In our foundry, we utilize a furan resin sand production line for manufacturing various castings, including ductile cast iron components. Recently, we encountered a persistent issue where certain ductile cast iron castings exhibited surface color differences due to poor spheroidization. This surface degradation led to compromised mechanical properties, failing to meet customer specifications. After thorough investigation, we identified that the sulfur (S) content in the molding sand, primarily originating from the sulfonic acid hardener used in the furan resin system, was reacting with magnesium (Mg) in the molten iron during pouring. This reaction depleted the Mg at the casting surface, causing graphite to form as flakes or vermicular structures instead of spheres, manifesting as color differences. This article details our comprehensive study, analysis, and the successful implementation of a sulfur-resistant coating to resolve this challenge, ensuring high-quality ductile cast iron castings.
The problem was most pronounced in heavy-section ductile cast iron castings, such as injection molding machine plates weighing around 500 kg, with a material grade of QT450-10. The machining allowance on the surface was 5–6 mm, and the allowable defect depth on as-cast surfaces was ≤0.2 mm. The required spheroidization level was grade 2. Initially, 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 a sulfur content of 0.12%–0.20%. We applied a type 800 coating, with specifications summarized in Table 1. The molding process involved top and bottom parting, one casting per mold, with four ingates. The resin used was FD305, added at 0.85%–0.90%, and the hardener was GH28, added at 25%–35%. Their technical specifications are listed in Tables 2 and 3. The molten iron was melted in a cupola, with a C-1 type nodularizer added at 1.2%–1.4% and a BS-1 type inoculant added at 0.4%–0.7%. The pouring temperature was controlled at 1,310–1,330 °C. The compositions of the base iron and the castings are shown in Table 4.
| Type | Color | Main Aggregates | Density (g/cm³) | Baumé Degree (°Bé) | Suspension Rate (2 h) | Applicability | Shelf Life |
|---|---|---|---|---|---|---|---|
| 800 | Red-brown | Graphite, Mullite | 1.60–2.00 | >90 | ≥90% | Ductile Cast Iron | 6 months |
| Type | Nitrogen Content (%) | Free Formaldehyde (%) | Viscosity at 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 at 20 °C (mPa·s) | Density (g/cm³) | Total Acidity (%) | Free Sulfuric Acid (%) | Non-crystallization Property | Shelf Life |
|---|---|---|---|---|---|---|
| GH28 | ≤25 | 1.1–1.3 | 27–29 | ≤12 | Non-crystallizing at -15 °C | 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 |
| Casting | 3.64 | 2.40 | 0.12 | 0.036 | 0.012 | 0.063 | 0.015 |
To diagnose the issue, we performed metallographic analysis on sections of the castings showing color differences. The problematic areas were typically at edges or internal cavity surfaces. The metallographic images revealed a mix of flake, vermicular, and spheroidal graphite at the surface, indicating poor spheroidization. Corresponding mechanical property tests showed unacceptable values: tensile strength around 380 MPa, yield strength around 275 MPa, and elongation below 10%, failing the requirements for ductile cast iron (≥420 MPa tensile strength, ≥290 MPa yield strength, ≥10% elongation). The spheroidization rate could not be rated due to the presence of non-spheroidal graphite. These results confirmed that the surface layer of the ductile cast iron castings was degraded.
We conducted further analysis on the molding materials. The raw sand had an acid demand value of 20–40 mL (at pH=7), LOI of 0.5%–1.1%, and grain size distribution between 20–70 mesh with an average fineness of 20–28. The reclaimed sand had a sulfur content of 0.12%–0.20% and LOI of 2.6%–3.2%. The sulfur primarily originated from the sulfonic acid hardener. The higher the acidity of the hardener and its addition rate, the greater the sulfur content in the sand. During pouring, the high temperature caused sulfur from the sand to diffuse into the molten iron surface, where it reacted with residual magnesium, leading to magnesium depletion and graphite degeneration. The reaction can be represented as:
$$ \text{S} + \text{Mg} \rightarrow \text{MgS} $$
This reaction reduces the effective magnesium available for graphite spheroidization at the casting surface. In ductile cast iron, the morphology of graphite is critically dependent on the balance between sulfur and nodularizing elements like magnesium. Sulfur is a surface-active element that adsorbs preferentially on the prismatic planes of graphite, reducing the interfacial energy between the iron melt and these planes. This promotes graphite growth along the [1010] direction, leading to flake formation. In contrast, magnesium neutralizes sulfur by forming MgS, altering the interfacial energy balance and promoting growth along the [0001] direction, resulting in spheroidal graphite. The condition for spheroidization can be expressed as:
$$ \omega(\text{Mg}_{\text{effective}}) > k \cdot \omega(\text{S}) $$
where $\omega(\text{Mg}_{\text{effective}})$ is the effective magnesium content, $\omega(\text{S})$ is the sulfur content, and $k$ is a constant dependent on other factors. When sulfur from the mold increases the surface sulfur content, this inequality fails, causing poor spheroidization.
To address the issue, we considered three strategies: reducing the sulfur source, blocking the transmission path, or counteracting sulfur’s effects. Reducing the sulfur source involved measures like lowering the LOI of reclaimed sand, using low-sulfur hardeners, reducing hardener addition, decreasing the sand-to-metal ratio, or adding new sand. Blocking the path included using sulfur-resistant coatings, increasing cooling rates, or lowering pouring temperatures. Counteracting sulfur meant increasing residual magnesium in the iron. After evaluation, we opted to block the transmission path by implementing a dedicated anti-sulfur coating, as it allowed us to maintain existing production parameters while effectively solving the problem.
