In the production process of ductile iron parts, after heat treatment, shot blasting or machining, some holes with a diameter of about 0.5-3mm and a spherical, oval or pinhole-like inner wall are often found . These holes are generally in the surface of the casting. Distributed under 2-3mm , this is the so-called subcutaneous stomata . The formation of subcutaneous pores is due to the high surface tension of the magnesium-containing iron liquid, which is easy to form an oxide film , which has a certain effect on hindering the discharge of precipitated gas and invading gas. These gases will form pores if they stay under the skin. In addition, the paste-like solidification characteristic of ductile iron causes the gas passage to be blocked earlier and also promotes the formation of subcutaneous pore defects.