In the production of heavy-section ductile iron castings, such as hydraulic components, the emergence of chunky graphite defects poses significant challenges to mechanical properties and structural integrity. Our investigation focuses on the root causes of these defects, particularly in thick-walled ductile iron casting parts, where prolonged solidification times and elemental segregation exacerbate graphite distortion. Through systematic analysis of melting materials, molding processes, and compositional adjustments, we have developed effective strategies to suppress chunky graphite formation. This article details our approach, incorporating theoretical models, experimental data, and practical solutions to enhance the quality of ductile iron casting products.
Chunky graphite, characterized by fragmented, irregular graphite morphologies, predominantly occurs in the thermal centers of heavy-section ductile iron casting components. These defects degrade tensile strength, elongation, and pressure tightness, leading to rejection rates exceeding 30% in some cases. Our study originated from issues observed in hydraulic cover castings with wall thicknesses ranging from 40 mm to 120 mm. Initial metallographic examinations revealed extensive chunky graphite zones, necessitating a thorough reevaluation of process parameters.
The solidification behavior of ductile iron casting is influenced by cooling rates and compositional factors. According to the Fe-C phase diagram, the presence of magnesium (Mg) shifts the eutectic point to the right, increasing the liquid-solid temperature range. The relationship can be expressed as: $$ \Delta T_{LS} = k \cdot w(Mg) $$ where $\Delta T_{LS}$ is the liquid-solid temperature interval, $k$ is a proportionality constant, and $w(Mg)$ is the residual magnesium content. Higher residual magnesium levels intensify undercooling, extending solidification time and promoting graphite degeneration in thick sections.
Rare earth elements (REE), such as cerium (Ce) and lanthanum (La), are commonly used in ductile iron casting to neutralize tramp elements like sulfur and oxygen. However, in heavy-section castings, these elements tend to segregate at austenite grain boundaries, delaying the encapsulation of graphite nodules by austenite shells. This allows carbon atoms to diffuse along open pathways, leading to distorted graphite growth. The segregation tendency can be modeled using the Scheil equation: $$ C_s = k_e \cdot C_0 \cdot (1 – f_s)^{k_e – 1} $$ where $C_s$ is the solute concentration in the solid, $C_0$ is the initial concentration, $k_e$ is the equilibrium distribution coefficient, and $f_s$ is the solid fraction. For REE, $k_e < 1$, resulting in enrichment at the end of solidification.
To quantify the impact of process variables, we conducted thermal analysis and MAGMA simulations. The temperature field analysis confirmed that thermal centers, such as neck regions of insulating risers, exhibit prolonged solidification times, fostering chunky graphite formation. The solidification time $t_s$ for a spherical segment can be approximated as: $$ t_s = \frac{R^2}{\alpha \cdot \pi^2} \ln \left[ \frac{T_p – T_m}{T_s – T_m} \right] $$ where $R$ is the characteristic length, $\alpha$ is thermal diffusivity, $T_p$ is pouring temperature, $T_m$ is mold temperature, and $T_s$ is solidus temperature. For sections exceeding 100 mm, $t_s$ increases exponentially, exacerbating graphite distortion.
Our experimental approach involved modifying the spheroidization treatment and alloy additions. The base composition for QT450 ductile iron casting is summarized in Table 1.
| Element | Composition Range (wt%) |
|---|---|
| C | 3.6–3.9 |
| Si | 2.0–2.5 |
| Mn | 0.3–0.6 |
| S | ≤0.03 |
| Cu | 0.4–1.0 |
| Sn | 0.04–0.10 |
| Mg | 0.020–0.050 |
| RE | 0.01–0.04 |
We tested three distinct processes to evaluate the effects of REE control and antimony (Sb) addition. Process A utilized conventional REE-containing spheroidizing core wire, while Process B employed REE-free wire. Process C combined REE-free wire with Sb inoculation. The experimental matrix and outcomes are detailed in Table 2.
| Process | Spheroidization Method | Sb Addition (wt%) | Chunky Graphite Presence |
|---|---|---|---|
| A (Original) | MgSiFe with REE | 0 | Severe (>30% area) |
| B (Test 1) | MgSiFe without REE | 0 | Moderate |
| C (Test 2) | MgSiFe without REE | 0.015 | None |
Process A, with REE-containing wire, resulted in pronounced chunky graphite due to Ce and La segregation. Process B, eliminating REE, reduced but did not fully eliminate the defect, indicating that REE control alone is insufficient. Process C, incorporating Sb, achieved complete suppression of chunky graphite. Antimony acts as a surface-active element, lowering the melting point of austenite surrounding graphite nodules and promoting earlier shell formation. The effectiveness of Sb can be described by the absorption energy at the graphite-austenite interface: $$ E_{ads} = -\Delta G_{Sb} + \gamma_{int} $$ where $\Delta G_{Sb}$ is the Gibbs free energy change due to Sb adsorption, and $\gamma_{int}$ is the interfacial energy. Negative $E_{ads}$ values indicate preferential segregation, stabilizing graphite growth.
Further analysis involved measuring nodule count and shape factor. For Process C, the nodule count increased by 40% compared to Process A, confirming enhanced nucleation. The shape factor $F$ is defined as: $$ F = \frac{4\pi A}{P^2} $$ where $A$ is the cross-sectional area and $P$ is the perimeter. Values closer to 1 indicate spherical graphite. Process C yielded $F \approx 0.85$, whereas Process A showed $F \approx 0.55$ in defective zones.
The mechanism of Sb in counteracting REE effects involves competitive segregation. While REE delay austenite encapsulation, Sb accelerates it by reducing the local solidus temperature. The combined effect can be modeled as a kinetic balance: $$ \frac{dC_{REE}}{dt} = -D_{REE} \nabla^2 C_{REE} + k_{Sb} C_{Sb} $$ where $D_{REE}$ is the diffusion coefficient of REE, and $k_{Sb}$ is the rate constant for Sb interaction. Optimal Sb addition balances the segregation kinetics, preventing graphite distortion.
In conclusion, the prevention of chunky graphite in heavy-section ductile iron casting requires a multifaceted approach. Reducing REE content minimizes segregation, while Sb addition counteracts residual REE effects and enhances graphite nucleation. Our findings demonstrate that Process C—using REE-free spheroidizing wire with 0.015% Sb—effectively eliminates chunky graphite, ensuring sound microstructure and mechanical properties. Future work will focus on optimizing Sb levels for varying section sizes and exploring synergistic effects with other inoculants in ductile iron casting applications.