The disc component is a critical element in high-speed mechanical presses, forming the core of the flywheel assembly. Its structural integrity is paramount to ensuring consistent stamping force and extending the operational lifespan of the entire press. Any deformation is unacceptable. Producing such a large, high-integrity component as a ductile iron casting presents significant foundry challenges. This article details the comprehensive process from initial analysis to production validation for this demanding ductile iron casting.
1. Casting Analysis and Requirements
The disc casting has a rough weight of 38,000 kg with overall dimensions reaching 5,820 mm in diameter and a height of 1,700 mm. The main wall thickness is 220 mm, with the thickest sections at the root of the bearing housing bosses, constituting major thermal centers. The material specification is QT500-7, requiring single cast test bar properties of tensile strength > 420 MPa, yield strength > 320 MPa, and elongation > 5%. A critical quality requirement is the absence of shrinkage porosity in the thread holes on the bottom surface, which mate with a cover. The large, open cavity area of this ductile iron casting complicates feeding and defect control.
The large modulus of the casting theoretically allows for a riserless gating approach, provided high-strength resin sand molds and effective late-stream inoculation are employed to promote a controlled, volume-compensating eutectic expansion. The key is to manage the solidification pattern to utilize this expansion effectively.
2. Foundry Process Design
2.1 Chemical Composition Design for Heavy-Section Ductile Iron
Controlling the chemical composition is the foundational step in achieving sound microstructure and mechanical properties in heavy-section ductile iron castings. The goal is to balance graphitization potential, minimize deleterious elements, and ensure effective nodularization throughout the extended solidification time.
The Carbon Equivalent (CE) is crucial for ensuring good castability and controlling shrinkage behavior. It is calculated as:
$$CE = \%C + \frac{1}{3}(\%Si + \%P)$$
For this application, a target CE between 4.0% and 4.2% was selected. Carbon content was maintained around 3.5-3.7% pre-treatment to provide a high graphitization potential.
Phosphorus is a positive segregating element and a strong carbide promoter. Its content must be minimized, with a strict requirement of ω(P) ≤ 0.02%.
Sulfur content directly influences the required residual magnesium level. To achieve a stable nodularizing effect (Grade 2 or better) with a moderate, safer residual magnesium range, the base iron sulfur must be very low. The target was ω(S) < 0.015%, preferably below 0.01%.
Trace elements are categorized and controlled meticulously:
- Mg-Consuming Elements: Ti, S, O, Te, Se. These interfere with magnesium’s nodularizing role and must be kept low via charge material selection.
- Positive Segregating Carbide Formers: Mn, Cr, Mo, V, P. These promote carbides in the last-to-freeze regions and are minimized.
- Controlled Addition Elements: Elements like Sb or Bi can be added in minute quantities (0.005-0.01%) to increase graphite nodule count and nodularity, helping to eliminate chunky graphite in heavy sections. Exceeding this range can reverse the beneficial effect.
The finalized target chemical composition ranges are summarized in the table below.
| Stage | C | Si | Mn | P (max) | S (max) | Cu | Sb | Res. Mg |
|---|---|---|---|---|---|---|---|---|
| Base Iron | 3.5-3.7 | 1.4-1.5 | 0.35-0.45 | 0.02 | 0.010 | – | – | – |
| Target (Final) | 3.4-3.6 | 2.0-2.4 | 0.35-0.45 | 0.02 | 0.010 | 0.5-0.8 | 0-0.01 | 0.035-0.055 |
2.2 Molding and Gating System Design
A three-part flask assembly using furan resin sand was employed. The parting plane was designed to place the critical bottom surface with the threaded holes in the drag (bottom flask). This positioning aims to enhance metallurgical soundness in that area and facilitates a bottom-gating scheme for calm filling.
The gating system was designed as a semi-choked, bottom-filling type to prevent turbulent entry of the iron, thereby reducing slag and gas entrainment—a common defect source in large ductile iron castings. Multiple ingates were positioned along the bottom circumference to ensure even distribution of metal. The gating ratio was set approximately to ΣAsprue : ΣArunner : ΣAingate = 1 : 1.2 : 0.8 to achieve high flow rates while maintaining a non-pressurized flow profile.
Given the large thermal modulus (M), calculated approximately by the volume-to-surface area ratio ($$M \approx \frac{V}{A}$$), and the extended solidification time, chills were strategically essential. They were placed at the thickest sections (bearing boss roots) where riser placement was geometrically impractical. The chills serve a dual purpose: (1) accelerating local cooling to refine the microstructure and prevent graphite degeneration, and (2) promoting earlier solidification in those areas to allow feed metal from the still-liquid gating system, effectively reducing the total shrinkage volume that must be accommodated by eutectic expansion.
2.3 Melting, Nodularizing, and Inoculation Practice
Melting was conducted to achieve a superheating temperature of 1500-1550°C, with a holding time of 5-8 minutes for refining and homogenization. The tapping temperature was controlled at 1430-1460°C.
Nodularizing Treatment: A sandwich method with a reaction chamber in the pouring ladle was used. For this heavy-section ductile iron casting, a Yttrium-bearing heavy rare earth (RE) nodularizer was selected. Yttrium provides a stronger, more fade-resistant nodularizing effect critical for the long solidification time. The target residual magnesium was carefully controlled between 0.035% and 0.055%. Higher residual Mg does not improve nodularity in heavy sections and significantly increases the risk of shrinkage porosity and dross defects. Post-treatment sulfur is targeted at 0.006-0.010%.
| Type | RE | Mg | Si | Addition Rate (wt.%) |
|---|---|---|---|---|
| Yttrium-based Heavy RE Nodularizer | 1.5-2.5 | 6-7 | ~45 | 1.0 – 1.2 |
Inoculation Practice: Multiple-stage inoculation is vital for heavy-section ductile iron castings to ensure a high nodule count and suppress degenerate graphite forms like chunky graphite.
