Abstract: This article focuses on the surface pitting issue in investment casting. It elaborates on the causes of surface pitting in investment casting, including factors related to the casting system design, shell-making materials, shell baking, and melting and pouring processes. Corresponding prevention measures are proposed for each cause. The article also specifically discusses the prevention measures for carbon steel and low alloy steel castings. Additionally, it emphasizes that the melting and pouring process is a crucial step in preventing pitting. Through a comprehensive understanding of these aspects, the quality of investment casting can be effectively improved.
1. Introduction
Investment casting is important manufacturing processes that can produce complex and high-precision metal parts. However, surface pitting is a common defect that can affect the quality and performance of investment casting. Understanding the causes of surface pitting and implementing effective prevention measures is essential for improving the quality of investment castings.
1.1 Importance of Surface Quality in Investment Casting
The surface quality of investment castings is crucial as it directly affects the mechanical properties, corrosion resistance, and aesthetic appearance of the final products. Pitting on the surface can act as stress concentrators, reducing the fatigue strength and overall durability of investment casting. In applications where corrosion resistance is important, such as in the aerospace and automotive industries, even small pits can provide initiation sites for corrosion, leading to premature failure of the component.
1.2 Common Materials Affected by Pitting
Pitting is commonly observed in 200 series stainless steel, 400 series stainless steel, carbon steel, and low alloy steel castings. These materials are widely used in various industries due to their excellent mechanical properties and corrosion resistance. However, their susceptibility to pitting makes it necessary to carefully control investment casting process to minimize this defect.
2. Defect Characteristics
The surface pitting defect in investment castings is characterized by the presence of numerous small, dot-like pits on investment casting surface. These pits can vary in size and depth, and may be distributed randomly or in clusters. In severe cases, the pitting can progress to form larger pits or even craters, which can significantly degrade the surface quality of investment casting.
2.1 Visual Appearance of Pitted Surfaces
As shown in Figure 1, the pitted surfaces of common 200 series and 400 series stainless steel castings exhibit a rough and uneven texture. The pits may appear as dark spots or indentations, depending on the severity of the defect. In some cases, the pitting may be accompanied by other surface defects, such as porosity or inclusions, further compromising the quality of investment casting.
2.2 Impact on Casting Performance
The presence of surface pitting can have a significant impact on the performance of investment castings. In addition to reducing the mechanical strength and corrosion resistance, pitting can also affect the dimensional accuracy and surface finish of investment casting. This can lead to problems during assembly and machining operations, resulting in increased production costs and reduced product quality.
3. Causes and Prevention Measures of Surface Pitting
3.1 Pouring System Design
| Cause | Description | Prevention Measure |
|---|---|---|
| Improper inner gate position | Inner gate placed in the center for convenience, may cause overheating and incomplete wax removal. | Design inner gate to deviate from the hot spot to reduce overheating and improve wax removal. Consider adding wax drainage channels or process ribs if necessary. |
| Unreasonable tree assembly orientation | Affects wax removal and may lead to residual wax, which reacts with alloy liquid during baking. | Optimize tree assembly orientation to ensure complete wax removal. For high-quality castings, consider secondary dewaxing and cleaning. |
3.2 Shell-making Materials
| Cause | Description | Prevention Measure |
|---|---|---|
| High metal oxide content in facing refractory material | Zircon sand with impurities can decompose and react with alloy elements, causing pitting. | Use zircon sand with low impurity content. Control impurity in zircon sand/powder to be less than 0.05%. Consider using neutral refractory materials like white corundum powder/sand for facing layer. |
| Poor quality of transition layer refractory material | Impurities in transition layer can penetrate into shell and investment casting surface, causing pitting. | Ensure the quality of transition layer refractory material. Regularly test and replace if necessary. Use materials with low impurity content, especially low melting point ferrosilicate and free iron. |
3.3 Shell Baking
| Cause | Description | Prevention Measure |
|---|---|---|
