In the production of ductile iron castings using furan resin sand, a significant challenge arises from the formation of abnormal graphite layers, such as flake or vermicular graphite, on the casting surface. This phenomenon is primarily attributed to sulfur penetration, which adversely affects the mechanical properties and service life of components like elevator traction wheels and agricultural machinery parts. Through extensive research and experimentation, we have developed innovative composite coatings designed to mitigate sulfur infiltration. This article details the mechanisms of sulfur penetration, the design principles of these coatings, and their practical application in enhancing the quality of ductile iron castings.
The issue of sulfur-induced graphite degeneration in ductile iron castings is a critical concern in foundry processes. Furan resin sand, while advantageous for its high dimensional accuracy and efficiency, decomposes under high temperatures to release sulfur-containing gases. These gases permeate the molten metal interface, leading to localized graphite abnormalities. Our study focuses on analyzing the sulfur penetration mechanism and formulating coatings that effectively block or absorb sulfur, thereby improving the microstructure and performance of ductile iron castings.
Sulfur Penetration Mechanism in Ductile Iron Castings
Sulfur penetration in ductile iron castings occurs through a series of high-temperature reactions during the pouring process. The furan resin sand, containing catalysts like p-toluenesulfonic acid, decomposes in the presence of oxygen to release sulfur dioxide (SO₂). The reaction can be represented as:
$$ \text{CH}_3\text{C}_6\text{H}_4\text{SO}_3\text{H} + X\text{O}_2 \rightarrow Y\text{CO}_2 + Z\text{CO} + P\text{H}_2\text{O} + Q\text{C}_m\text{H}_n + \text{SO}_2 \uparrow $$
Subsequently, SO₂ gas adsorbs onto the iron melt surface, dissociating into sulfur and oxygen atoms:
$$ \text{SO}_2 (g) \rightarrow \text{S} + 2\text{O} $$
The sulfur atoms then diffuse into the molten iron, establishing a concentration gradient and reacting with elements like magnesium (Mg), rare earth (RE), and manganese (Mn), which have high affinity for sulfur. This results in the formation of sulfides and degrades the spheroidization of graphite in the surface layer of ductile iron castings. The depth of this affected zone varies, but it can compromise the fatigue strength and longevity of the castings.
Design Principles for Anti-Sulfur Penetration Coatings
To address sulfur penetration in ductile iron castings, we designed coatings based on three core principles: blocking, absorption, and shielding. Each type targets different aspects of sulfur interaction with the casting surface.
Blocking-Type Coatings: These coatings form a dense, sintered layer between the mold and molten metal, preventing SO₂ gas from reaching the iron interface. The effectiveness depends on the coating’s integrity, uniformity, and sintering density. Key components include quartz powder and fluxing agents that promote sintering at high temperatures.
Absorption-Type Coatings: By incorporating alkaline materials like dolomite powder, these coatings chemically react with SO₂ to form solid sulfates, thus capturing sulfur before it penetrates the casting. The reactivity and adsorption capacity of the alkaline components are crucial for performance.
Shielding-Type Coatings: Combining both blocking and absorption mechanisms, shielding coatings offer the most robust protection. They integrate sintering promoters and alkaline absorbers to create a multi-layered defense against sulfur infiltration in ductile iron castings.
The following table summarizes the key characteristics of each coating type:
| Coating Type | Mechanism | Key Components | Advantages |
|---|---|---|---|
| Blocking | Forms sintered barrier | Quartz powder, flux agents | High density, prevents gas contact |
| Absorption | Chemical reaction with SO₂ | Dolomite powder, alkaline compounds | Captures sulfur, reduces penetration |
| Shielding | Combined blocking and absorption | Quartz, dolomite, flux agents | Enhanced protection, minimal graphite degeneration |
Selection and Formulation of Coating Materials
The choice of refractory materials is pivotal in developing effective coatings for ductile iron castings. We selected quartz powder (SiO₂ ≥ 98%) as the primary refractory due to its low thermal conductivity, affordability, and ability to sinter with fluxing agents. Dolomite powder (CaMg(CO₃)₂) was incorporated for its alkaline properties, decomposing at 700–900 °C to CaO and MgO, which react with SO₂ to form stable sulfates. Graphite powders, both flake and earthy types, were added for their high refractoriness and non-wettability, though they require flux agents to improve sintering.
