In the production process of ductile iron castings, some holes with a diameter of about 0.5-3 mm and a spherical, elliptical or pinhole-shaped inner wall are often found in heat treatment, shot blasting or machining. These holes are generally in the casting skin. Under the 2-3mm distribution, this is called the subcutaneous stomata.
The formation of subcutaneous pores is due to the large tension of the surface of the magnesium-containing iron liquid, which tends to form an oxide film, which has a certain influence on the discharge of the precipitated gas and the invading gas. These gases are retained under the skin to form pores. In addition, the paste-like solidification characteristics of ductile iron make the gas passage blocked earlier, and also promote the formation of subcutaneous pore defects.