In the production of high manganese steel castings using the lost foam casting process, the selection of coatings is critical to avoid defects such as severe sand sticking, especially in thermal sections or thick-walled castings. Traditional zircon powder coatings, being weakly acidic, can react with the alkaline nature of high manganese steel, leading to adhesion issues. To address this, I developed an alcohol-based coating system that ensures high performance and cost-effectiveness for high manganese steel castings. This article details the formulation, testing, and application of this coating, focusing on its components, properties, and industrial validation.
The coating was formulated using brown alumina powder as the refractory aggregate, phenolic resin as the binder, lithium-based bentonite as the suspending agent, and industrial ethanol as the solvent. Additionally, calcium fluoride (CaF₂) was incorporated to enhance the coating’s peelability. Brown alumina, with a melting point of 2,050°C and low expansion rate, offers excellent thermal and chemical stability, resisting reactions with acids, bases, and oxides, which prevents sand sticking in high manganese steel castings. Phenolic resin provides strong adhesion, while lithium-based bentonite ensures uniform suspension. The bentonite was pre-treated by mixing with a small amount of water, allowing it to sit for 24 hours to form a paste, then dispersed with industrial ethanol using a high-speed mixer to create a膏状 suspending agent.
To evaluate the coating’s performance, several key properties were tested: suspension stability, gas permeability, coating strength, high-temperature thermal shock resistance, and thixotropy. Suspension stability was measured using the sedimentation method, where the volume fraction of precipitate in a 100 mL cylinder after 24 hours was recorded. Gas permeability was determined on an STZ direct permeability tester by assessing the dry-state permeability of the coating layer using standard samples. Coating strength was evaluated by dropping sand (50/100 mesh) from a viscosity cup onto a coated glass plate until the coating was abraded to expose the glass; the total mass of sand used indicated the surface strength. High-temperature thermal shock resistance was tested by coating a φ50 mm water glass sand sample to a thickness of 2 mm, drying it, and then placing it in a furnace at 1,200°C for 2 minutes to observe cracking. Thixotropy was measured with an NDN-1 rotational viscometer, plotting the apparent viscosity against time to obtain the thixotropy curve.
The composition of the coating was optimized by varying the components relative to the refractory aggregate, which was set at 100%. Phenolic resin content ranged from 1% to 3%, lithium-based bentonite from 1% to 6%, and CaF₂ from 2% to 4%. The suspension stability was significantly influenced by these components; for instance, phenolic resin at 1.5% to 2% and lithium-based bentonite at 2% to 4% yielded optimal results. The relationship between component addition and suspension stability can be expressed using a simplified formula for suspension efficiency: $$ S = k_1 \cdot P + k_2 \cdot B $$ where \( S \) is the suspension percentage, \( P \) is the phenolic resin content, \( B \) is the bentonite content, and \( k_1 \) and \( k_2 \) are constants derived from experimental data. This highlights the importance of balanced formulation for high manganese steel castings.
An orthogonal experimental design was employed to identify the optimal配方, with factors and levels as shown in Table 1. The L9(3^4) orthogonal array was used, focusing on phenolic resin (A), lithium-based bentonite (B), and CaF₂ (C) as the primary variables. Each factor was tested at three levels to assess their impact on suspension stability, gas permeability, coating strength, and high-temperature thermal shock resistance. The results, summarized in Table 2, were analyzed using range analysis to determine the best combination. The optimal formulation was identified as A3B3C3, corresponding to 2% phenolic resin, 4% lithium-based bentonite, and 4% CaF₂. This combination achieved a density of 1.69 g/cm³, an average coating strength of 850 g, a gas permeability of 0.85, a suspension stability of 96%, a high-temperature thermal shock resistance of Grade I, and a pH of 9, indicating an alkaline coating that promotes leveling. The thixotropy value was 46, exceeding the minimum requirement of 20 for effective application.
