This article focuses on the analysis and improvement of nitrogen porosity defects in gray cast iron produced by iron – clad sand casting. Through EDS spectrometer analysis, the causes of nitrogen pores are explored in combination with the casting production process. Measures such as replacing coated sand, adjusting sand ratios, and optimizing production parameters are implemented. The results show significant improvements in reducing the occurrence of nitrogen porosity defects.
1. Introduction
The iron – clad sand casting process has been widely used in the production of various castings due to its unique advantages. However, the occurrence of nitrogen porosity defects has been a significant challenge, affecting the quality and performance of the castings. Understanding the formation mechanism of these defects and implementing effective improvement measures is crucial for the production of high – quality castings.
2. Casting Process Overview
2.1 Melting and Molding Process
The casting production employs a medium – frequency induction furnace. The charge materials mainly consist of recycled materials, scrap steel, and a certain proportion of additives. The proportion of recycled materials and scrap steel is carefully controlled, and the addition sequence of materials in the furnace is also crucial. The melting temperature and holding time are maintained within specific ranges to ensure the quality of the molten iron. For example, the furnace temperature is controlled between 1500 – 1530°C for melting and held at 1530 – 1550°C for 5 minutes with repeated slag removal. The molten iron is tapped at a temperature between 1490 – 1530°C and is inoculated with 75″ ferrosilicon during tapping. The pouring temperature of the last piece is not lower than 1360°C, and the entire pouring process for a package should not exceed 15 minutes.
2.2 Pouring System
The pouring system is designed as a semi – closed type. The width of the runner is consistent with that of the ingate, and the thickness is determined according to the wall thickness of the casting flange. The sprue cup is of a funnel type, which can buffer the molten iron entering the cavity, prevent turbulent flow, and store the initial low – temperature metal liquid containing gas and slag, thus playing a role in slag blocking. The pressure angle of the sprue cup is not less than 40°, and a 15PPI ceramic filter is installed inside. The ratio of the cross – sectional areas of the ingate, runner, and sprue is 1:4.17:2.04. There are 4 or 5 ingates according to the product structure, and the cross – sectional area of a single ingate is not less than 240 mm². The pouring time for a single casting is not more than 25 seconds, and two stress grooves are set on both sides of the ingate for easy removal.
3. Defect Analysis
3.1 EDS Spectral Analysis
EDS analysis of the defect surface reveals the presence of dendritic crystals and irregularly shaped defects perpendicular to the casting surface, penetrating 2 – 5 mm into the casting. The defect contains elements such as C, N, Fe, O, and Si, with significant nitrogen content.
3.2 Nitrogen Porosity Formation Mechanism
Nitrogen in the molten iron forms an interstitial solid solution. During the solidification process of the molten iron, as the temperature decreases, the solubility of nitrogen also decreases. In the iron – clad sand casting process, due to the rapid cooling rate, the surface layer of the molten iron solidifies quickly, forming a shell, and the nitrogen gas cannot be discharged smoothly, resulting in the formation of nitrogen porosity defects.
3.3 Causes of Nitrogen Porosity
3.3.1 Nitrogen Source in Raw Materials
The nitrogen content in different raw materials varies. In scrap steel, the nitrogen content in some types is relatively high, such as in ordinary carbon steel and high – manganese steel. In addition, different types of carburizers also have different nitrogen contents. For example, coal – based carburizers have a relatively high nitrogen content, while graphite – based carburizers have a lower nitrogen content.
3.3.2 Role of Coated Sand
The coated sand used in the process contains phenolic resin and hexamethylenetetramine (urotropine). Urotropine decomposes when heated above 230°C, releasing a large amount of NH₃, which can contribute to the formation of nitrogen porosity defects.
4. Improvement Measures
4.1 Improvement of Coated Sand Quality
4.1.1 Optimization of Resin and Urotropine Quality
By improving the quality of phenolic resin and urotropine, the nitrogen content in the coated sand can be reduced, thereby reducing the risk of nitrogen porosity defects.
4.1.2 Adjustment of Sand Ratio
The proportion of new sand is increased from 5% – 20% to 20% – 40%, reducing the residual nitrogen content in the recycled sand. At the same time, due to the different performance requirements of coated sand in different production lines, the sand should be managed separately to avoid process problems caused by mixing recycled sand.
4.2 Improvement of Production Process
4.2.1 Addition of Exhaust Channels
Adding exhaust channels in the production process is beneficial for the gas discharge during the curing of the coated sand and also improves the sand filling quality.
4.2.2 Optimization of Temperature Parameters
The temperature of the mold is set between 230 – 250°C, and the temperature of the iron mold is set between 240 – 280°C. This is because urotropine in the coated sand decomposes at 230°C, and by setting appropriate temperatures, the gas release of the coated sand can be accelerated.
4.2.3 Extension of Time Interval
The time interval from the end of sanding to the start of pouring is extended from 10 minutes to not less than 20 minutes. This allows the gas invading the molten iron to float upward, reducing the formation of porosity defects in the casting.
5. Results and Discussion
After implementing the above improvement measures, continuous production of more than 100,000 products was carried out, and no nitrogen porosity defects occurred. This indicates that the improvement measures are effective in reducing the occurrence of nitrogen porosity defects and improving the quality of the castings. The analysis and improvement process of nitrogen porosity defects in this article can provide a reference for the production of similar castings, helping to optimize the casting process and improve product quality.
6. Conclusion
In conclusion, the nitrogen porosity defects in iron – clad sand cast gray cast iron have been effectively addressed through a combination of measures including raw material adjustment and process optimization. The understanding of the defect formation mechanism and the implementation of appropriate improvement measures are crucial for ensuring the quality of castings. Future research may focus on further optimizing the casting process and exploring more effective methods to prevent the occurrence of various defects.
6.1 Melting and Molding Process
The medium – frequency induction furnace is a crucial equipment in the melting and molding process. It provides the necessary heat for melting the charge materials. The proper control of the furnace temperature and the addition sequence of materials is essential for obtaining high – quality molten iron. As shown in the picture, the furnace is designed to ensure efficient heat transfer and uniform melting of the materials.
6.2 Pouring System
The pouring system is designed to ensure the smooth flow of the molten iron into the mold cavity. The funnel – shaped sprue cup and the ceramic filter help to control the flow rate and quality of the molten iron. The ratio of the cross – sectional areas of different parts of the pouring system is carefully designed to meet the requirements of the casting process.
6.3 Defect Analysis
6.3.1 EDS Spectral Analysis
The EDS spectral analysis provides detailed information about the elemental composition of the defect. The presence of nitrogen and other elements can be clearly seen from the analysis result, which helps to understand the nature of the defect.
6.3.2 Nitrogen Porosity Formation Mechanism
The schematic diagram illustrates how the nitrogen gas is trapped during the solidification process of the molten iron due to the rapid cooling rate of the iron – clad sand casting process. This helps to understand the root cause of the nitrogen porosity defect.
6.4 Improvement Measures
6.4.1 Improvement of Coated Sand Quality
6.4.1.1 Optimization of Resin and Urotropine Quality
By improving the quality of the resin and urotropine, the chemical stability of the coated sand can be enhanced, reducing the release of nitrogen – containing gases.