| Incomplete shell baking | Residual moisture, organics, and volatiles in shell cause secondary oxidation of metal liquid during pouring, leading to pitting. | Bake shell at 950 – 1200 °C for 1 – 0.5 h or more to completely remove free water, crystallization water, and inorganic salts. Ensure high permeability and low gas evolution of shell. Cover with casting steel insulation agent immediately after pouring to prevent secondary oxidation. |
3.4 Melting and Pouring
| Cause | Description | Prevention Measure |
|---|---|---|
| Incomplete deoxidation | Metal oxides in liquid metal due to excessive oxides in furnace charge, insufficient deoxidation, or improper melting process. | Use dry and clean furnace charge. Control the proportion and usage times of return furnace charge. Select appropriate deoxidizer and ensure sufficient addition amount. Follow proper melting process with sufficient pre-deoxidation, static time, and final deoxidation. |
| Improper binder selection | Acidic binders used in investment casting can cause pitting in titanium and chromium alloy steel castings. | Consider using neutral or alkaline binders when other measures are ineffective. |
| Improper pouring process | High temperature of metal liquid and shell, slow cooling, and long interaction time can cause pitting. | Control the temperature of metal liquid and shell within a reasonable range. Optimize pouring process to reduce interaction time. |
4. Prevention Measures for Carbon Steel and Low Alloy Steel Castings
4.1 Improving Metal Liquid Quality
| Measure | Description |
|---|---|
| Furnace charge selection | Use dry and clean furnace charge. Avoid using rusty or heavily oxidized materials. Limit the use of return furnace charge and control its quality. |
| Deoxidation process | Adopt complete deoxidation process. Add manganese iron, silicon iron, and silicon calcium in sequence. Then, perform static power outage for 2 minutes and add aluminum or composite deoxidizer for final deoxidation. Consider using secondary addition method for final deoxidation. |
| Melting and pouring process control | Develop and strictly follow reasonable melting and pouring processes. Ensure sufficient static time before tapping to allow oxides to float up. Remove slag thoroughly. Consider using vacuum pouring to prevent secondary oxidation. |
4.2 Shell Baking Process Optimization
| Measure | Description |
|---|---|
| Baking temperature and time | For water glass shell, bake at 950 – 1050 °C for 0.5 – 2 h. For silica sol shell, bake at 1050 – 1200 °C for 30 minutes. Ensure shell is baked thoroughly to remove volatile substances and improve shell quality. |
4.3 Binder Selection
| Measure | Description |
|---|---|
| Reasonable binder choice | Select binder according to casting material. For stainless steel castings, consider using neutral or alkaline binder when other prevention measures are not effective enough to reduce oxide content and prevent pitting. |
4.4 Pouring Process Control
| Measure | Description |
|---|---|
| Temperature control | Strictly control the temperature of metal liquid and shell during pouring. Keep them within a reasonable range to avoid excessive interaction between metal liquid and shell facing material. |
5. Key Points in Melting and Pouring Process
5.1 Importance of Steel Liquid Quality
The quality of the steel liquid, especially the degree of deoxidation and slag removal, is the most critical factor affecting the formation of pitting. Complete deoxidation and effective slag removal can significantly reduce the amount of metal oxides in the liquid metal, thereby minimizing the occurrence of pitting.
5.2 Prevention of Secondary Oxidation
During the pouring process, measures should be taken to prevent the secondary oxidation of investment casting surface. This includes avoiding blowing air on the molten metal and high-temperature shell, using high-quality filling sand, and implementing timely sealing and cooling measures after pouring.
5.3 Shell Baking Requirements
The shell should be baked at a suitable temperature and for an appropriate duration to ensure complete removal of volatile substances and to improve the shell’s properties. A well-baked shell can reduce the occurrence of pitting by minimizing the release of gases and the reaction with the metal liquid.
5.4 Refractory Material Quality
The quality of the refractory material, especially the impurity content in the facing layer, is crucial. Low impurity content can reduce the oxidation atmosphere at the interface and prevent the formation of pitting.
6. Conclusion
Surface pitting in investment casting is a complex issue that requires a comprehensive understanding of the casting process and the implementation of appropriate prevention measures. By addressing the causes related to the pouring system design, shell-making materials, shell baking, and melting and pouring processes, the quality of investment casting can be significantly improved. Continued research and innovation in this area, such as the development and application of new materials and technologies, will further enhance the surface quality and performance of investment casting in the future.