In the field of metal casting, resin sand casting has emerged as a predominant method for producing carbon steel components, particularly low carbon steel castings. However, a significant challenge associated with resin sand casting is the tendency for carburization to occur on the surface of low carbon steel castings. This phenomenon leads to an increase in carbon content at the casting surface, resulting in microstructural alterations such as increased pearlite, decreased ferrite, and the potential formation of acicular martensite. These changes detrimentally affect the mechanical properties of the castings, including reduced plasticity, toughness, machinability, and weldability. Therefore, developing effective anti-carburizing coatings is paramount for the resin sand casting process to ensure product quality and performance.
The conventional approach employs zircon flour as the primary refractory material in anti-carburizing and anti-burning-on coatings for resin sand casting. While effective, zircon flour is characterized by scarcity, high cost, and the presence of low concentrations of radioactive elements like uranium and thorium in associated minerals such as monazite. These elements pose health risks through ionizing radiation, potentially causing DNA damage and increased chromosomal aberration rates. Consequently, there is an urgent need to identify and develop alternative, cost-effective, and environmentally friendly refractory materials for use in resin sand casting coatings.
This article, from my research perspective, delves into the development, application, and mechanistic analysis of a novel water-based, self-drying coating formulated with zirconium-containing composite silicate mineral powder. This material serves as a direct replacement for pure zircon flour in resin sand casting applications. The core objective is to present a comprehensive study on a coating that not only prevents carburization and sand burning-on but also offers advantages in cost reduction, improved working environment, and enhanced casting surface finish in resin sand casting processes.
The foundation of any effective coating for resin sand casting lies in its meticulous composition design. A casting coating is typically a multicomponent system comprising a refractory aggregate, binder, sintering agent, suspending agent, carrier liquid (solvent), and various additives. Each component plays a critical role in determining the final coating’s performance during the resin sand casting process.
1. Refractory Aggregate: The skeletal structure of the coating is provided by the refractory aggregate. It must possess high refractoriness to prevent mechanical penetration of molten metal (burning-on) and exhibit stable chemical behavior at high temperatures to avoid interfacial reactions with the resin sand mold/core and the molten steel. For this coating system, I selected zirconium-containing composite silicate mineral powder as the primary refractory aggregate. This material is a complex silicate predominantly based on aluminosilicates, with a light white appearance. Its typical chemical composition range is as follows: $Al_2O_3 \geq 50\%$, $SiO_2 \leq 35\%$, $Fe_2O_3 \leq 1\%$, $TiO_2 \leq 5\%$, $ZrO_2 \leq 9\%$. Radiometric analysis indicates its specific activity for U is approximately 1.671 Bq/g and for Th is 0.413 Bq/g, which is about half that of typical Australian or South African zircon flours, making it a safer choice for the resin sand casting workshop environment.
X-ray diffraction (XRD) phase analysis reveals that the primary crystalline phases in this powder are corundum ($\alpha-Al_2O_3$), quartz ($SiO_2$), and zircon ($ZrSiO_4$), along with a minor amount of a specific phase referred to as Phase A. The thermal behavior of Phase A is crucial. It exhibits a high coefficient of thermal expansion and undergoes an irreversible thermal decomposition above approximately 1200°C, transforming into mullite ($3Al_2O_3 \cdot 2SiO_2$) and a high-viscosity silica glass (cristobalite glassy aggregate). This transformation is accompanied by significant volume expansion. By 1450°C, the decomposition is essentially complete. The formed mullite is highly refractory (stable up to 1800°C) and chemically inert. The irreversible expansion during heating and the subsequent differential contraction during cooling between the coating (now containing glassy phase and mullite) and the steel casting create substantial interfacial shear stresses. This promotes the automatic, flaky peeling of the coating after casting solidification and cooling, a highly desirable feature in resin sand casting for easy cleanup. The high-viscosity glassy phase formed during sintering also resists metal penetration, aiding in the prevention of mechanical burn-on. The market price of this composite powder is only about one-third to one-quarter that of pure zircon flour, offering significant cost savings for resin sand casting operations.
The particle morphology, however, differs from spherical zircon flour particles. The composite silicate powder particles are primarily angular or multi-angular. While this can influence packing density, it is managed through careful particle size distribution control.
