In the field of foundry technology, the V-process, or vacuum sealed molding, represents a significant advancement as a green and physical molding method. This technique involves using dry quartz sand without binders in a specialized flask, sealing it with a plastic film, and evacuating air to create a pressure differential that compacts the sand. It is particularly valuable for producing complex castings, including high manganese steel casting, which is known for its wear resistance and toughness but poses challenges like severe burn-on or penetration defects in thick sections or hot spots. Traditional coatings, such as zircon-based or magnesia-based alcohol coatings, often fail to prevent these issues due to low strength or bubbling when binders like polyvinyl butyral (PVB) are increased. Therefore, developing a new, cost-effective coating tailored for high manganese steel casting in V-process applications is crucial. This study focuses on formulating an alcohol-based coating using innovative materials to enhance performance and reduce costs.
Our investigation began with the selection of refractory aggregates. We utilized a by-product from ferrochromium production, known as chromium iron slag, which was crushed and ground into a powder with a fineness of 200 mesh. This material, referred to as high-chromium corundum, contains over 85% Al2O3 and more than 10% Cr2O3, giving it a melting point of 1830–2000°C, a density of 3.68 g/cm3, a Mohs hardness of 9, and a thermal expansion coefficient approximately half that of quartz. Its thermal conductivity is about twice that of quartz, making it a neutral material resistant to chemical reactions with molten high manganese steel casting. The cost-effectiveness, at roughly one-third the price of conventional materials like corundum or zircon flour, further justified its use. The properties can be summarized using the formula for thermal stress resistance, which relates to performance under thermal cycling: $$ R = \frac{\sigma_f (1 – \nu)}{\alpha E} $$ where \( R \) is thermal shock resistance, \( \sigma_f \) is fracture strength, \( \nu \) is Poisson’s ratio, \( \alpha \) is thermal expansion coefficient, and \( E \) is Young’s modulus. For high-chromium corundum, the low \( \alpha \) enhances \( R \), benefiting coating durability.
Other components were carefully chosen to optimize coating performance. As binders, we employed phenolic resin and PVB, both dissolved in industrial alcohol prior to use. Phenolic resin offers excellent heat resistance, while PVB acts as both a binder and suspending agent. To improve anti-sagging properties and promote self-hardening on plastic films, we incorporated a phosphate additive, prepared by reacting 85% industrial phosphoric acid with MgO and Al(OH)3 in a water bath at 90–100°C, followed by dilution and cooling. The final H3PO4 concentration was adjusted to 65%. As a suspending agent, lithium-based bentonite was used; it was pre-hydrated with a small amount of water, aged for 24 hours to form a paste, and then dispersed in ethanol with high-speed mixing. Minor additives included surfactants and defoamers like n-octanol. The solvent was industrial ethanol, and EVA plastic film with a thickness of 0.05–0.1 mm served as the molding membrane. The coating preparation process followed a systematic sequence: lithium bentonite + additives + PVB + phenolic resin + refractory filler + phosphate +适量 industrial alcohol, ball-milled for 30–60 minutes, followed by additional alcohol and further ball-milling for 15–20 minutes before discharge.
To evaluate coating properties, we employed standard testing methods. Suspension stability was measured by the sedimentation volume percentage in a 100 mL cylinder after 24 hours. Permeability was assessed using an STZ direct permeability tester with standard samples, calculated as: $$ P = \frac{V \cdot h}{A \cdot t \cdot \Delta p} $$ where \( P \) is permeability, \( V \) is air volume, \( h \) is sample height, \( A \) is cross-sectional area, \( t \) is time, and \( \Delta p \) is pressure difference. Coating strength was determined by sand abrasion test: sand was dropped from an Engler viscosity cup onto a coated glass plate until the coating wore through, with the total sand mass indicating strength. High-temperature crack resistance was evaluated by coating a Ø50 mm water glass sand sample, drying it, and heating at 1200°C for 2 minutes to observe cracking. Thixotropy was analyzed using an NDJ-1 rotational viscometer to plot apparent viscosity versus shearing time, with thixotropy value computed as: $$ \text{Thixotropy} = \frac{s – s’}{s} \times 100 $$ where \( s \) is viscosity at 30 seconds and \( s’ \) at 10 minutes. This is critical for ensuring good brushability and resistance to running in high manganese steel casting applications.
