In our foundry, we faced a challenge with producing a small ductile iron casting—a head mandrel with a specified material grade of QT500-7. The raw casting weight was 68 kg, with a geometric modulus of 1.69 cm. Traditional casting theories and empirical knowledge suggest that small castings, especially ductile iron castings, require risers for effective feeding to prevent shrinkage defects. However, the inherent properties of ductile iron, such as a wide solidification range, mushy freezing behavior, surface non-crusting before internal solidification, and graphite expansion causing mold wall movement, make riser-based compensation inefficient. Initial calculations showed a perimetrical quotient (Qm) of 17.9 kg/cm³, which, according to established references, does not meet the criteria for riserless casting of ductile iron castings. Similarly, the modulus was below the recommended threshold. This led us to explore alternative approaches to achieve sound castings without risers, aiming to reduce production costs and improve yield.
Our investigation focused on manipulating the solidification characteristics of ductile iron castings through process modifications. We conducted a series of experiments with three distinct process designs, progressively moving from conventional riser-based methods to innovative riserless techniques. The key was to enhance the perimetrical quotient artificially and promote balanced solidification, where shrinkage and expansion phases overlap effectively. Below, we detail each process, supported by data, formulas, and comparative analysis, emphasizing the repeated theme of optimizing ductile iron casting processes for small-modulus components.
In the first process, we adopted a horizontal molding and vertical pouring approach to address issues with the long, slender core (φ40 mm × 480 mm) that could bend due to molten metal buoyancy. The casting was oriented with the flange upward, and two risers (φ85 mm × 120 mm) were placed on the top surface for feeding. A bottom-gating system was used, resulting in a process yield of 73%. However, upon dissection, internal shrinkage porosity was observed, failing to meet technical requirements. This outcome highlighted the limitations of riser-dependent methods for ductile iron castings, where graphite expansion is not fully utilized.
The modulus (M) for a casting is defined as the ratio of volume (V) to surface area (A), expressed as: $$ M = \frac{V}{A} $$ For the initial mandrel, with a modulus of 1.69 cm, traditional criteria suggest that riserless casting is feasible only for higher moduli, typically above 2.5 cm for ductile iron castings. Additionally, the perimetrical quotient (Qm) is a critical parameter influencing solidification behavior. We define Qm as: $$ Q_m = \frac{m}{M^3} $$ where m is the mass of the casting. Initially, Qm was 17.9 kg/cm³, which is below the recommended threshold of 20 kg/cm³ for riserless ductile iron casting processes. This reinforced the need for risers in Process 1, but the results were unsatisfactory due to the inefficiency in handling the expansion-shrink dynamics of ductile iron.
Building on this, we shifted to a riserless approach in Process 2, inspired by the concept that ductile iron castings can achieve soundness through balanced solidification, even with small moduli, by artificially increasing Qm. We added external ribs to the casting wall, which enhanced the surface area and altered the cooling dynamics. This modification increased Qm to 37 kg/cm³, and we maintained the horizontal molding and vertical pouring with a bottom-gating system. The process yield improved to 82%. Comparative trials with Process 1, using the same heat of molten iron, showed that Process 2 produced castings free of shrinkage defects, while Process 1 did not. This demonstrated that for ductile iron castings, increasing Qm through design changes can enable riserless casting by promoting early expansion and overlapping with shrinkage.
The underlying principle involves the interaction between thin and thick sections in the casting. Thin sections solidify rapidly, contracting and drawing liquid from thicker areas, while thicker sections undergo graphite expansion later, compensating for micro-shrinkage. The equilibrium solidification condition can be modeled using the following relation for ductile iron castings: $$ \Delta V_{\text{shrinkage}} = \Delta V_{\text{expansion}} – \Delta V_{\text{feeding}} $$ where ΔV represents volume changes. By adjusting the geometry, we ensure that ΔV_expansion occurs timely to offset ΔV_shrinkage, making risers unnecessary for ductile iron casting processes.
In Process 3, we further optimized this by replacing the ribs with external chills, which act as localized heat sinks and mimic the effect of thin sections. The chills were arranged intermittently on the casting surface, creating a pattern of alternating thin and thick zones that facilitate balanced solidification. This approach boosted the process yield to 90%, and dissected castings confirmed the absence of internal defects. The success of Process 3 underscores that for small-modulus ductile iron castings, strategic use of chills can elevate the effective Qm and achieve riserless production reliably.
To summarize the evolution, the table below compares the key parameters and outcomes of the three processes for ductile iron castings:
| Process | Riser Usage | Modulus (cm) | Perimetrical Quotient, Qm (kg/cm³) | Process Yield (%) | Internal Defects |
|---|---|---|---|---|---|
| Process 1 | Yes | 1.69 | 17.9 | 73 | Shrinkage present |
| Process 2 | No | 1.69 (with ribs) | 37.0 | 82 | None |
| Process 3 | No | 1.69 (with chills) | 37.0 (effective) | 90 | None |
The data clearly shows that riserless methods for ductile iron castings are viable when Qm is enhanced above 20 kg/cm³. The modulus, while important, is not the sole determinant; instead, the ability to manipulate solidification through design changes is crucial. We derived a generalized condition for riserless ductile iron casting: $$ Q_m \geq 20 \, \text{kg/cm}^3 $$ This ensures that graphite expansion is harnessed effectively to counteract shrinkage, a fundamental aspect of ductile iron casting processes.
In our experiments, the transition from Process 1 to Process 3 involved a deep understanding of ductile iron’s behavior. The formula for solidification time (t) based on modulus is: $$ t = k \cdot M^2 $$ where k is a constant dependent on material properties. For ductile iron castings, k accounts for graphite expansion, and by increasing Qm, we reduce the effective solidification time disparity between sections, leading to more uniform cooling. Additionally, the heat transfer equation supports the use of chills: $$ q = h \cdot A \cdot \Delta T $$ where q is heat flux, h is heat transfer coefficient, A is surface area, and ΔT is temperature difference. Chills increase local q, accelerating solidification in specific zones and promoting the desired expansion-shrinkage overlap.
From a practical perspective, Process 3 has been adopted for mass production, consistently yielding high-quality ductile iron castings without defects. This approach not only improves economic efficiency by raising yield but also aligns with sustainable practices by reducing metal waste. Our findings challenge conventional wisdom, proving that small-modulus ductile iron castings can successfully employ riserless processes through deliberate design interventions.
In conclusion, the journey from riser-dependent to riserless casting for ductile iron components demonstrates the importance of innovative process design. By focusing on parameters like modulus and perimetrical quotient, and leveraging the unique properties of ductile iron, we achieved significant improvements. The key takeaway is that for ductile iron castings, even those with small moduli, riserless casting is attainable by artificially increasing Qm through methods like rib addition or chill usage. This paradigm shift enhances the competitiveness and sustainability of ductile iron casting operations, paving the way for broader applications in the industry.