Analyzing Defects and Prevention Measures of Cast Steel Valves

Cast steel valves play a crucial role in various industrial applications, including petroleum, chemical, power generation, and water supply. Their reliable performance directly affects the overall safety of system operations. However, due to the complex structure of cast steel valves, various casting defects can arise during the manufacturing process, such as porosity, shrinkage cavities, and shrinkage porosity. These defects not only impair the mechanical performance and service life of cast steel valves but can also lead to severe consequences like leakage. Statistics show that the main causes of leakage in cast steel valves are often closely related to shrinkage cavities and shrinkage porosity that occur during the casting process. This article will delve into the causes of these defects in cast steel valves and discuss effective preventive measures to improve their inherent quality.

Causes of Shrinkage Cavities and Porosity in Cast Steel Valves

During the casting process of valves, the cooling of the castings goes through three stages of shrinkage.

1. Liquid Shrinkage

As the molten steel cools inside the mold, its volume gradually decreases before crystallization occurs. This process is influenced by the pouring temperature; the higher the temperature, the greater the liquid shrinkage. Controlling liquid shrinkage is crucial for the success of casting steel valves since it directly impacts the filling and final shaping of the casting.

2. Solidification Shrinkage

After crystal nuclei form, the molten steel transitions from liquid to solid, continuing to shrink in volume. The extent of this shrinkage is affected not only by the pouring temperature but also by the alloy composition. For example, higher carbon and silicon content usually reduces shrinkage, while increased manganese and sulfur content can lead to greater shrinkage. Properly balancing the alloy composition can help reduce shrinkage cavities and porosity in cast steel valves.

3. Solid Shrinkage

Once the valve is fully solidified, its volume further decreases as the temperature drops. While solid shrinkage does not directly cause shrinkage cavities and porosity, it remains a significant factor in the overall cooling process. The effects of solid shrinkage are mainly observed in the later cooling stages; rapid cooling can increase internal stress and lead to other defects.

Research indicates that shrinkage cavities and shrinkage porosity in cast steel valves primarily form during the solidification shrinkage stage. If the design of the pouring system and compensating risers is inadequate, and if the shrinkage at hot spots of the valve is not timely compensated, it can easily lead to the formation of cavities or porosity at these hot spots. The complex structure of valves, particularly at the connecting points of the flange and the body, makes these defects more common. In these areas, poor heat dissipation conditions increase the likelihood of defects during final solidification.

Key Defect Areas in Cast Steel Valves

The hot spots during the cooling process of cast steel valves are the most likely areas for shrinkage cavities and porosity to develop. The following locations are particularly critical.

1. Flange and Body Connectors

This area tends to have poor heat dissipation, increasing the likelihood of defects, especially during uneven cooling of the cast steel valve. The complexity of the connection can impede airflow, exacerbating the formation of defects.

2. Internal Walls and Corners

These areas often experience poor flow conditions, creating dead corners where metal flow is obstructed, further increasing the risk of defects.

In these areas, failing to implement effective compensation measures during the casting process may lead to leaks during valve operation, impacting normal functioning. Optimizing the casting process in these key areas is crucial to reducing defects.

Measures to Prevent Defects in Cast Steel Valves

To effectively prevent the occurrence of shrinkage cavities and porosity, here are some improvement suggestions for cast steel valves.

1. Optimizing Risers Design

Setting risers on the flange of the cast steel valve and selecting high-efficiency insulated risers can help. This design allows the riser to provide secondary heating to the cast steel valve after pouring, promoting sequential solidification and enhancing the riser's ability to compensate for shrinkage, thereby significantly reducing the likelihood of defects.

2. Using External Chills

Installing external chills in the flange, boss, and central part of the valve can shorten the effective compensation distance of the risers and prevent shrinkage areas in the valve's middle section. The external chills help lower local temperatures and guide more uniform heat distribution, reducing defect formation.

3. Strengthening Inlet Design

Incorporating inner runners on the flange and bosses ensures smooth filling during the pouring process, avoiding defects such as sand inclusions and slag traps. A well-designed inlet effectively directs metal flow, ensuring that every part is adequately filled, thus enhancing the overall quality of the cast steel valve.

4. Strictly Controlling Pouring Parameters

Careful management of pouring temperature, time, and speed is crucial in preventing defects in cast steel valves. Too low a pouring temperature can lead to inadequate filling and cold shuts, while too high a temperature might cause sand sticking defects. Therefore, it is essential to adhere to process requirements to ensure reasonable control of temperature and speed, avoiding other casting defects.

5. Improving Mold Design

Properly incorporating features such as chill edges and cooling channels in the lower and upper parts of the mold enhances sequential solidification and ensures that the molten metal has a good path for compensating for shrinkage. Well-designed molds can improve the heat dissipation of cast steel valves, promoting effective heat transfer and reducing defect occurrence.

6. Optimizing Alloy Composition

Adjusting the alloy composition of cast steel valves can optimize their solidification properties, effectively reducing the formation of shrinkage cavities and porosity. For example, slightly increasing the carbon and silicon content can improve fluidity, thus lowering the risk of defects.

Conclusion

Preventing defects in cast steel valves is not only crucial for product quality but also for the safe operation of industrial equipment. By thoroughly researching the causes of shrinkage cavities and porosity and implementing effective preventive measures, we can significantly enhance the quality of cast steel valves and reduce the likelihood of defects, ensuring their stability and reliability in industrial applications. As technology continues to advance, optimizing casting processes will provide a solid foundation for meeting industry production needs. Through scientifically sound process design, material selection, and strict production control, we can contribute to the sustainable development of the cast steel valve industry.