We conducted orthogonal experiments to determine the optimal coating formulation, focusing on four factors at three levels: phenolic resin (A), PVB (B), lithium bentonite (C), and phosphate (D). The levels are detailed in Table 1. Each trial tested suspension, permeability, coating strength, and high-temperature crack resistance, with results in Table 2. Based on performance权衡, the optimal combination was A3B1C3D2, corresponding to 2.0% phenolic resin, 0.6% PVB, 4% lithium bentonite, and 1.0% phosphate. This配方 maximized suspension stability and strength while maintaining adequate permeability for high manganese steel casting.
| Factor | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
| A: Phenolic Resin | 1.0 | 1.5 | 2.0 |
| B: PVB | 0.6 | 0.7 | 0.8 |
| C: Lithium Bentonite | 2 | 3 | 4 |
| D: Phosphate | 0.5 | 1.0 | 1.5 |
| Trial | Suspension (%) | Permeability | Coating Strength (g) | High-Temperature Crack Resistance | pH |
|---|---|---|---|---|---|
| A1B1C1D1 | 90.2 | 0.32 | 1094 | I | 8 |
| A1B2C2D2 | 91.6 | 0.33 | 844 | II | 9 |
| A1B3C3D3 | 92.0 | 0.41 | 628 | I | 8 |
| A2B1C2D3 | 91.0 | 0.34 | 726 | I | 8 |
| A2B2C3D1 | 90.7 | 0.37 | 666 | I | 9 |
| A2B3C1D2 | 91.5 | 0.36 | 874 | II | 8 |
| A3B1C3D2 | 92.3 | 0.38 | 727 | I | 8 |
| A3B2C1D3 | 88.0 | 0.34 | 512 | I | 8 |
| A3B3C2D1 | 86.0 | 0.35 | 810 | II | 9 |
With the optimal formulation, we characterized the coating’s comprehensive properties. Application tests via brushing, dipping, and spraying confirmed excellent涂挂性, forming uniform layers on EVA films. Thixotropy analysis, as shown in Figure 1, revealed a sharp decline in apparent viscosity under constant shear, from 82 Pa·s at 30 seconds to 40 Pa·s at 10 minutes. The thixotropy value calculated was: $$ \text{Thixotropy} = \frac{82 – 40}{82} \times 100 = 51 $$ exceeding the minimum requirement of 20 for V-process coatings, ensuring optimal flow during application and stability thereafter. This high thixotropy is vital for preventing sagging on vertical surfaces in molds for high manganese steel casting. Permeability remained around 0.38, sufficient to allow gas escape during pouring, while suspension stability exceeded 92%, minimizing settling during storage. Coating strength averaged 727 g, adequate to withstand handling and metal冲击, and high-temperature crack resistance was rated Grade I, indicating no cracks after thermal shock, crucial for withstanding the high temperatures of molten high manganese steel casting.
To validate practical efficacy, we conducted production trials on ZGMn13 high manganese steel casting components, such as球墨机衬板. Coatings were applied using brushing, dipping, and spraying methods onto plastic films in V-process molds. After pouring and shakeout, castings exhibited smooth, clean surfaces with sharp edges, no burn-on, and high dimensional accuracy. The coating sintered layer peeled off in flakes, facilitating easy cleaning. This success underscores the coating’s suitability for high manganese steel casting, particularly in large or complex geometries where traditional coatings fail. Further analysis involved measuring the coating’s thermal properties using the formula for heat transfer: $$ q = -k \frac{dT}{dx} $$ where \( q \) is heat flux, \( k \) is thermal conductivity, and \( \frac{dT}{dx} \) is temperature gradient. The high \( k \) of high-chromium corundum, approximately double that of quartz, enhances heat dissipation, reducing thermal stress and improving surface quality in high manganese steel casting.
In discussion, we compared our coating with commercial alternatives. Cost analysis showed a reduction of about 60% due to the use of chromium iron slag, while performance metrics matched or exceeded those of zircon-based coatings. The neutral nature of high-chromium corundum minimizes interfacial reactions, as described by the thermodynamic potential: $$ \Delta G = \Delta H – T \Delta S $$ where \( \Delta G \) is Gibbs free energy, \( \Delta H \) is enthalpy change, \( T \) is temperature, and \( \Delta S \) is entropy change. For high manganese steel casting, the negative \( \Delta G \) for slag-metal interactions is reduced, preventing adhesion. Additionally, the coating’s pH stabilized around 8–9, aligning with the alkaline nature of high manganese steel casting to avoid chemical incompatibility. We also explored scalability: the coating can be adapted for other steel castings by adjusting filler ratios, but its primary advantage lies in high manganese steel casting applications where粘砂 is prevalent.
In conclusion, this study successfully developed an alcohol-based coating for V-process molding of high manganese steel casting. The optimal formulation, comprising high-chromium corundum, phenolic resin, PVB, lithium bentonite, and phosphate, delivered superior suspension, permeability, strength, and thermal resistance at low cost. Production trials confirmed excellent铸件 quality with no defects, validating the coating’s industrial potential. Future work could optimize particle size distribution using the Andreasen equation: $$ CPFT = \left( \frac{d}{D} \right)^n \times 100 $$ where \( CPFT \) is cumulative percentage finer than, \( d \) is particle diameter, \( D \) is maximum diameter, and \( n \) is distribution modulus, to further enhance performance for diverse high manganese steel casting scenarios. This research contributes to sustainable foundry practices by leveraging industrial by-products and advancing coating technology for demanding applications like high manganese steel casting.