This article focuses on the nitrogen porosity defects in gray cast iron produced by iron-clad sand casting. Through EDS spectrometer for qualitative and quantitative analysis, combined with the casting production process, the causes of nitrogen porosity are explored. Measures such as improving the quality of coated sand, adjusting the production process, and optimizing various parameters are implemented. The results show that these improvements effectively reduce the defect risk, and continuous production has achieved good results without nitrogen porosity defects.
1. Introduction
Iron-clad sand casting is a widely used casting process, especially in the production of some key components such as automotive brake drums. The resulting castings have the advantages of dense microstructure and high mechanical properties. However, nitrogen porosity defects may occur during the production process, which seriously affects the quality of the castings. This article aims to analyze and solve this problem.
2. Casting Process Overview
2.1 Melting and Molding Process
The casting production adopts an intermediate frequency induction electric furnace. The raw materials mainly include return scraps and new materials. The proportion of return scraps is about 60% – 70%, and the rest is new materials. The quality requirements for the raw materials are strict, such as carbon content, sulfur content, ash content, and volatile content. The charging sequence of the furnace materials is also crucial to ensure the formation of a high-carbon molten pool at the bottom of the furnace. During the melting process, the temperature control is very important. For example, the temperature is maintained at 1500 – 1530 °C for a certain period and then adjusted to 1530 – 1550 °C for 5 minutes with repeated slag removal. The pouring temperature is also carefully controlled, with the final pouring temperature of the last piece not lower than 1360 °C for 15 minutes.
The old sand used is regenerated coated sand, and the new sand is Inner Mongolia Tongliao sand. The coated sand is regenerated by a thermal method, and its mechanical properties and gas generation amount have specific requirements. The tensile strength, bending strength at different temperatures, and 灼烧减量 are all within a certain range to ensure the quality of the sand.
2.2 Pouring System
The pouring cup adopts a funnel type, and the pressure angle formed between the cup top and the maximum outer circle of the casting upper plane is not less than 40°. A 15PPI ceramic filter sheet is placed in the pouring cup to buffer the molten iron entering the cavity and avoid high turbulence. The pouring system is a semi-closed type, and the ratio of the cross-sectional areas of the internal runner, the sprue, and the runner is 1:4.17:2.04. According to the product structure, 4 or 5 internal runners are set, and the cross-sectional area of a single internal runner is not less than 240 mm². The single-mold pouring time of the casting is 25 seconds. The runner width is consistent with the internal runner width, and the thickness is determined according to the flange wall thickness of the casting to ensure the smooth flow of the molten iron and the function of slag blocking.
3. Defect Analysis
3.1 EDS Spectrometer Analysis
From the defect surface scanning map of the EDS spectrometer, it can be seen that there are dendritic crystals in the defect part, and the defect size is about 2 – 5 mm. The elements detected include C, N, Fe, O, and Si. Nitrogen and carbon are precipitated from the graphite film and are connected to the decarburized matrix around them.
3.2 Nitrogen Porosity Formation Mechanism
Nitrogen in the molten iron forms an interstitial solid solution with iron. 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, the cooling speed is fast, resulting in the surface layer of the molten iron solidifying and forming a shell, and the nitrogen gas cannot be discharged smoothly, thus forming nitrogen porosity defects.
3.3 Causes of Nitrogen Porosity
There are mainly three aspects:
- Raw material factors: The nitrogen content in some raw materials is relatively high. For example, in some ordinary carbon steels and high manganese steels used as gray cast iron casting materials, the nitrogen content is about 40 – 60 ppm, and in some special steels such as threaded steel and guide rail steel, the nitrogen content is even higher. In addition, the nitrogen content in the coated sand is also an important factor. The resin in the coated sand may release ammonia gas during the heating and curing process and the pouring process.
- Production process factors: The production process may also affect the formation of nitrogen porosity. For example, if the gas generated during the curing process of the coated sand cannot be discharged in time, it may be trapped in the casting and form pores.