- In-the-Stream Inoculation: A Ba-containing high-efficiency inoculant was added during tapping at 0.5-0.6%.
- Late-Stream Inoculation: A long-lasting RE-containing inoculant was added during casting at 0.1-0.2% to counteract inoculation fade during the extended pouring time.
Care was taken to ensure base iron sulfur and oxygen were not excessively low (>0.006% S, >0.001% O) to maintain inoculation effectiveness. Total silicon was capped to prevent excessive ferrite or reduced toughness.
| Type | Si | Ca | Al | Ba | RE | Addition Rate (wt.%) |
|---|---|---|---|---|---|---|
| Ba-bearing Efficient Inoculant | 70-75 | 1.5-2.0 | 1.0-2.0 | 1.0-2.0 | – | 0.5-0.6 |
| Long-lasting Late Inoculant | 55-65 | – | – | – | 1.0-2.0 | 0.1-0.2 |
Pouring Temperature: A lower pouring temperature (1300-1330°C) was chosen to shorten the total solidification time, which helps improve graphite morphology and reduce total liquid contraction. This temperature represents a balance against the risk of cold shuts and impaired slag floatation.
3. Simulation Verification of the Casting Process
AnyCasting simulation software was used to analyze the filling and solidification of the designed process for the ductile iron disc, providing a virtual verification before costly production trials.
3.1 Filling Sequence and Velocity Field
The simulation confirmed a generally calm filling pattern. At 12 seconds into the fill, isolated liquid zones were noted near the ingate entries, but the time difference to the furthest filling point was 24 seconds, indicating low risk of cold shuts. By 43 seconds, the metal front advanced uniformly. Velocity analysis showed a peak of 160 cm/s at ingate entries during early fill (14%), dropping to an average of 51 cm/s within the casting cavity by 40% fill, confirming minimal risk of sand erosion or turbulent defect formation.
3.2 Oxide Formation and Temperature Field
Oxide formation simulation indicated a potential risk zone (4.2 g/cm³) at the bearing boss pads during mid-fill (17%). However, by the end of filling, the oxide content on the top surface reduced to 3.58 g/cm³, suggesting most oxides floated out, validating the bottom-gating design for these large ductile iron castings. The thermal analysis showed a maximum temperature drop of 50°C from the ingate to the end of fill, with a gradient of only 30°C at 26% fill, confirming adequate thermal uniformity to avoid mist runs.
4. Production Validation and Results
The process was implemented in production based on the design and simulation feedback. The chemical composition of the resulting ductile iron casting is shown below, aligning well with the targets.
| C | Si | Mn | P | S | Cu | Sb | Res. Mg |
|---|---|---|---|---|---|---|---|
| 3.43 | 2.09 | 0.42 | 0.020 | 0.010 | 0.65 | 0.005 | 0.045 |
Mechanical properties and microstructure were evaluated on separately cast keel block coupons (as the disc itself was too large for standard testing). The results surpassed the QT500-7 requirements.
| Property | Tensile Strength (MPa) | Elongation (%) | Graphite Shape (ISO 945) | Nodularity (%) |
|---|---|---|---|---|
| Specification | > 420 | > 5 | ≥ Type IV (Nodular) | ≥ 90 |
| Result | 535 | 5.5 | Type VI (Spheroidal), Size 6 | 95.4 |
Metallographic examination revealed a fully ferritic matrix with well-formed, finely distributed spheroidal graphite and a high nodularity percentage. The first casting was fully machined without revealing any subsurface shrinkage or porosity in the critical threaded hole areas, confirming the effectiveness of the chilling and gating design. The process was subsequently standardized for series production.
5. Conclusions
The successful production of the high-speed press disc, a critical large-scale ductile iron casting, was achieved through a synergistic approach integrating precise metallurgical control, optimized foundry engineering, and virtual process simulation.
Key findings for manufacturing such heavy-section, high-integrity ductile iron castings include:
- Chemical Composition Control: A critical window exists for heavy-section ductile iron. A Carbon Equivalent of 4.0-4.2%, combined with low levels of carbide promoters (P, Cr, etc.) and a controlled residual magnesium level (0.035-0.055%) is essential. The use of minute amounts of structure-modifying elements like Sb (0.005-0.01%) can be beneficial in suppressing degenerate graphite.
- Nodularization and Inoculation Strategy: Employing a Yttrium-based heavy rare earth nodularizer provides fade resistance for long solidification times. Multiple-stage inoculation (in-stream and late-stream) with efficient inoculants is non-negotiable for achieving a high, uniform nodule count and ensuring the desired microstructure throughout the massive section of these ductile iron castings.
- Gating and Cooling Design: A bottom-fed, multi-ingate gating system is paramount for the calm filling of large ductile iron castings, minimizing oxide and gas defects. Strategic use of chills in non-feedable hot spots is equally crucial. Chills not only refine the local microstructure but also sequence solidification to maximize the use of the gating system for liquid feeding and the eutectic expansion for internal feeding, effectively eliminating shrinkage porosity.
The integration of simulation tools provided valuable insights into flow patterns, thermal gradients, and potential defect sites, allowing for process optimization before the first melt. This comprehensive methodology from design to validation ensures the reliable production of high-performance ductile iron castings for the most demanding applications.