Effects of Various Elements in Ductile Iron Castings
Ductile iron has grown in prominence as the chosen material in castings. It is highly tensile, has excellent castability and better mechanical properties than cast iron or gray iron. It also is low cost casting material.
The properties of ductile iron castings depend on the charge materials used. Therefore, it is necessary to make the proper choice of charge material while casting ductile iron. Here are a few elements that affect the properties of ductile iron castings. Careful selection and proportion of the charge material used changes the microstructure and properties of the castings.
Predominant among the elements that are used in ductile iron castings are manganese and copper.
Manganese is a strong pearlite ( a two phased and laminar structure with alternate layers of ferrite and cementite) forming element. It is added to ductile iron to increase its tensile strength. However, adding too much manganese can result in reduction of plasticity. Manganese retards the onset of the eutectoid transformation. When the manganese content is between 0.8 to 0.9 %, high strength and high hardness ductile iron can be produced, however, the ductile strength of the casting is reduced. If lesser manganese is added, the plasticity increases, but the strength and hardness decreases.
Copper has a moderate density, decent elastic modulus and a melting temperature of 1083 C. It forms many useful alloys to provide a wide variety of engineering property combinations. Like Manganese, copper too is a strong pearlite promoter. It increases the proof stress, the tensile strength and hardness with no embrittlement in the casted matrix. The addition of copper also increases the amount of pearlite sharply.
The addition of silicon improves the stability of ductile iron castings at higher temperatures. It also forms a surface layer that inhibits oxidiation and gives protection from corrosion.
Wherever the ductile iron casting is to be used under corrosive conditions (e.g. valves in submersible pumps), Nickel is added. Nickel reduces the corrosion and also provides wear resistance. However, it is expensive and used in castings only for special purposes. If added in large quantities, it can make the casting brittle.
Molybdenum is carbide former, as well as a pearlite stabilizer. It does so by depressing the pearlite transformation temperature. It improves mechanical properties of the ductile iron casting like creep resistance and hardness. However, Molybdenum can also make the casting brittle.
Chromium prevents corrosion of the ductile iron casting by forming a protective layer of chromium oxide on the surface.
Phosphorus and Sulphur are some of the other elements that may be added to ductile iron castings.