2. Sintering Agent: The sintering agent is vital for facilitating the formation of a continuous, dense sintered layer on the coating surface upon exposure to molten metal heat during resin sand casting. This sintered layer acts as a barrier against both sand burn-on and carbon diffusion. I utilized a fluoride-based compound as the sintering agent. The mechanism involves fluoride ions ($F^-$) disrupting the aluminum-oxygen and silicon-oxygen networks in the refractory aggregate due to their similar ionic radius to $O^{2-}$ but much lower electrostatic potential. The reaction can be conceptually represented as:
$$ \text{Si-O-Si} + 2F^- \rightarrow \text{Si-F} + \text{Si-O}^- \quad \text{(Network Breaking)} $$
$$ \text{Al-O-Al} + 2F^- \rightarrow \text{Al-F} + \text{Al-O}^- $$
This network breaking lowers the melting point of the aggregate, promoting sintering. However, an excess can be detrimental. Experimental findings indicate an optimal addition range of 1% to 3% of the total refractory aggregate weight.
Additionally, a sintering aid was incorporated. This aid not only enhances sintering but also thermally decomposes at temperatures exceeding 1000°C to generate a reducing gas with high vapor pressure and good high-temperature chemical stability. This gas, in conjunction with the dense sintered layer, helps to prevent contact between solid carbonaceous materials from the resin sand mold/core and the molten steel at the interface, thereby inhibiting the solid-state diffusion of carbon into the steel surface layer during resin sand casting.
3. Particle Size and Gradation: The particle size distribution of the powder components profoundly affects coating properties in resin sand casting. Finer particles generally improve suspension stability, brushability, permeability into the sand mold, and the shielding effectiveness against carbon. However, excessively fine particles can lead to cracking in the coating layer. Therefore, a moderate particle size range, typically between 200 to 500 mesh, was targeted for the refractory base and sintering agent.
Furthermore, achieving a dense coating requires an optimized particle size gradation. A well-designed, broad size distribution allows finer particles to fill the voids between coarser ones, minimizing interstitial spaces. The packing density ($\phi$) can be conceptually related to the particle size distribution. For a binary mixture of coarse and fine particles, the maximum packing fraction can be estimated if the fine particle diameter ($d_f$) is much smaller than the coarse particle diameter ($d_c$). A simple model for the volume fraction of solids in a densely packed coating layer might be expressed as:
$$ \phi_{max} \approx \phi_c + (1 – \phi_c) \phi_f $$
where $\phi_c$ is the packing fraction of the coarse particles alone and $\phi_f$ is the packing fraction of the fine particles within the remaining voids. In practice, continuous grading is used. The particle size distribution was engineered to follow a modified Fuller curve for optimal density, which for powders can be represented by the equation:
$$ P(d) = 100 \times \left( \frac{d}{d_{max}} \right)^n $$
where $P(d)$ is the cumulative percentage finer than size $d$, $d_{max}$ is the maximum particle size, and $n$ is an exponent typically between 0.3 and 0.5 for dense packing. This careful gradation is essential for producing a pore-free, impermeable barrier in resin sand casting coatings.
4. Binder System: The coating must possess adequate strength both at room temperature (for handling) and at elevated temperatures (to resist erosion by molten metal). A composite binder system was adopted, combining an organic low-temperature binder for room temperature strength and an inorganic high-temperature binder. Considering the anti-carburizing requirement, the content of the organic binder—which upon pyrolysis can be a source of carbon—was minimized while still ensuring sufficient green strength for the resin sand casting process. The inorganic binder provides strength after the organic components have burned off during metal pouring.
5. Carrier Liquid (Solvent): Common solvents are water or alcohols. Alcohol-based coatings often rely on organic binders. Since both the alcohol solvent and organic binders can pyrolyze to leave carbon residues, potentially increasing carburization risk, water was chosen as the solvent for this coating formulation aimed at resin sand casting of low carbon steel.
The table below summarizes the key technical properties of the developed zirconium-containing composite silicate water-based, self-drying coating, as tested in our laboratory. These properties are critical for its successful application in resin sand casting.
