One of the most common defects in the production of nodular cast iron is subcutaneous porosity. Subcutaneous porosity is most likely to occur in wet casting, especially in small castings with large specific surface area. The subcutaneous pores are usually located 0,5-1mm below the surface of the casting, most of which are pinholes with a pore diameter of 0,5-2mm. The inner wall is smooth (sometimes with a graphite film on the inner surface), which are evenly distributed on the upper surface of the casting or far away from the place where the casting is poured in. However, the surface and the bottom of the casting are occasionally in place. In the as cast condition, the subcutaneous porosity is not suitable to be found; however, it is exposed after heat treatment or machining. The subcutaneous porosity affects the surface quality of the casting, and in the place where the subcutaneous porosity occurs, it is often accompanied by flake graphite, which worsens the mechanical properties of the place.
After the casting surface is removed, many small pinholes will be found, which are full of hydrogen sulfide gas. Therefore, it can be inferred that when the magnesium sulfide in the molten iron meets the water in the mold, hydrogen sulfide will be produced. The result of MGS + H2O → MgO + N2S is that the formed N2S gas stays close to the surface of the casting before it floats during the rapid solidification of the casting. Therefore, these bubbles are not only spherical, but also raindrops. The tip of these raindrops extends into the casting.
Subcutaneous pores may also be caused by the hydrogen produced. In the spheroidized molten iron, the following reactions will take place:
(Fe, Mg) C + N2O (in mold) → (Fe, Mg) O + C2H2
C2H2 → 2C (graphite film) + H2 ↑ (bubble)
Mg + N2O (in mold) → MgO + H2 ↑ (bubble)
In addition, slag inclusions are sometimes found in the subcutaneous pores, and the contents of Si, Al, ng and Ca in these slag are relatively high. As a result, Al2O3, Cao, MgO, SiO2, MnO and so on can be used to provide crystalline backing for heterogeneous nucleation of bubbles.
When the temperature is higher than 1530 ℃, the reaction will take place: SiO2 + 2C → Si + 2CO ↑ when the temperature is higher than 1400 ℃, the reaction will take place: MnO + C → Mn + 2CO ↑ when the temperature is higher than 720 ℃, the reaction will take place: FeO + C → Fe + CO ↑
All of the above three reactions will form CO gas, which will lead to supersaturation of CO gas in molten iron and generate excessive precipitation pressure, thus increasing the formation of subcutaneous pores.