Abstract
Orange peel defects are prevalent on the surfaces of large steel castings, primarily caused by chemical reactions between corundum powder coating on the sand mold and molten steel at high temperatures. To address this issue, an optimized casting process is proposed in this paper, utilizing a combination of silica sand, chromite sand, and zircon powder coating to prevent contact between corundum powder and molten steel. Research demonstrates that applying three layers of zircon powder coating effectively eliminates orange peel defects, enhances casting surface quality, reduces cleaning difficulty, and significantly improves production efficiency. For castings with minor orange peel tendencies, applying two layers of zircon powder coating can reduce production costs. This method provides a practical and effective solution for improving the surface quality of steel castings.
1. Introduction
Large steel castings are widely used in various industries due to their excellent mechanical properties and large size. However, orange peel defects, characterized by rough, uneven surfaces resembling orange peel, frequently occur on the surfaces of these castings after sand removal and shot blasting. These defects not only affect the aesthetic appearance of the castings but also hinder subsequent processes such as ultrasonic flaw detection due to excessive roughness. Therefore, exploring effective methods to prevent orange peel defects is crucial for improving the quality and production efficiency of steel castings.
2. Analysis of Orange Peel Defects
2.1 Occurrence and Characteristics
Orange peel defects typically appear as spots with depths of approximately 1-2 mm on the surfaces of steel castings after sand removal and shot blasting. These spots vary in size, ranging from small pits to larger areas, and are characterized by a rough, uneven surface.
2.2 Chemical Composition Analysis
Chemical composition measurements were conducted on the surfaces of castings with orange peel defects after shot blasting. The results revealed that the Al content in both high-alloy and low-alloy products was significantly higher, with an average value of approximately 6%, while the actual Al content in the casting body was within 0.1%. This indicated that the higher Al content on the surfaces with orange peel defects was primarily introduced by the corundum powder coating.
Table 1: Chemical Composition Analysis Results
| Material Type | Al Content (%) | Average Al Content (%) |
|---|---|---|
| High-Alloy Casting | 5.5 – 6.5 | 6.0 |
| Low-Alloy Casting | 5.8 – 6.2 | 6.0 |
| Casting Body | < 0.1 | – |
2.3 Cause Analysis
The corundum powder coating used on the sand mold surface is mainly composed of Al2O3, with a mass fraction of ≥98%. During the casting process, due to the large weight, high heat storage, and prolonged high temperature exposure of the castings, especially alloy castings with high pouring temperatures and complex chemical compositions, the molten steel exhibits weak acidity at high temperatures. Although the melting point of the corundum powder coating used in the casting field is generally high, reaching 2000°C-2050°C, it becomes weakly alkaline at high temperatures and reacts with the weakly acidic molten steel, resulting in orange peel defects.
3. Process Design and Improvement
3.1 Initial Process Design
Based on the analysis of defect composition, it was determined that the main factor causing orange peel defects was the reaction between the corundum powder coating on the sand mold surface and molten steel at high temperatures, forming difficult-to-remove defects. To reduce orange peel defects, zircon powder coating with higher performance requirements was applied on top of the corundum powder coating to isolate the latter from molten steel. The sand mold generally uses two layers of corundum powder coating. To reduce orange peel defects, the final layer of corundum powder coating was replaced with zircon powder coating, resulting in a molding and coating process of silica sand + chromite sand + corundum powder + zircon powder.
3.2 Process Improvement Through Experimentation
To more effectively reduce orange peel defects on the surfaces of castings, experiments were conducted with different thicknesses of corundum powder and zircon powder. The results showed that when only 2-3 layers of zircon powder were used without corundum powder, the casting surfaces exhibited excellent performance, with almost no orange peel defects.
After testing, it was found that when only zircon powder was used without corundum powder, there were very few orange peel defects on the casting surfaces, which was better than the process of using one layer each of corundum powder and zircon powder. This was mainly because in the 1+1 process, zircon powder was difficult to completely isolate corundum powder from molten steel, leading to contact and reaction between them. Eliminating corundum powder could fundamentally solve the problem of orange peel defects on casting surfaces. However, using zircon powder alone for molding would increase costs. Experiments showed that applying three layers of zircon powder could solve the orange peel problem for most products, and for some products, the number of layers could be reduced to two to lower costs.
4. Experimental Results and Analysis
4.1 Elimination of Orange Peel Defects
By using three layers of zircon powder coating without corundum powder, orange peel defects were eliminated on most castings. This significantly improved the surface quality of the castings, reduced the difficulty of subsequent cleaning, and increased overall production efficiency. For castings with minor orange peel tendencies, applying two layers of zircon powder coating was sufficient to reduce production costs.
Table 2: Comparison of Casting Surface Quality Before and After Process Improvement
| Process | Orange Peel Defects | Surface Quality | Cleaning Difficulty | Production Efficiency |
|---|---|---|---|---|
| Before Improvement (Corundum Powder) | Severe | Poor | High | Low |
| After Improvement (Zircon Powder) | Minimal to None | Excellent | Low | High |
4.2 Cost Considerations
Although using zircon powder alone for molding increased costs compared to using corundum powder, the improved process significantly reduced the need for subsequent cleaning and defect repair, thereby offsetting the additional cost of zircon powder to some extent. Additionally, by adjusting the number of zircon powder layers based on the severity of orange peel tendencies in different castings, costs were further controlled.
5. Conclusion
In summary, by using three layers of zircon powder coating without corundum powder, orange peel defects on most steel castings can be effectively eliminated. This not only improves the surface quality of the castings but also reduces the difficulty of subsequent cleaning and significantly enhances production efficiency. For castings with minor orange peel tendencies, applying two layers of zircon powder coating is sufficient to reduce production costs. This method provides a practical and effective technical pathway for improving the surface quality of steel castings and has important application value in the casting industry.