Nodular cast iron, known for its excellent mechanical properties, has been widely used in various industrial fields. However, during the casting process, defects such as shrinkage porosity, porosity, and cold shut can occur, which affect the quality and performance of the castings. In this article, we will focus on the analysis of casting defects and the optimization of the casting process for nodular cast iron, taking the example of the front cover casting.
I. Introduction
Nodular cast iron is a type of cast iron with spherical graphite inclusions, which gives it superior mechanical properties compared to ordinary gray cast iron. It has high strength, good toughness, and excellent wear resistance, making it suitable for applications in the automotive, machinery, and other industries.
The front cover, as an important component, requires high-quality nodular cast iron to ensure its reliability and performance. However, in the initial casting process, there were some problems such as high scrap rate, unstable quality, and low production efficiency, which needed to be addressed through process optimization.
II. Initial Casting Process Design
The front cover is made of QT700 – 2 nodular cast iron and is produced using the green sand casting process. The basic wall thickness is 9 mm, the bottom flange plate has a wall thickness of 40 mm, and the top has a wall thickness of 23 mm. It has four independent lugs with uneven wall thickness (see Table 1 for details).
| Part | Dimension | Wall Thickness (mm) |
|---|---|---|
| Bottom Flange Plate | Diameter approximately 450 mm | 40 |
| Top | – | 23 |
| Lugs | Four independent | – |
The 分型面 is conventionally designed to be at the flange surface near the middle. The small end of the casting is placed in the upper box, and the large flange and the core head are placed in the lower box (see Figure 1 for the initial process). The inner pouring gate enters the mold cavity from the large flange surface, and the inner cavity uses a handmade self – hardening sand core.
According to the structural characteristics of the front cover, the outer circular flange plate has a large annular hot spot with an average wall thickness of 40 mm. Therefore, the process design uses a combination of risers and external chill to eliminate the hot spot at this location. Two side risers are designed as 60 mm × 80 mm × 70 mm pressure – edge risers, and six external chills are set in the sand core for quenching to eliminate the hot spot at the flange ring and achieve sequential solidification of the entire flange ring. At the top flange, the hot spots are concentrated at the four independent lugs, and internal chills are placed to solve the shrinkage porosity problem.
During the trial production stage, the internal structure of the product showed no shrinkage porosity defects after dissection, and it was approved by the customer. However, in the mass production stage, there were significant fluctuations in product quality:
- Quality instability: There were a high proportion of defective products, such as shrinkage porosity at the top cold iron location (see Figure 2), porosity, and cold iron deviation, with a comprehensive scrap rate of up to 15%. After dissection, it was found that at the position of the four independent lugs at the top, although internal chills were used, there was always a certain proportion of products with shrinkage porosity at the cold iron location. In addition, the porosity at the internal cold iron location was also relatively concentrated.
- Low production efficiency: In the production of this product, six cold irons needed to be placed in the core (see Figure 3), which resulted in low core – making efficiency. During the molding process, manual insertion of the internal cold iron into the mold cavity was required, which greatly affected the production efficiency.
III. Result Analysis
The main reasons for the low production efficiency of this process are the extensive use of cold irons. The external cold irons need to be manually inserted into the core box during the self – hardening sand core – making process, which severely restricts the core – making efficiency. The internal cold irons at the four independent lugs at the top need to be manually inserted into the mold cavity during the molding process, which greatly affects the production efficiency.
The main reasons for the unstable product quality and the concentrated casting defects at the four lugs at the top are as follows:
- The quality of the internal cold iron inserted into the mold cavity is uncontrollable. According to the product structure, the internal cold iron needs to be vertically inserted into the upper box mold cavity, and there are strict requirements for the depth and perpendicularity of the cold iron insertion. However, due to the limitations of the current production line, the internal cold iron is inserted into the mold cavity manually, and there is a large deviation in the quality of the cold iron insertion: if the internal cold iron is inserted too short, it cannot eliminate the shrinkage porosity; if it is too long or too inclined, it cannot be completely processed, resulting in the scrapping of the casting.
- The quality of the cold iron is uncontrollable. The internal cold iron is a purchased finished part, and its size has a certain deviation. When the deviation is too large, it will affect the quenching effect of the cold iron. Before use, the internal cold iron needs to be baked to remove the surface moisture. However, during the production, storage, and use process, the surface will inevitably rust and be contaminated, and a large amount of gas will be generated during the pouring of the molten iron, resulting in porosity around the cold iron. When the area is too large, it will lead to the scrapping of the casting.
- Damage to the mold cavity: When the internal cold iron is manually inserted into the mold cavity, it is easy to damage the sand mold at the root of the cold iron, resulting in sand hole defects around the cold iron.
IV. Process Optimization Scheme
To address the above problems, the process optimization is carried out from the following two aspects:
- Reduce the use of cold irons: Try to use risers for feeding the hot spots at the flange ring. Change the internal cold iron to external cold iron.
- Change the overall sand core to the hanging sand core: This can reduce costs and improve production efficiency.
