Alloying Effects of Ductile Iron

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Carbon (C):

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# Carbon is a Primary Graphitizing element.

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Silicon (Si):

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# It helps to acheive carbide free structures.

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#Silicon helps to increase elongation in Ferritic grades. Its effect is less in Pearlitic-ferritic and Pearlitic grades.

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 Phosporous (P):

 

# It is a embrittling element. It also forms inclusions. Increasing it will reduce elongation, reduce toughness, increase tendency to embrittlement and increase tendency for dispersed porosity. It stablizes perlite increasing hardness but reduces elongation.

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# The effect of Phosporous can be reduced but not be eliminated.

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Carbon Equivalent (CE):

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# Carbon equivalent is derived by,

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    Carbon Equivalent = Carbon + 0.33(Silicon) + 0.33(Phosporous)

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# Carbon equivalent is generally maintained low for Heavy sectioned castings to reduce Graphite flotation and for better feeding.

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# Carbon equivalents are generally maintained High for Thin walled castings to avoid formation of carbides and Surface Shrinkage. Higher Carbon along with higher silicon & Cerium Promote Chunky Graphite. High carbon content also causes gas related defects.

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Magnesium (Mg):

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# It Forms graphite into Spheroidal nodules. Higher level of Magnesium increases solidification and promote carbides. 

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# Magnesium is Volatile. It loses while holding.

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# Before Nodularising, it first oxidies then desulphurizes. Magnesium oxides ang Forms Magnesium oxide(MgO) which has high melting point and low density .They stay as solid , floats and causes inclusions.

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# Magnesium reacts with Sulphur to form Magnesium Sulphide(MgS). MgS is Unstable, so it again reacts with Oxygen to Form Magnesium Oxide(MgO). MgO is white in colour. MgS should be removed after Treatment, Before inoculation and before pouring.

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Manganese (Mn):

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# It Promotes Ferritic structure and reduce brittleness. It is a carbide forming element.

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# It's level is tolerated by Silicon level and section thickness of casting. Below 30mm thickness castings, the carbide promoting tendency of Manganese(Mn) is reduced by addition of Silicon(Si). In higher thickness, the carbide forming tendency effect cannot be controlled by silicon as silicon freezes with iron.

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Sulphur (S):

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# Sulphur(S) readily reacts with (Magnesium)Mg to form Magnesium Sulphide(MgS). Magnesium Sulphide combines with Oxygen(O) and Silicon(Si) to form MgSi03(Slag). This slag cause dross inclusion.

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# High level of Sulphur(S) cause Graphite reversal . This causes Faded structure in Casting.

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Copper (Cu):

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# It is a graphitizing element, promote pearlite and does not causes Carbides. It is 10 to 15 times Stronger than Manganese(Mn). It retards the decomposition of pearlite during cooling. It is Maintained at Maximum of 0.3% in Ferritic grades. Copper(Cu) should be used as pure.

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# It increases As-cast hardness.

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Tin (Sn):

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# It is a powerfull pearlite promoting element with out forming carbides. It is 10 times effective than Copper(Cu).

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# Higher level of Tin(Sn) Causes Cracking of Pearlite.

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Nickel(Ni):

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# It reduces Chilling and Carbide forming tendency.

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