| Property | Value / Description | Significance in Resin Sand Casting |
|---|---|---|
| Solvent | Water | Reduces carbon source, cost-effective, environmentally friendly. |
| Conditional Viscosity (Ford Cup #4, s) | 12 | Indicates good flow and application characteristics. |
| Density ($\times 10^3$ kg/m³) | 1.92 | Optimal for suspension and coating thickness control. |
| Suspension Stability (24 h, %) | 99 | Excellent; minimizes settling, ensures consistent composition during application in resin sand casting. |
| Thixotropy Index / Ratio | 37.2% | High value indicates good leveling and brushability, though lower anti-sagging. |
| Brushability Index ($\eta_6 / \eta_{60}$) | 7.6 | High ratio denotes strong shear-thinning, making brushing easy and allowing thicker application without runs. |
| Gas Evolution (mL/g) | 12.7 | Low gas generation reduces the risk of gas-related casting defects like pinholes in resin sand casting. |
| High-Temperature Cracking Resistance | Grade I (Excellent) | Minimizes cracks during heating, ensuring a continuous barrier. |
The brushability index is defined as the ratio of the apparent viscosity at a low shear rate (6 rpm, $\eta_6$) to that at a high shear rate (60 rpm, $\eta_{60}$). A higher index signifies more pronounced shear-thinning behavior, which is beneficial for brushing. The thixotropy rate indicates the change in viscosity with stirring time; a higher rate correlates with better leveling but slightly reduced resistance to sagging. These rheological properties are finely tuned for the manual or automated application processes common in resin sand casting foundries.
To evaluate the practical performance of this coating in a real-world resin sand casting scenario, a production trial was conducted. The test casting was a stainless steel impeller made of material ZG1Cr18Ni9Ti, weighing approximately 30 kg, with wall thicknesses ranging from 15 to 22 mm. The molten steel was tapped at around 1700°C. The molds and cores were produced using acid-cured furan resin sand—a standard resin sand casting process. For controlled comparison, two identical rectangular test block molds (50 mm × 90 mm × 110 mm) were also prepared from the same resin sand. One set of the impeller mold and a test block mold was coated with the novel composite silicate coating, while another set was coated with a conventional commercial zircon flour-based water-based coating. The coating was applied to a total thickness of 0.5 to 0.8 mm. Importantly, no pre-baking was performed before pouring to avoid any potential carbon deposition from burner flames affecting the carburization results—a critical consideration in resin sand casting studies.
After casting, cooling, and shakeout, the coatings on the impellers cast with the novel material exhibited excellent self-peeling behavior, detaching in large flakes. The casting surfaces were smooth and free from burn-on, sand inclusion, gas holes, or coating penetration defects. This significantly improves finishing efficiency in resin sand casting. Test blocks were sectioned at identical locations using wire electrical discharge machining (EDM). The chemical composition at the center and at various depths from the surface was analyzed using a direct reading optical emission spectrometer (OES). The results are summarized in the table below. The original molten steel carbon content was between 0.106% and 0.110%. The target for the casting surface layer carbon content was to keep it below 0.12%.
| Coating Type | Measurement Location | Carbon Content, C (%) | Notes on Carburization |
|---|---|---|---|
| Zirconium-containing Composite Silicate Coating | Test Block Surface (0 mm) | 0.136 | Surface carburization is present but significantly lower than with zircon flour coating. The effective carburized layer thickness (where C > 0.12%) is only about 1 mm. |
| 1 mm below surface | 0.122 | ||
| 2 mm below surface | 0.115 | ||
| Center of test block | 0.110 | ||
| Commercial Zircon Flour Coating | Test Block Surface (0 mm) | 0.178 | Severe carburization observed. The surface carbon exceeds the 0.12% limit. The carburized layer (C > 0.12%) extends beyond 2 mm. |
| 1 mm below surface | 0.137 | ||
| 2 mm below surface | 0.132 | ||
| Center of test block | 0.106 |
The data clearly demonstrates the superior anti-carburizing performance of the novel coating in this resin sand casting application. For the composite silicate coating, the surface carbon content was 0.136%, representing a maximum carburization rate of:
$$ \text{Max. Carburization Rate} = \frac{0.136 – 0.110}{0.110} \times 100\% \approx 23.6\% $$
The depth at which the carbon content fell below the 0.12% threshold was approximately 1 mm. In contrast, for the zircon flour coating, the surface carbon was 0.178%, giving a maximum carburization rate of about 67.9%, nearly three times higher. The carburized layer exceeded 2 mm. This confirms that the novel coating provides an effective barrier against carbon pickup during the resin sand casting process.
Further validation was obtained using Energy Dispersive Spectroscopy (EDS) line scan analysis across the subsurface region (up to 6 mm deep) of test blocks from the same heat. The EDS results plotted the incremental ratio of carbon content, corroborating the OES findings and reinforcing the conclusion that the zirconium-containing composite silicate coating offers excellent protection against surface carburization in resin sand casting.