| Level | Phenolic Resin (A) / % | Lithium-Based Bentonite (B) / % | CaF₂ (C) / % |
|---|---|---|---|
| 1 | 1 | 2 | 2 |
| 2 | 1.5 | 3 | 3 |
| 3 | 2 | 4 | 4 |
| Experiment | Suspension / % | Gas Permeability | Coating Strength / g | High-Temperature Thermal Shock Resistance |
|---|---|---|---|---|
| A1B1C1 | 88.9 | 0.48 | 980 | I |
| A1B2C2 | 91.0 | 0.63 | 744 | I |
| A1B3C3 | 92.9 | 0.66 | 528 | II |
| A2B1C2 | 90.0 | 0.64 | 626 | I |
| A2B2C3 | 90.5 | 0.39 | 566 | I |
| A2B3C1 | 92.3 | 0.71 | 474 | II |
| A3B1C3 | 92.5 | 0.80 | 537 | I |
| A3B2C2 | 89.9 | 0.55 | 512 | I |
| A3B3C1 | 91.0 | 0.45 | 420 | II |
The gas permeability of the coating is a critical parameter for high manganese steel castings, as it affects the escape of gases during casting, reducing defects. The permeability \( k \) can be modeled using Darcy’s law for porous media: $$ k = \frac{Q \cdot L}{A \cdot \Delta P} $$ where \( Q \) is the flow rate, \( L \) is the coating thickness, \( A \) is the cross-sectional area, and \( \Delta P \) is the pressure difference. In our tests, the optimal配方 achieved a permeability of 0.85, indicating efficient gas venting. Similarly, coating strength \( S_c \) was correlated with the sand mass \( m_s \) required to abrade the coating: $$ S_c = m_s $$ which, for the best sample, was 850 g, demonstrating robust surface integrity.
Thixotropy, a measure of the coating’s viscosity change under shear, is vital for application uniformity. The thixotropy index \( TI \) was calculated from the viscosity-time curve: $$ TI = \frac{\eta_0 – \eta_t}{\eta_t} $$ where \( \eta_0 \) is the initial viscosity and \( \eta_t \) is the viscosity after a specified time. A value of 46 indicates good recovery and ease of brushing or dipping, essential for complex high manganese steel casting geometries. The alkaline nature (pH 9) further enhances flowability and reduces reactivity with the steel.
In production validation, the coating was applied to high manganese steel castings such as tooth plates using a ball mill for mixing and dip-coating for application. A single coat achieved a thickness of 0.5 to 1.5 mm, with drying at temperatures below 50°C before subsequent coats. The results showed excellent collapsibility, smooth surface finish, and no sand sticking or other defects, confirming the coating’s suitability for large-scale production of high manganese steel castings. The optimized formulation not only meets performance criteria but also reduces costs compared to traditional zircon-based coatings.
Further analysis of the suspension stability revealed that the interaction between components can be described using a multivariate regression model. For instance, the suspension percentage \( S \) as a function of phenolic resin \( P \), bentonite \( B \), and CaF₂ \( C \) can be approximated by: $$ S = \alpha + \beta P + \gamma B + \delta C + \epsilon PB + \zeta PC + \eta BC $$ where \( \alpha, \beta, \gamma, \delta, \epsilon, \zeta, \eta \) are coefficients derived from experimental data. This model helps in predicting performance for different formulations, ensuring consistency in high manganese steel casting applications.
The high-temperature performance was assessed through thermal cycling tests, where the coating maintained integrity up to 1,200°C. The thermal shock resistance \( R_{ts} \) can be quantified using the formula: $$ R_{ts} = \frac{\sigma_f \cdot E}{\alpha \cdot \Delta T} $$ where \( \sigma_f \) is the fracture strength, \( E \) is the Young’s modulus, \( \alpha \) is the thermal expansion coefficient, and \( \Delta T \) is the temperature change. Grade I resistance indicates no cracking, which is crucial for high manganese steel castings subjected to rapid cooling during casting.
In conclusion, the developed alcohol-based coating for high manganese steel castings offers superior properties, including high suspension stability, excellent gas permeability, strong coating strength, and exceptional thermal shock resistance. The use of brown alumina as the refractory aggregate ensures chemical inertness, while the optimized配方 minimizes defects in production. This coating is particularly effective for large and complex high manganese steel castings, providing a reliable solution for the foundry industry. Future work could focus on scaling up the production process and exploring alternative eco-friendly solvents to further enhance sustainability.