In the production of ductile iron castings using furan resin sand, surface color differences due to poor spheroidization have been a persistent challenge. These color variations often indicate underlying metallurgical issues, such as the presence of flake or vermicular graphite in the surface layer, which compromise mechanical properties and customer satisfaction. This study investigates the root causes and implements effective countermeasures, focusing on the role of sulfur infiltration from molding sand into the casting surface. Through systematic analysis and optimization, we demonstrate how targeted interventions can mitigate these defects, ensuring high-quality ductile iron castings.
The problem arises primarily from the use of sulfonic acid as a hardener in the furan resin sand system. High acidity in the hardener increases free sulfuric acid and sulfur content, leading to elevated sulfur levels in reclaimed sand. During pouring, high-temperature reactions between the mold and molten iron allow sulfur to react with magnesium at the casting surface, depleting residual magnesium and causing spheroidization degradation. This results in color differences and reduced surface integrity in ductile iron castings. Our research delves into the mechanisms and presents a solution involving anti-sulfur coatings to block or neutralize sulfur transfer.
Problem Analysis and Initial Findings
Our company produces ductile iron castings, such as injection molding machine plates, with material grade QT450-10. These ductile iron castings have a machining allowance of 5–6 mm on surfaces, with allowable defect depths ≤ 0.2 mm, and weigh approximately 500 kg. The required spheroidization grade is level 2. Originally, we used a blend of 30/50 mesh scrubbed sand and reclaimed sand in a 1:9 ratio. The reclaimed sand had a loss on ignition (LOI) of 2.6%–3.2% and sulfur content of 0.12%–0.20%. Type 800 coating was applied, with parameters detailed in Table 1. The molding process involved top and bottom parting, one casting per mold, with four ingates. Resin type FD305 was added at 0.85%–0.90%, and hardener type GH28 at 25%–35%; their specifications are listed in Tables 2 and 3. Melting was done in a cupola, with spheroidizer C-1 added at 1.2%–1.4% and inoculant BS-1 at 0.4%–0.7%. The pouring temperature was controlled at 1,310–1,330 °C. Base iron and casting compositions are shown in Table 4.
| Type | Color | Main Aggregates | Density (g/cm³) | Baumé Degree (°Bé) | Suspension Rate (2 h) | Application | Shelf Life |
|---|---|---|---|---|---|---|---|
| 800 | Red-brown | Graphite, Mullite | 1.60–2.00 | > 90 | ≥ 90% | Ductile Iron | 6 months |
| FQ30pro | Yellow | Graphite, Forsterite | 1.20–1.50 | 60–70 | ≥ 98% | Ductile Iron | 6 months |
| Type | Nitrogen Content (%) | Free Formaldehyde (%) | Viscosity (20°C, mPa·s) | Density (g/cm³) | Free Furfuryl Alcohol (%) | pH Value | Shelf Life |
|---|---|---|---|---|---|---|---|
| FD305 | ≤ 3.0 | ≤ 0.3 | ≤ 65 | 1.12–1.20 | ≥ 75 | 7.5–8.5 | 6 months |
| Type | Viscosity (20°C, mPa·s) | Density (g/cm³) | Total Acidity (%) | Free Sulfuric Acid (%) | Non-Crystallization | Shelf Life |
|---|---|---|---|---|---|---|
| GH28 | ≤ 25 | 1.1–1.3 | 27–29 | ≤ 12 | -15°C no crystallization | 6 months |
| Material | C | Si | Mn | P | S | Mg | Cu |
|---|---|---|---|---|---|---|---|
| Base Iron | 3.82 | 1.47 | 0.12 | 0.034 | 0.053 | 0.001 | 0.015 |
| Ductile Iron Casting | 3.64 | 2.40 | 0.12 | 0.036 | 0.012 | 0.063 | 0.015 |
Metallographic analysis of discolored areas on the ductile iron castings revealed significant spheroidization issues. The casting structure and common sulfur infiltration zones are illustrated in a representative image, where edges and inner holes showed color differences. Corresponding metallographic photos at 100× magnification displayed mixed graphite structures, including flake, vermicular, and spherical graphite, making spheroidization rating impossible in affected areas. Mechanical properties from these regions fell below requirements, as shown in Table 5.
| Position | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Spheroidization Rate (%) | Graphite Size |
|---|---|---|---|---|---|
| Customer Requirement | ≥ 420 | ≥ 290 | ≥ 10 | ≥ 90 | 5–7 |
| Position ① | 382 | 276 | 8.3 | Unratable (mixed graphite) | N/A |
| Position ② | 379 | 271 | 7.9 | Unratable (mixed graphite) | N/A |
| Position ③ | 386 | 279 | 9.2 | Unratable (mixed graphite) | N/A |
Further analysis of raw materials showed that the base sand had an acid demand value of 20–40 mL (at pH=7), LOI of 0.5%–1.1%, and grain size distribution of 20–70 mesh with an average fineness of 20–28. The reclaimed sand’s sulfur content (0.12%–0.20%) and LOI (2.6%–3.2%) were linked to the hardener’s acidity and addition rate. The relationship between sulfur content and spheroidization can be expressed using the following formula for critical sulfur levels: $$ \omega(S)_{\text{critical}} = k \cdot \omega(Mg)_{\text{residual}} $$ where $k$ is a proportionality constant dependent on process conditions. Excessive sulfur leads to magnesium depletion: $$ \text{S} + \text{Mg} \rightarrow \text{MgS} $$ This reaction reduces effective magnesium, promoting flake graphite formation and causing color differences in ductile iron castings.