To understand the fundamental mechanism behind the anti-carburizing and anti-burning-on effects, the microstructure of the coating sintered skin after casting and cooling was examined using Scanning Electron Microscopy (SEM). The SEM analysis revealed distinct differences. The sintered layer from the novel coating showed a more extensively fused and interconnected microstructure. The particles appeared to be more thoroughly sintered together with widespread neck formation and surface melting, creating a denser, more continuous, and glassy matrix. This contrasts with the sintered structure of the zircon flour coating, which appeared less fused and more porous.
The enhanced sintering and densification in the composite silicate coating can be attributed to the action of the sintering agent (fluoride) and the presence of Phase A. The thermal decomposition and phase transformation of Phase A, as described earlier, contribute to liquid phase sintering at high temperatures. The volume expansion associated with this transformation helps to close pores, and the resulting high-viscosity silica glass effectively seals the coating’s structure. The density of the sintered layer ($\rho_{sintered}$) and its porosity ($\epsilon$) are critical parameters. The effectiveness of the coating as a diffusion barrier can be related to its density and the tortuosity ($\tau$) of any remaining pore pathways. The flux of carbon atoms ($J_C$) through a porous barrier can be modeled by an equation considering diffusion through the solid phase and through pores:
$$ J_C \propto \frac{D_{eff} \cdot \Delta C}{\delta} $$
where $D_{eff}$ is the effective diffusion coefficient in the porous layer, $\Delta C$ is the carbon concentration gradient, and $\delta$ is the coating thickness. $D_{eff}$ is significantly reduced in a dense, glassy matrix compared to a porous, particulate one. It can be approximated for a porous solid as:
$$ D_{eff} = D_s \cdot \frac{(1 – \epsilon)}{\tau} $$
Here, $D_s$ is the diffusion coefficient in the solid sintered material (which is very low for a glassy silicate), $\epsilon$ is the porosity, and $\tau$ is the tortuosity factor (>>1 for a dense, winding pore structure). The novel coating achieves a very low $\epsilon$ and high $\tau$, minimizing $D_{eff}$ and thus the carbon flux. Furthermore, the reducing gas generated by the sintering aid creates a local atmosphere at the coating/molten metal interface that hinders the activity of carbonaceous species, adding a chemical barrier to the physical one. This multi-faceted barrier mechanism is highly effective in the context of resin sand casting.
The angular particle shape of the composite silicate powder, while initially a concern, is effectively compensated by the optimized particle size distribution and the sintering process that leads to a dense, glassy phase. The differential thermal contraction between this glassy/mullite coating layer and the steel casting, calculated based on their coefficients of thermal expansion ($\alpha_{coating}$ and $\alpha_{steel}$), generates the interfacial shear stress ($\tau_{interface}$) that promotes self-peeling:
$$ \tau_{interface} \propto E_{coating} \cdot (\alpha_{steel} – \alpha_{coating}) \cdot \Delta T $$
where $E_{coating}$ is the elastic modulus of the cooled coating layer and $\Delta T$ is the temperature drop from the sintering temperature to room temperature. The designed composition ensures a sufficient difference in $\alpha$ values.
In conclusion, the development and application of this zirconium-containing composite silicate mineral powder-based coating represent a significant advancement for the resin sand casting industry, particularly for producing low carbon and stainless steel castings. The coating formulation, through careful selection and balance of components—including the cost-effective and low-radioactivity refractory aggregate, efficient sintering system, optimized particle gradation, and water-based binder system—delivers outstanding performance. Its excellent suspension, brushability, permeability, leveling, and crack resistance ensure easy and reliable application in resin sand casting processes. The low gas evolution minimizes casting defects. Most notably, the coating provides a dual function: superior anti-burning-on performance evidenced by smooth, peelable casting surfaces, and effective anti-carburization protection, limiting surface carbon increase to acceptable levels with a shallow affected layer. This performance surpasses that of traditional zircon flour-based coatings in the specific resin sand casting trial described. By replacing expensive, imported zircon flour with a more economical and safer alternative, this coating technology offers a path to reduce production costs significantly, improve the environmental and safety conditions in foundries, and enhance the quality of castings produced via resin sand casting. The successful integration of such materials addresses a critical industry need and paves the way for more sustainable and efficient resin sand casting operations.
The principles established here—focusing on dense sintering through tailored phase transformations and composite material design—can be extended to other coating systems for different alloy families within the broad domain of resin sand casting. Future work may involve further optimization of the particle size distribution using advanced modeling, exploration of other synergistic sintering aids, and testing on a wider range of low-carbon steel grades and casting geometries in resin sand casting production environments. The ultimate goal remains the continuous improvement of coating technology to support the production of high-integrity, cost-effective castings through the versatile and widely used resin sand casting method.