and bottom holes for connections. The primary challenge lies in preventing deformation and defects such as shrinkage porosity, given the large cavity area and substantial wall thickness of 220 mm, with the bearing seat root being the thickest section acting as a hot spot. The material specification requires QT500-7 grade ductile iron, with mechanical properties including tensile strength greater than 420 MPa, yield strength above 320 MPa, and elongation exceeding 5%. Additionally, the bottom surface contains threaded holes that must be free from looseness defects. To address these issues, we developed a comprehensive casting process design, incorporating chemical composition optimization, molding strategies, and melting techniques, supported by simulation validation using AnyCasting software to ensure quality and performance.
The chemical composition is crucial for achieving the desired microstructure and mechanical properties in ductile iron castings. Based on the carbon equivalent (CE) principle, we aimed for a CE range of 4.0% to 4.2%, with carbon content around 3.5% to balance graphitization and strength. Phosphorus, a positive segregation element that promotes carbide formation, was minimized to below 0.02%, while sulfur content was controlled to less than 0.015% to facilitate effective magnesium treatment and prevent issues like fading spheroidization. Residual magnesium was maintained between 0.035% and 0.055%, and rare earth elements like antimony were added in trace amounts of 0.005% to 0.01% to enhance graphite nodule count and spheroidization rate without causing degeneration. Copper was included to improve strength and hardness. The chemical composition design is summarized in Table 1, which outlines the target ranges for key elements before and after spheroidization.
| Element | Pre-Spheroidization | Post-Spheroidization |
|---|---|---|
| C | 3.5–3.7 | 3.4–3.6 |
| Si | 1.4–1.5 | 2.0–2.4 |
| Mn | 0.35–0.45 | 0.35–0.45 |
| P | ≤ 0.02 | ≤ 0.02 |
| S | ≤ 0.01 | 0.006–0.01 |
| Cu | – | 0.5–0.8 |
| Sb | – | 0–0.01 |
| Mg_res | – | 0.035–0.055 |
The carbon equivalent can be calculated using the formula: $$ CE = C + \frac{1}{3}Si $$ which helps in predicting the casting behavior and solidification characteristics. For instance, with C at 3.5% and Si at 2.2%, the CE is approximately 4.23%, aligning with our target range to ensure proper graphitization and reduce the risk of chilling in thick sections.
For the molding process, we employed furan resin sand in a three-box molding system to accommodate the large size of the ductile iron castings. The parting surface was designed to place the bottom face in the lower box, facilitating a bottom-gating system that ensures smooth filling and minimizes turbulence, thereby reducing slag inclusion and gas defects. The gating system was designed with a ratio of sprue:runner:ingate cross-sectional areas of approximately 1:1.2:0.8, allowing for high flow rates and stable metal entry. Chills were strategically placed in the bottom hot spots to accelerate cooling, shorten solidification time, and prevent graphite degeneration and shrinkage porosity. This approach is essential for large modulus ductile iron castings, as it enhances the cooling rate in critical areas and promotes a denser microstructure. The overall casting process schematic illustrates the arrangement of gates, runners, and chills to achieve uniform filling and solidification.
In the melting and treatment phase, we focused on spheroidization and inoculation to achieve high-quality ductile iron castings. The spheroidization process used a冲入法 (impact method) with yttrium-based heavy rare earth spheroidizer, containing 1.5–2.5% RE, 6–7% Mg, and 45% Si, added at 1.0–1.2% of the iron weight. This choice helps maintain stable spheroidization in thick sections by resisting fading, especially when sulfur levels are kept low. Inoculation was performed multiple times to ensure fine graphite formation; we used a barium-containing high-efficiency inoculant (0.5–0.6% addition) for initial treatment and a long-lasting stream inoculant (0.1–0.2% addition) during pouring. The inoculant compositions are detailed in Table 2, highlighting elements like Si, Ca, Al, Ba, and RE that aid in nucleation and graphite distribution.
