1. Classification and Characteristics of Porosity Defects
1.1. Intrusive porosity (localized porosity)
During the heat treatment of the molten metal, gases generated by the mold (or core) infiltrate into the iron liquid, resulting in localized porosity in certain areas of the casting during the cooling process. It should be emphasized that the interaction between the molten metal and the mold/core only occurs during casting, allowing gases produced by the mold/core at high temperatures to infiltrate into the iron liquid. (Physical reaction)
◆Features of Ingress Porosity:
- Manifests as localized porosity occurring in specific regions of the casting.
- The surface of the pores is relatively smooth, presenting as individual or honeycombed voids.
- The color of the pores is white or may have a dark layer, occasionally covered with an oxidized skin.
- In the case of nodular/compacted graphite iron, it may emit an odor reminiscent of carbide.See Figure 1.
Shrinkage Porosity:
- Exhibits characteristics of shrinkage and porosity.
- Refer to Figure 2.
1.2 Precipitation Porosity (Sieve-like Porosity):
Gases dissolved in the liquid form pores during the cooling process as their solubility decreases. These pores often have circular, elliptical, or needle-like shapes. It is essential to note that gas formation in the iron liquid occurs during the smelting and processing stages. As the temperature of the iron liquid rises, the solubility of gases increases, resulting in an increased gas content due to physical and chemical reactions during the smelting process. (The presence of gas within the iron liquid is a consequence of the physical and chemical reactions involving all substances participating in the smelting process).
Characteristics of Precipitation Porosity:
The characteristic feature is that it is numerous, dispersed, and relatively evenly distributed throughout the entire or a significant portion of the cross-section of the casting. See Figure 3.
1.3 Reaction Porosity:
Porosity generated as a consequence of chemical reactions between the molten metal and the mold interface. In this process, the iron liquid undergoes a cooling stage, causing the gases to be released and trapped solely on the surface of the casting.
Characteristics of Reaction Porosity:
This type of porosity predominantly appears on the surface of the casting, around 1-3mm away from the casting surface. It presents as a densely distributed pattern of small, closely spaced pores, which become more evident after heat treatment and shot blasting. Typically, these pores exhibit a needle-like or tadpole-like shape. It is also known as subsurface porosity. See Figure 4.
A. Spheroidizing Agent Slag Type**
Defect Characteristics: Spherical depressions appear on the casting surface, containing inclusions. These depressions often occur near the inner gating system. Scanning electron microscopy reveals uneven surfaces inside the pores. Spectral analysis of the pore content detects Si, Mg, Al, Ba, and O. The presence of Mg, which is specific to spheroidizing agents, indicates that the inclusions are slag formed through the participation of spheroidizing agents. CO gas pinholes result from the reaction between carbon in the iron liquid and slag.
B. Slag Type Resulting from Inoculant Defect Characteristics: The cross-section displays several depressions. Scanning electron microscopy and spectral analysis reveal uneven internal surfaces within the depressions, along with the presence of Si, Ca, Ba, and O in the inclusions. Ba is a unique element of the inoculant. This indicates that residual silicon-iron inoculant forms slag, and the reaction between carbon in the iron liquid and the oxide in the slag leads to the generation of CO gas, causing pinhole defects. Cause: Incomplete melting of the inoculant during flow results in slag formation. Countermeasures: Use dry inoculants to prevent splashing of the iron liquid and slag porosity during inoculation.
C Defect: Slag and Sand Inclusion Type Defect Appearance: Multiple depressions on the surface of the casting near the sprue. Scanning electron microscopy shows the presence of slag and sand within the depressions. Spectral analysis indicates the presence of Si, O, Al in the sand, and elements like Mg, Ce, Mn in the slag. This suggests that the defect is formed due to the interaction between the inoculant and the sand. Solution: Increase the cross-sectional area of the sprue runner and reduce the flow velocity in the sprue.
D Defect: Moisture-Induced Sand Mold Defect Defect Appearance: Depressions on the surface of the casting after machining. Scanning electron microscopy reveals no defects within the depressions. Spectral analysis shows that the main elements are C, O, Si, and Fe. This is a pinhole defect caused by water vapor generated from moisture in the wet-type mold. Solution: Reduce the moisture content in the molding sand, improve the permeability of the molding sand, and increase the proportion of coal powder in the molding sand. Decrease the resin moisture content in the cold box core manufacturing process.
2.1 Analysis of causes for invasive porosity:
1. Reasons for invasive porosity:
- Unreasonable design of the pouring system, leading to poor gas exhaust or vortex formation, resulting in entrapped gases during pouring.
- Excessive compactness of the sand mold, reducing its permeability.
- Inadequate gas venting in the sand core or blockage of the air passages.
- High moisture content in the molding sand (core). During humid weather conditions, the moist air can be absorbed by the mold/core and react with the molten iron, resulting in the generation of a large amount of gas being trapped in the mold cavity.
- Contamination of the core support and core iron with oil.
- Excessive volatile substances present in the molding sand.
- High resin nitrogen (N) content in coated sand, leading to the decomposition of NH3 and the formation of N and H gases.
- Uneven pouring, insufficient filling, resulting in the entry of a large amount of gas.
