The most common problem in the production of ductile iron manhole covers is subcutaneous pores. Subcutaneous pores are most likely to occur in green sand castings, especially small castings with large surface areas. Subcutaneous pores are often located ~ ~ below the surface of the casting. The pores are mostly pinholes with a smooth inner wall. They are evenly distributed on the upper surface of the casting or away from the sprue, but occasionally exist on the sides and bottom of the casting. In the as-cast state, subcutaneous pores are not easily found. However, they are exposed after the casting is heat treated or machined. Subcutaneous pores affect the surface quality of castings, and where subcutaneous pores appear, they are often accompanied by flake graphite, thus deteriorating the mechanical properties of the part.
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