The physicochemical properties of these materials are outlined below:
| Material | Main Composition | Density (g/cm³) | Linear Expansion Coefficient (×10⁻⁶/°C) | Melting Point (°C) | Chemical Nature |
|---|---|---|---|---|---|
| Quartz Powder | SiO₂ | 2.65 | 12.3 | 1,713 | Acidic |
| Dolomite Powder | CaMg(CO₃)₂ | 2.90 | 1.2 | 1,850 | Alkaline |
| Graphite Powder | C | 2.25 | – | >3,000 | Neutral |
Additional components included magnesium aluminum silicate as a suspending agent for stability, phenolic resin and rosin as binders to enhance cohesion, polyvinyl butyral (PVB) as a thickener, and ethanol as the carrier liquid for easy application and drying. The optimal composition of the refractory base was determined through iterative testing, resulting in the following blend:
| Component | Mass Percentage (%) |
|---|---|
| Quartz Powder | 52 |
| Flake Graphite | 15 |
| Flux Agent A | 6 |
| Dolomite Powder | 15 |
| Earthy Graphite | 8 |
| Flux Agent B | 4 |
Three distinct coatings were formulated based on this base, as shown in the table below. Coating 1 represents the blocking type, Coating 2 the absorption type, and Coating 3 the shielding type, each with varying proportions of key ingredients to evaluate their anti-sulfur performance in ductile iron castings.
| Component | Coating 1 (Blocking) % | Coating 2 (Absorption) % | Coating 3 (Shielding) % |
|---|---|---|---|
| Quartz Powder | 61.2 | 57.8 | 52 |
| Flake Graphite | 17.6 | 16.7 | 15 |
| Earthy Graphite | 9.4 | 8.8 | 8 |
| Dolomite Powder | 0 | 16.7 | 15 |
| Flux Agent A | 7.1 | 0 | 6 |
| Flux Agent B | 4.7 | 0 | 4 |
| Phenolic Resin | 0.6 | 0.6 | 0.6 |
| PVB | 0.1 | 0.1 | 0.1 |
| Magnesium Aluminum Silicate | 6 | 6 | 6 |
| Rosin | 1.4 | 1.4 | 1.4 |
| Ethanol | As needed | As needed | As needed |
Experimental Methodology and Coating Application
To validate the effectiveness of these coatings, we conducted production trials using elevator traction wheels made of QT700-2 ductile iron. The chemical composition of the iron is provided in the table below, ensuring consistency in material properties across tests.
| Element | Content (wt%) |
|---|---|
| C | 3.5–3.7 |
| Si | 2.0–2.4 |
| Mn | 0.4–0.8 |
| P | ≤0.07 |
| S | ≤0.02 |
| Mg | 0.04–0.06 |
| RE | 0.02–0.04 |
| Cu | 0–1.0 |
The molten iron was prepared in a medium-frequency induction furnace using Q10 pig iron and carbon steel scrap, with a tapping temperature of 1,420 °C ± 10 °C and a pouring temperature of 1,350 °C ± 10 °C. Spheroidization was achieved via the sandwich method with 1.2% nodulizer. Furan resin sand molds were employed, with resin added at 1.5% of sand weight and catalyst at 40% of resin weight. Coatings were applied using a flow coating process to ensure uniformity, with a Baume degree of approximately 48. The dried coating thickness ranged from 200 to 250 μm, and the molds were dried prior to pouring.