- High clay content in the molding sand, poor permeability, causing "blowholes" on the surface of the casting, which is also considered as invasive porosity.
2.2 Analysis of the Causes of Porosity:
1. High gas content, severe corrosion, and excessive surface grease in the furnace charge result in a higher gas content in the molten iron.
2. Insufficient drying of the molten iron mold.
3. Insufficient drying of the alloy.
4. Silicon and rare earth elements in the furnace charge can easily generate hydrogen gas holes, while aluminum or alumina can generate gas.
5. Low pouring temperature, causing the generated gas to not have enough time to rise and escape.
6. Unstable pouring.
7. High sand temperature exceeding 35°C or high core temperature can lead to moisture absorption on the mold cavity surface and excessive water content in the surface layer.
8. Reaction porosity: Gas produced from the chemical reaction between the chemical elements of the molten iron and the mold/core infiltrates into the liquid. The gas pores are formed during the cooling process when the gas does not have enough time to be released.
9. High residual magnesium content: Excessive magnesium content exacerbates the hydrogen absorption tendency of the molten iron. Residual magnesium content greater than 0.05% in the molten iron can cause subcutaneous gas porosity. High-nickel austenitic ductile iron with residual magnesium content greater than 0.07% is more prone to subcutaneous gas porosity.
10. Low pouring temperature.
11. High sulfur content in the molten iron: When sulfur content exceeds 0.094%, subcutaneous gas porosity occurs, and the higher the sulfur content, the more severe the subcutaneous gas porosity.
12. Rare earth content: Excessive rare earth content increases the oxide content in the molten iron, leading to an increase in foreign bubble nuclei and subcutaneous gas porosity. The residual rare earth content should be controlled within 0.043%.
13. Aluminum content: Aluminum in the molten iron is the main cause of hydrogen gas porosity in castings. When the residual aluminum content in wet-type ductile iron is between 0.03% and 0.05%, subcutaneous gas porosity occurs.
14. Casting wall thickness: Thin-walled and thick-sectioned castings are less prone to subcutaneous gas porosity.
15. Moisture content in molding sand: With an increase in moisture content, the tendency of nodular cast iron to produce subcutaneous gas porosity increases. When the moisture content in the molding sand is controlled below 4.8%, the subcutaneous gas porosity rate approaches zero.
Additionally, compactness of the molding sand and pouring temperature also play a role.
Magnesium vapor escaping from the molten iron and magnesium sulfide on the surface of the molten iron react with the water vapor in the mold as follows: Mg + H2O → MgO + 2[H] and MgS + H2O → MgO + H2O. The generated hydrogen, magnesium oxide, and magnesium sulfide gases can potentially infiltrate into the casting through the surface of the molten iron.
3.Methods to Prevent Porosity Defects:
1. Clean the furnace charge thoroughly to remove excessive gas content, severe corrosion, and surface grease before use.
2. Strictly control the molten iron temperature when it is taken out of the furnace and during pouring. Avoid excessively low pouring temperatures.
3. Fully dry the furnace crucible, ladle, and molten iron mold. Preheat the ladle before use.
4. Preheat spheroidizing agents and inoculants adequately to reduce the amount of gas introduced by rare earth and ferrosilicon.
5. Properly design the pouring system to ensure smooth venting inside the mold cavity and a steady flow into the cavity.
6. Ensure uniform compactness of the molding sand, avoiding excessive tightness.
7. Reduce the clay content in the core sand appropriately and increase its permeability.
8. Ensure proper venting of the sand core and seal the gaps between the cores to prevent molten iron from entering and blocking the air passages.
9. Set risers or vents at the highest points of the casting. Pay attention to venting during the pouring of large castings.
10. Tilt the casting slightly for large flat castings, with the vent holes positioned slightly higher to facilitate venting.
11. Dry and clean the chaplets and chills, ensuring they are free from rust and oil contamination.
12. Reduce moisture content in the molding sand, create vent slots on parting surfaces, and increase the amount of coal powder added if necessary.
13. Reduce binder content appropriately. For large castings, add materials that increase permeability, such as sawdust.
14. Use round sand grains to enhance permeability.
15. Reduce residual magnesium content while ensuring proper nodularization. Minimize the sulfur content in the original molten iron.
16. Control the sand temperature and pour as soon as possible after closing the mold.
17. Use dried sand cores and prevent moisture absorption inside the mold. Do not use sand cores with severe moisture absorption.
18. Spray carbonaceous materials like ingot oil on the mold surface to create a reducing atmosphere between the molten iron and the mold interface. Sprinkling a small amount of fluorspar powder or sodium fluoride on the molten iron-mold interface can reduce or eliminate subcutaneous porosity.
19. Increase the pouring temperature appropriately in rainy weather.
20. Reduce magnesium sulfide inclusions. Use low sulfur pig iron or add a small amount of soda ash during spheroidization treatment for desulfurization. After spheroidization, skim slag multiple times and let it stand briefly to allow MgS slag to float up.
21. Control the pouring temperature. For thin-walled castings, the temperature should not be less than 1320°C; for medium wall thickness castings, it should not be less than 1300°C; for thick-walled components like guide plates, it should not be less than 1280°C. Silicon molybdenum cast iron and high-nickel austenitic ductile iron require even higher temperatures.
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