| Type | Si | Mg | RE | Ca | Al | Ba | Addition Amount |
|---|---|---|---|---|---|---|---|
| Yttrium-based Spheroidizer | 45 | 6–7 | 1.5–2.5 | – | – | – | 1.0–1.2 |
| Barium Inoculant | 70–75 | – | – | 1.5–2.0 | 1.0–2.0 | 1.0–2.0 | 0.5–0.6 |
| Stream Inoculant | 55–65 | – | 1.0–2.0 | – | – | – | 0.1–0.2 |
The melting temperature was controlled between 1,500°C and 1,550°C, with a holding time of 5–8 minutes for purification. The tapping temperature was set at 1,430–1,460°C, and the pouring temperature was maintained at 1,300–1,330°C to balance fluidity and solidification time. Lower pouring temperatures help reduce liquid contraction and shrinkage defects, but must not compromise filling completeness. The relationship between temperature and solidification time can be expressed as: $$ t_s = k \cdot M^n $$ where \( t_s \) is solidification time, \( M \) is modulus, and \( k \) and \( n \) are constants dependent on material and process conditions. For ductile iron castings, optimizing this equation is vital to prevent defects.
Simulation using AnyCasting software provided insights into the filling and solidification behavior of the ductile iron castings. The filling sequence analysis showed that at 12 seconds, isolated liquid islands formed near the ingates, indicating potential splashing, but the time difference of 24 seconds to the farthest points ensured safety. By 43 seconds, the metal filled the cavity uniformly, with no significant turbulence. Velocity field simulations revealed that at 14% filling, the velocity at ingates was 160 cm/s, which stabilized to 51 cm/s at 40% filling, minimizing erosion risks. Oxide formation simulations indicated initial risks in bearing seat areas (4.2 g/cm³ at 17% filling), but this reduced to 3.58 g/cm³ by the end, lowering inclusion probabilities. Temperature field simulations demonstrated a gradual drop of 30°C at 26% filling and 50°C at completion, with no cold shut issues. These results validated the gating and cooling design, ensuring that the ductile iron castings would achieve the desired quality.
Production trials confirmed the effectiveness of the process for ductile iron castings. The actual chemical composition of the castings, as shown in Table 3, met the specified ranges, with C at 3.43%, Si at 2.09%, Mn at 0.42%, P at 0.020%, S at 0.010%, Cu at 0.65%, Sb at 0.005%, and residual Mg at 0.045%. Mechanical properties from attached specimens exceeded requirements: tensile strength of 535 MPa, elongation of 5.5%, graphite grade 6, and spheroidization rate of 95.42%. Microstructural analysis revealed well-distributed graphite nodules and a pearlitic matrix, free from defects like shrinkage or degenerate graphite. The successful machining of the first piece, with no issues in threaded holes, demonstrated the process reliability. This outcome underscores the importance of controlled chemistry, proper inoculation, and simulation-backed design in producing high-integrity ductile iron castings.
| Parameter | Value | Requirement |
|---|---|---|
| C (%) | 3.43 | 3.4–3.6 |
| Si (%) | 2.09 | 2.0–2.4 |
| Mn (%) | 0.42 | ≤ 0.5 |
| P (%) | 0.020 | ≤ 0.02 |
| S (%) | 0.010 | ≤ 0.015 |
| Cu (%) | 0.65 | 0.5–0.8 |
| Sb (%) | 0.005 | 0–0.01 |
| Mg_res (%) | 0.045 | 0.035–0.055 |
| Tensile Strength (MPa) | 535 | ≥ 420 |
| Elongation (%) | 5.5 | ≥ 5 |
| Graphite Grade | 6 | ≥ 4 |
| Spheroidization Rate (%) | 95.42 | ≥ 90 |
In conclusion, the design and simulation of the ductile iron punch disc casting process highlight key strategies for achieving high-performance ductile iron castings. The chemical composition, with a carbon equivalent of 4.0–4.2%, residual magnesium of 0.035–0.055%, and rare earth elements of 0.01–0.02%, ensures optimal graphitization and mechanical properties. Multiple inoculation stages and the use of yttrium-based spheroidizers reduce solidification time and improve graphite morphology. The bottom-gating system with dispersed ingates promotes smooth filling, while chills enhance cooling in critical zones, minimizing shrinkage and porosity. Simulation tools like AnyCasting provide valuable validation, reducing trial costs and ensuring defect-free production. This comprehensive approach can be applied to other large-scale ductile iron castings, emphasizing the importance of integrated process design for industrial applications.
The success of this project demonstrates that through careful control of metallurgical parameters and advanced simulation techniques, ductile iron castings can meet stringent requirements for applications such as punch presses. Future work could explore optimizing chill placement and inoculant types for even better performance in ductile iron castings. Overall, the methodologies described here serve as a robust framework for manufacturing high-quality ductile iron components in demanding environments.