Solution Strategy and Implementation
To address sulfur-induced color differences in ductile iron castings, we focused on interrupting sulfur transfer pathways. While reducing sulfur sources—such as lowering LOI in reclaimed sand, using low-sulfur hardeners, minimizing hardener addition, reducing sand-to-metal ratio, and adding new sand—could help, we prioritized coating optimization for immediate impact. Specifically, we adopted FQ30pro anti-sulfur coating, which blocks and neutralizes sulfur infiltration. The coating was applied in two steps: first at 50–55 °Bé, followed by ignition and drying, then a second coat at 40–45 °Bé, achieving a thickness of 0.3–0.5 mm. Key parameters of FQ30pro are listed in Table 1.
The mechanism involves forsterite (Mg₂SiO₄) in the coating reacting with sulfur compounds at high temperatures: $$ \text{SO}_2 + \text{MgO} \rightarrow \text{MgSO}_3 $$ $$ \text{SO}_3^{2-} + \text{Mg}^{2+} \rightarrow \text{MgSO}_3 $$ $$ \text{S}^{2-} + \text{Mg}^{2+} \rightarrow \text{MgS} $$ These reactions absorb sulfur, preventing it from reaching the molten iron and preserving magnesium for spheroidization. This approach ensures that ductile iron castings maintain uniform surface quality without color variations.
Results and Discussion of Optimized Process
After implementing FQ30pro coating, we observed a significant reduction in color differences on ductile iron castings. Metallographic examination confirmed well-spheroidized graphite in surface layers, meeting grade 2 requirements. Mechanical properties improved substantially, as shown in Table 6. For instance, tensile strength exceeded 420 MPa, yield strength surpassed 290 MPa, and elongation reached over 19%, with spheroidization rates of 90%–95% and graphite sizes of 5–6.
| Position | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Spheroidization Rate (%) | Graphite Size |
|---|---|---|---|---|---|
| Customer Requirement | ≥ 420 | ≥ 290 | ≥ 10 | ≥ 90 | 5–7 |
| Position ① | 428 | 301 | 21.6 | 95 | 6 |
| Position ② | 433 | 308 | 22.8 | 95 | 6 |
| Position ③ | 426 | 297 | 19.5 | 90 | 5–6 |
The effectiveness of FQ30pro coating stems from its dual action of physical barrier formation and chemical neutralization. Sulfur, a surface-active element, adsorbs on graphite prismatic planes, reducing interfacial energy and promoting flake growth: $$ \Delta G_{\text{interface}} = \gamma_{\text{prism}} – \gamma_{\text{basal}} $$ where $\Delta G_{\text{interface}}$ is the change in interfacial energy, and $\gamma$ denotes surface energies. Spheroidizing elements like magnesium counteract this by eliminating sulfur’s influence, enabling isotropic growth along [0001] directions: $$ \text{Mg} + \text{S} \rightarrow \text{MgS} \quad \Delta H < 0 $$ The coating ensures that sulfur from the mold does not increase the effective sulfur content in the iron, thus maintaining spheroidization integrity in ductile iron castings.
Our analysis confirms that with controlled sulfur levels in molding sand (reclaimed sand sulfur ≤ 0.20%), FQ30pro coating effectively mitigates infiltration. The coating’s high suspension rate (≥98%) and forsterite content enhance its protective capability. This solution is particularly vital for ductile iron castings with high surface quality requirements, as it addresses both aesthetic and functional defects.
Conclusion and Recommendations
In summary, color differences in ductile iron castings produced with furan resin sand are primarily caused by sulfur infiltration leading to surface spheroidization degradation. Key measures include optimizing sand properties—such as adding new sand and controlling LOI and sulfur content—and employing advanced coatings like FQ30pro to block sulfur transfer. The coating’s ability to absorb and neutralize sulfur through chemical reactions ensures that graphite remains spherical, eliminating color variations and restoring mechanical performance.
For future production of ductile iron castings, we recommend regular monitoring of reclaimed sand parameters, use of low-sulfur hardeners, and adoption of anti-sulfur coatings as a standard practice. Additionally, controlling pouring temperatures and increasing residual magnesium can provide supplementary benefits. By implementing these strategies, manufacturers can consistently produce high-quality ductile iron castings free from surface defects, meeting stringent customer specifications and enhancing competitiveness in the market.
The successful resolution of this issue underscores the importance of integrated process control in foundry operations. Continuous improvement in material selection and application techniques will further advance the reliability of ductile iron castings in critical applications.