We switched to a sulfur-resistant coating designated FQ30pro. This coating was applied twice: first with a Baume degree of 50–55 °Bé, followed by drying and ignition, then a second coat at 40–45 °Bé. The total coating thickness was controlled at 0.3–0.5 mm. The technical specifications of FQ30pro are summarized in Table 5. The main aggregates in this coating are graphite and forsterite (magnesium olivine), which provide both barrier and chemical absorption properties against sulfur.
| Type | Color | Main Aggregates | Density (g/cm³) | Baumé Degree (°Bé) | Suspension Rate (2 h) | Applicability | Shelf Life |
|---|---|---|---|---|---|---|---|
| FQ30pro | Yellow | Graphite, Forsterite | 1.20–1.50 | 60–70 | ≥98% | Ductile Cast Iron | 6 months |
After implementing the FQ30pro coating, we re-evaluated the castings. No visible color differences were observed on the machined surfaces. Metallographic analysis showed well-formed spheroidal graphite throughout the surface layer, with a spheroidization rate exceeding 90% and graphite size of 5–6. Mechanical properties improved significantly, meeting all specifications: tensile strength above 420 MPa, yield strength above 290 MPa, and elongation over 19%. The results are summarized in Table 6. This confirmed that the sulfur-resistant coating effectively prevented sulfur infiltration, preserving the spheroidization quality of the ductile cast iron castings.
| Location on Casting | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Spheroidization Rate (%) | Graphite Size |
|---|---|---|---|---|---|
| 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 |
| Customer Requirement | ≥420 | ≥290 | ≥10 | ≥90 | 5–7 |
The effectiveness of the FQ30pro coating stems from its dual mechanism: physical barrier and chemical neutralization. The coating forms a dense layer that impedes the diffusion of sulfur-containing gases or species from the sand into the molten iron. Moreover, the forsterite (Mg₂SiO₄) in the coating reacts with sulfur oxides or sulfides at high temperatures, absorbing and neutralizing them. The reactions can be represented as:
$$ \text{SO}_2 + \text{MgO} \rightarrow \text{MgSO}_3 $$
$$ \text{SO}_3^{2-} + \text{Mg}^{2+} \rightarrow \text{MgSO}_3 $$
$$ \text{S}^{2-} + \text{Mg}^{2+} \rightarrow \text{MgS} $$
These reactions occur within the coating layer, preventing sulfur from reaching the ductile cast iron surface. Thus, the residual magnesium in the iron remains available for graphite spheroidization, ensuring high-quality ductile cast iron properties. This approach is particularly advantageous as it does not require major changes in sand composition or melting practice, making it cost-effective and easy to integrate into existing production lines for ductile cast iron castings.
To further optimize the process, we also monitored and controlled other factors. Maintaining the reclaimed sand’s LOI below 3.0% and sulfur content below 0.20% is essential. Regular sand testing ensures consistency. The hardener addition rate was optimized to the minimum necessary for adequate curing, typically around 30% of the resin weight, to minimize sulfur input. Additionally, we ensure proper sand compaction and venting to reduce gas pressure that might drive sulfur penetration. The pouring temperature is kept within the range of 1,310–1,330 °C to balance fluidity and minimize reaction time. These measures, combined with the sulfur-resistant coating, provide a robust solution for producing defect-free ductile cast iron castings.
In conclusion, the surface color difference issue in ductile cast iron castings produced with furan resin sand is primarily caused by sulfur from the hardener infiltrating the casting surface and reacting with magnesium, leading to graphite degeneration. Through systematic analysis, we identified that using a specialized sulfur-resistant coating, FQ30pro, effectively blocks and neutralizes sulfur, preserving spheroidization. The coating’s mechanism involves both physical barrier formation and chemical absorption via magnesium-containing compounds. This solution has been successfully implemented in our production, eliminating color differences and ensuring that the ductile cast iron castings meet all mechanical and metallographic specifications. Future work may explore further refinements in coating formulations or alternative low-sulfur binder systems to enhance sustainability. This case underscores the importance of comprehensive material control and tailored solutions in achieving high-quality ductile cast iron components.
The successful resolution of this problem highlights key principles in ductile cast iron foundry practice. First, understanding the interaction between molding materials and molten metal is crucial. Second, proactive monitoring of sand parameters like sulfur content and LOI can prevent issues. Third, targeted interventions, such as functional coatings, can solve specific problems without disrupting overall production. For ductile cast iron, maintaining graphite spheroidization is paramount, and any factor affecting it must be carefully managed. Our experience demonstrates that with proper analysis and innovation, challenges like surface color differences can be overcome, ensuring reliable performance of ductile cast iron castings in demanding applications.
In summary, the production of high-integrity ductile cast iron castings requires attention to detail across the entire process chain. From sand preparation and binder selection to coating application and pouring control, each step influences the final quality. By addressing the sulfur infiltration issue with a scientifically designed coating, we have enhanced the consistency and reliability of our ductile cast iron products. This approach not only solves the immediate problem but also contributes to broader knowledge in foundry technology, particularly for ductile cast iron manufacturing. As industries demand higher performance from ductile cast iron components, such advancements will remain essential for meeting stringent quality standards and customer expectations.