- Equipment and environment factors: The equipment used in the production process and the production environment may also have an impact on the formation of nitrogen porosity. For example, if the temperature and humidity of the production environment are not properly controlled, it may affect the quality of the coated sand and the solidification process of the molten iron.
4. Improvement Measures
4.1 Improvement of Coated Sand Quality
- Quality control of resin and additives: The quality of phenolic resin and urotropine used in the coated sand is crucial. By improving their quality, the gas generation amount during the curing process can be reduced.
- Adjustment of new sand proportion: Increasing the proportion of new sand from 5% – 20% to 20% – 40% can reduce the nitrogen porosity risk.
- Separate management of different sands: Since the performance requirements and particle sizes of the coated sands used in different production lines are different, and the amount of new sand added is also different, it is necessary to separate the management of the recycled sands of the shell line and the iron line to avoid process problems caused by mixing.
4.2 Improvement of Production Process
- Increasing exhaust channels: Adding exhaust channels in the mold and iron type is beneficial for the gas discharge during the curing process of the coated sand and also conducive to the filling of the coated sand, improving the quality of sand shooting.
- Optimizing temperature parameters: Adjusting the temperature parameters of the curing process and the iron type temperature. For example, reducing the curing temperature from 230 °C to 220 – 240 °C, and increasing the iron type temperature from a certain value to 230 – 250 °C or 240 – 280 °C.
- Prolonging curing time: Increasing the curing time from 8 – 20 minutes to 20 minutes can help the gas to be discharged more thoroughly.
- Optimizing pouring temperature: By optimizing the pouring temperature, the gas in the molten iron can be made to float up, reducing the formation of porosity defects in the casting.
5. Results and Discussion
After implementing the above improvement measures, continuous production has achieved good results. More than 100,000 products have been produced continuously without nitrogen porosity defects. This shows that the analysis of the causes of nitrogen porosity defects is accurate, and the improvement measures are effective.
6. Conclusion
This article has analyzed the nitrogen porosity defects in gray cast iron produced by iron-clad sand casting through a series of methods. By improving the quality of coated sand and optimizing the production process, the defect risk has been effectively reduced. The successful experience of this research can provide a reference for the production of similar castings in the future, helping to improve the quality of castings and reduce production costs.
In order to better illustrate the content of this article, the following tables and pictures are provided:
| Material | Carbon Content | Sulfur Content | Ash Content | Volatile Content | Grain Size |
|---|---|---|---|---|---|
| Return Scraps | >98% (by mass fraction) | ≤0.5% | ≤2% | ≤1% | 1 – 5 mm |
| Inner Mongolia Tongliao Sand | – | – | – | – | 70 – 140 µm |
| Property | Value |
|---|---|
| Normal Temperature Tensile Strength | ≥2.5 MPa |
| Normal Temperature Bending Strength | ≥6 MPa |
| Hot State Tensile Strength | ≥1.3 MPa |
| Hot State Bending Strength | ≥3.8 MPa |
| Burning Loss | ≤2.6% |
| Gas Generation Amount | ≤20 mL at 95 – 110 °C |
| Parameter | Value |
|---|---|
| Pressure Angle of Pouring Cup | ≥40° |
| Ratio of Cross-sectional Areas of Internal Runner, Sprue, and Runner | 1:4.17:2.04 |
| Number of Internal Runners | 4 or 5 |
| Cross-sectional Area of a Single Internal Runner | ≥240 mm² |
| Single-mold Pouring Time of Casting | 25 seconds |
| Improvement Measure | Before Improvement | After Improvement |
|---|---|---|
| Proportion of New Sand | 5% – 20% | 20% – 40% |
| Curing Temperature | 230 °C | 220 – 240 °C |
| Iron Type Temperature | – | 230 – 250 °C or 240 – 280 °C |
| Curing Time | 8 – 20 minutes | 20 minutes |