The specific improvement scheme is as follows:
- Improvement in the shrinkage prevention design of the casting: For the annular hot spot at the flange ring, the internal cold iron is canceled, and a large riser is set on the side of the flange for feeding. Through the calculation of the modulus and hot spot of the casting, to meet the feeding requirements of the casting, the riser size is designed to be Φ80 mm × 140 mm, and the riser diameter size is designed to be Φ25 mm × 20 mm. For the hot spot at the top flange lug, the internal cold iron is canceled and replaced by external cold iron quenching, combined with the side riser for feeding. Since the four lugs are four independent thick hot spots relative to the top flange, the external cold iron cannot be simply designed as an annular sheet – shaped cold iron because the annular cold iron has the same quenching effect on the flange and the lugs, and at this time, the four lugs are still in a relatively hot spot state. Therefore, the cold iron is designed as four independent block – shaped cold irons to quench the four lugs separately.
- Change the overall sand core to the hanging sand core: After canceling the cold iron at the flange ring, only the oil groove in the inner cavity of the front cover requires the use of a sand core, and the other parts can be realized through the hanging sand process. Therefore, the process of this product is redesigned. The 分型面 is still selected at the flange surface, and the casting is placed entirely in the lower box. The inner cavity of the casting is formed by the hanging sand, and a small sand core is placed in the local oil groove. This design reduces the weight of the sand core from 33 kg to 0.5 kg, greatly reducing the core – making cost.
After the process optimization, through production verification, the shrinkage porosity problem of the casting is better solved, and the scrap rate of sand holes and porosity in the product is also significantly reduced. The quality of the product in mass production is stable, which well meets the needs of customers.
V. Conclusion
- The manual insertion of internal cold iron into the mold cavity is not suitable for mass production. When using internal cold iron to solve the shrinkage porosity problem of the casting, there are high requirements for the depth and perpendicularity of the insertion into the mold cavity. It is only suitable for single – piece or small – batch production verification. In mass production, internal cold iron can easily cause unstable product quality, such as deviation, porosity, and sand hole defects.
- The use of external cold iron can reduce the production efficiency of the product. Therefore, in the design of the casting shrinkage prevention, it is preferable to use hot risers for feeding as much as possible, which can greatly improve the production efficiency.
In summary, the optimization of the casting process for nodular cast iron is crucial to improve the quality and production efficiency of the castings. By reducing the use of cold irons, improving the design of risers, and adopting the hanging sand core process, the problems of casting defects and low production efficiency can be effectively solved. This not only meets the requirements of customers but also enhances the competitiveness of the enterprise in the market.
In the future, further research and development can be conducted to explore more advanced casting technologies and processes to further improve the quality and performance of nodular cast iron castings. At the same time, the use of simulation software and advanced testing equipment can help optimize the process and predict potential defects, providing a more scientific basis for the production of nodular cast iron castings.
VI. Further Analysis of Nodular Cast Iron Properties
Nodular cast iron possesses several advantageous properties that make it a preferred choice in many applications. Its high strength and toughness allow for the design of lighter and more durable components. The spherical graphite inclusions also contribute to improved fatigue resistance and machinability.
In addition to its mechanical properties, nodular cast iron exhibits good thermal conductivity and damping capacity. These characteristics make it suitable for use in applications where heat dissipation and vibration reduction are important considerations.
However, the properties of nodular cast iron can be influenced by various factors, such as the composition of the molten metal, the cooling rate during solidification, and the heat treatment process. Controlling these factors is crucial to ensure the desired properties in the final casting.
VII. Case Studies on Nodular Cast Iron Casting
To further illustrate the importance of proper casting process and defect prevention, let’s look at some case studies. In one example, a manufacturer experienced frequent porosity defects in nodular cast iron parts. Upon investigation, it was found that the pouring temperature was too high, leading to excessive gas entrapment. By adjusting the pouring temperature and implementing proper degassing procedures, the porosity problem was significantly reduced.
In another case, a company faced challenges with shrinkage porosity in thick sections of the casting. By optimizing the riser design and using chills strategically, the shrinkage porosity was effectively eliminated, resulting in improved casting quality.
These case studies highlight the need for careful process control and continuous improvement in nodular cast iron casting.
VIII. Advanced Techniques in Nodular Cast Iron Casting
In recent years, there have been significant advancements in nodular cast iron casting techniques. For instance, the use of computer simulation software has enabled manufacturers to predict and optimize the casting process, reducing the likelihood of defects.
Additionally, the development of new molding materials and processes has improved the accuracy and surface finish of the castings. Advanced heat treatment methods also play a crucial role in enhancing the properties of nodular cast iron.
IX. Future Trends and Challenges
Looking ahead, the demand for high-quality nodular cast iron castings is expected to continue to grow. However, there are also several challenges that need to be addressed. For example, the need for more sustainable and environmentally friendly casting processes is becoming increasingly important.
Furthermore, the development of new alloys and composite materials may offer even better properties and performance for specific applications. Keeping up with these trends and meeting the evolving requirements of the market will require continuous innovation and investment in research and development.
X. Conclusion
In conclusion, nodular cast iron is a versatile and valuable material in the field of casting. By understanding and addressing the common casting defects and optimizing the casting process, manufacturers can produce high-quality castings that meet the diverse needs of various industries. Continued research and development in this area will undoubtedly lead to further improvements in the properties and performance of nodular cast iron castings, opening up new possibilities for their application.
It is essential for manufacturers to stay updated with the latest advancements and best practices in nodular cast iron casting to remain competitive in the global market. By focusing on quality, efficiency, and innovation, the future of nodular cast iron casting looks promising.