Abstract
Focus on the problem of porosity defects in cast steel casting, with a focus on analyzing the dissolution behavior of gas in steel, porosity nucleation, growth mechanism, etc. Combined with practical cases, the actual impact of the production environment and sample preparation process on porosity defects in cast steel is specifically analyzed. At the same time, further research is conducted on the application schemes of porosity defect control technologies such as metal liquid treatment and casting temperature and speed control. The aim is to verify the effectiveness of advanced melting technology and new casting process methods through case analysis and provide a reference for improving casting quality in the steel casting industry.
Keywords: cast steel casting; pore defects; dissolution precipitation; nucleation and growth; investment casting
1. Formation Mechanism of Pore Defects in Steel Casting
1.1 Dissolution and Precipitation of Gas in Steel
| Gas Type | Dissolution Behavior | Precipitation Condition |
|---|---|---|
| Hydrogen | Soluble in liquid steel | Precipitates during solidification |
| Nitrogen | Soluble in liquid steel | Precipitates when undersaturated |
The formation of pores in steel casting is closely related to the solubility and precipitation behavior of gases in molten steel. During the melting process, the molten steel absorbs gases such as hydrogen and nitrogen. When the molten steel cools, the solubility of these gases decreases, leading to their precipitation. If the gases absorbed during cooling cannot be effectively removed, pores will form inside the casting.
1.2 Nucleation and Growth Mechanism of Pores
Pore formation undergoes two critical stages: nucleation and growth. Nucleation typically occurs in the solid-liquid coexistence region, such as between dendrite arms or at columnar grain boundaries. Gas atoms accumulate in these regions to form stable bubble nuclei. Once formed, the nuclei gradually absorb more gas atoms and grow larger in the molten steel. The final size and distribution of pores depend on nucleation location, number, cooling rate, and gas content in the molten steel.
2. Case Analysis of Pore Defects in Steel Casting
2.1 Production Environment and Sample Preparation
A company specializing in steel casting production using investment casting technology was investigated. The company employs a medium-temperature wax-silica sol casting process, using zircon sand/powder and mullite sand/powder as transitional and reinforcing materials, with silica sol as the binder. During shell firing, the melting temperature reaches 1,150 °C, using an intermediate frequency furnace for melting.
| Defect Type | Number of Defective Pieces | Percentage |
|---|---|---|
| Invasive Pores | 625 | 86.8% |
| Entrained Pores | 67 | 9.3% |
| Slag Pores | 28 | 3.9% |
A detailed profile analysis of 90 samples revealed:
| Defect Type | Number of Samples | Percentage |
|---|---|---|
| Invasive Pores | 68 | 75.6% |
| Entrained Pores | 13 | 14.4% |
| Precipitated Pores | 9 | 10% |
2.2 Defect Analysis
Invasive pores are the primary cause of rejection in investment cast steel parts. They form due to the precipitation of gases such as hydrogen and nitrogen during solidification. Hydrogen has a solubility of 0.0025% in liquid steel and only 0.001% in solid steel. When the hydrogen content in the molten steel exceeds this critical value, hydrogen precipitates during solidification, forming pores.
3. Technologies for Controlling Pore Defects in Steel Casting
3.1 Metal Liquid Treatment
| Treatment Method | Description | Effect |
|---|---|---|
| Vacuum Degassing | Establishes a vacuum environment to remove gases | High |
| Alkali Melting | Adds alkaline substances to neutralize acidic oxides | Medium |
| Ultrasonic Treatment | Generates microbubbles to aid in gas removal | Low-Medium |
To effectively control pore defects, metal liquid treatment technologies can be introduced. Vacuum degassing reduces gas content by creating a vacuum environment. Alkali melting changes the viscosity and surface tension of the molten metal, facilitating gas removal. Ultrasonic treatment uses microbubbles to aggregate and remove gases.
3.2 Control of Casting Temperature and Speed
| Parameter | Formula/Guideline | Importance |
|---|---|---|
| Casting Temperature (T) | T = T_liquidus – ∆T | High |
| Casting Speed (v) | v = (A × g) / (ρ × μ) | Medium |
Choosing the appropriate casting temperature and speed is crucial. High temperatures increase gas solubility, while low temperatures impair metal fluidity. The ideal casting speed ensures smooth filling of the mold cavity, avoiding gas defects.
3.3 Mold Design and Manufacturing Technology
| Mold Material | Characteristics |
|---|---|
| High-Strength Alloy Steel | Good thermal fatigue resistance, stability at high temperatures |
High-strength, high-thermal conductivity alloy steel is used for mold materials. Computer-aided design and high-precision numerical control machining technologies optimize mold structure and surface roughness, reducing pore defects.
Gas Venting System Design
| Parameter | Formula/Guideline |
|---|---|
| Vent Slot Area (A_1) | A_1 = (V × n) / v_1 |
| Cooling Channel Diameter (D) | D = (4 × π × Q) / (v_2) |
Effective gas venting and cooling system designs further reduce pore defects.
3.4 Mold Coating Technology
| Coating Material | Characteristics |
|---|---|
| Zircon Powder | Low thermal conductivity |
Mold coating technology reduces pore defects by selecting materials with good thermal insulation and moderate permeability. Precise control of coating thickness ensures coating integrity and continuity.
4. Improvement Schemes for Pore Defect Control Technology
4.1 Application of Advanced Melting Technology
| Melting Technology | Description |
|---|---|
| Vacuum Melting | Conducts melting in a vacuum environment |
| Electromagnetic Stirring | Promotes uniform distribution and removal of gases |
Advanced melting technologies, such as vacuum melting and electromagnetic stirring, significantly reduce gas content in the molten metal.
4.2 Optimization of Pouring and Solidification Processes
| Optimization Measure | Description |
|---|---|
| Low-Pressure Pouring | Controls metal flow speed and filling pattern |
| Computational Simulation | Optimizes gate and riser locations and sizes |
Optimizing pouring and solidification processes, such as using low-pressure pouring and computational simulation, further minimizes pore defects.
Conclusion
The control and process improvement of pore defects in steel casting have become crucial for enhancing casting quality. By combining theoretical analysis and case studies, comprehensive treatment schemes, including metal liquid treatment, casting temperature and speed control, advanced melting technology, and new casting processes, effectively reduce pore defects. The application of investment casting technology further underscores the importance of these advancements in achieving high-quality steel casting.