Results and Analysis of Coating Performance
After shakeout and shot blasting, samples were extracted from the castings at specified locations (20 mm × 40 mm areas) and prepared for metallographic analysis. The samples were ground, polished, and etched with 4% nital to examine the graphite structure at depths of 0–1 mm, 1–2 mm, and 2–3 mm from the surface. The thickness of the abnormal graphite layer was measured to assess the degree of sulfur penetration in each ductile iron casting.
For Coating 1 (blocking type), the abnormal graphite layer extended up to 1.1–1.15 mm, indicating limited effectiveness. The sintering barrier alone was insufficient to fully block SO₂ permeation. In contrast, Coating 2 (absorption type) reduced the layer thickness to 0.65–1.0 mm, as the dolomite component captured sulfur through reactions like:
$$ \text{CaO} + \text{SO}_2 + \frac{1}{2}\text{O}_2 \rightarrow \text{CaSO}_4 $$
$$ \text{MgO} + \text{SO}_2 + \frac{1}{2}\text{O}_2 \rightarrow \text{MgSO}_4 $$
Coating 3 (shielding type) demonstrated the best performance, with the abnormal layer limited to 0.25–0.3 mm. The combination of flux agents and dolomite created a synergistic effect, where the sintered barrier impeded gas flow while the alkaline materials absorbed residual sulfur. This dual mechanism significantly enhanced the protection of ductile iron castings from graphite degeneration.
The sulfur content in the coatings before and after pouring was analyzed to quantify absorption. The relationship between dolomite content and sulfur capture can be expressed as:
$$ S_{\text{absorbed}} = k \cdot C_{\text{dolomite}} $$
where \( S_{\text{absorbed}} \) is the sulfur absorbed, \( k \) is a reaction constant, and \( C_{\text{dolomite}} \) is the dolomite concentration. Experimental data showed that beyond 15% dolomite, the rate of sulfur absorption plateaued, justifying the selected proportion.
The following table compares the performance metrics of the three coatings in ductile iron castings production:
| Coating Type | Abnormal Graphite Layer Thickness (mm) | Sulfur Absorption Efficiency | Overall Effectiveness |
|---|---|---|---|
| Blocking (Coating 1) | 1.1–1.15 | Low | Moderate |
| Absorption (Coating 2) | 0.65–1.0 | High | Good |
| Shielding (Coating 3) | 0.25–0.3 | Very High | Excellent |
Discussion on Coating Mechanisms and Industrial Implications
The superior performance of the shielding-type coating in protecting ductile iron castings stems from its multi-faceted approach. The flux agents, such as Flux A and B, lower the sintering temperature of quartz and graphite, promoting the formation of a dense, continuous layer that physically blocks SO₂ diffusion. Simultaneously, dolomite acts as a chemical scavenger, reacting with sulfur species to form harmless compounds. This is particularly important in high-temperature environments where sulfur mobility is high.
In industrial settings, the application of these coatings can lead to significant improvements in the quality of ductile iron castings. For instance, components like traction wheels and bridge shells exhibit enhanced fatigue resistance and longer service life due to the reduction in surface defects. The cost-effectiveness of the materials, such as quartz and graphite, also makes this solution viable for large-scale production. Moreover, the use of ethanol as a carrier ensures environmental safety and easy handling.
Further optimization could involve adjusting the particle size distribution of refractory powders to enhance coating density or exploring alternative alkaline materials for even higher sulfur affinity. The principles established here can be extended to other casting processes where sulfur penetration is a concern, underscoring the versatility of this approach for ductile iron castings.
Conclusion
Our research demonstrates that composite coatings incorporating quartz powder, graphite, and dolomite can effectively mitigate sulfur penetration in ductile iron castings produced with furan resin sand. The shielding-type coating, which combines sintering promoters and alkaline absorbers, offers the best protection by reducing the abnormal graphite layer to minimal thickness. This advancement not only improves the mechanical properties of ductile iron castings but also supports the production of more reliable and durable industrial components. Future work will focus on refining the coating formulations and expanding their application to other alloy systems.