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SAI CHARAN
Silver
Asked: 2020-04-17T03:23:40+05:30 In:

Why the free carbon present in the cast-iron?why not it forms total Fe3C?

Why the free carbon present in the cast-iron?why not it forms total Fe3C?
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    7 Answers

    1. Platinum⭐ GATE Metallurgy Coaching

      Free carbon present in cast iron is present in graphite form when silicon content is 2-3 wt% and provided the cooling has been slow. Fe3C is present in white cast iron which contains less than 1 wt% Si content.

      Free carbon present in cast iron is present in graphite form when silicon content is 2-3 wt% and provided the cooling has been slow. Fe3C is present in white cast iron which contains less than 1 wt% Si content.

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    2. Platinum GATE Metallurgy Coaching | Faculty with 7 years of Teaching Experience | 50% of Selections in all PSUs
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      Hello Sai Charan Ji, Free carbon (graphite) will be present in Cast Iron depending on the Cooling Rate. Generally, the Cooling Rates in casting are lower and not rapid. As Cementite is a metastable phase, it decomposes into Carbon & Iron if given sufficient heat and time as per-  Fe3C  = Fe + 3CRead more

      Hello Sai Charan Ji,

      Free carbon (graphite) will be present in Cast Iron depending on the Cooling Rate. Generally, the Cooling Rates in casting are lower and not rapid.

      As Cementite is a metastable phase, it decomposes into Carbon & Iron if given sufficient heat and time as per-  Fe3C  = Fe + 3C

      If the cooling is fast enough then Cementite remains as it is. Please see the diagram below which explains the effect of cooling rate on Cast Iron and final microstructure.

       

       

      You can see the extremes- if you cool fast the initial cast liquid converts to Pearlite & Cementite. If you cool very slowly, even the Cementite in the Pearlite converts to Graphite leaving behind Ferrite (Pure Iron)

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    3. Silver

      100% of Fe3C requires only 6.67% of carbon so when the carbon present is more than 6.67% it will remain in the form of free carbon (graphite) in the cast iron since it is the most stable form of carbon. Fe3C is metastable when we allow it for longer times it will decompose into 3Fe and graphite.

      100% of Fe3C requires only 6.67% of carbon so when the carbon present is more than 6.67% it will remain in the form of free carbon (graphite) in the cast iron since it is the most stable form of carbon. Fe3C is metastable when we allow it for longer times it will decompose into 3Fe and graphite.

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    4. Gold Metallurgist, Standard Provider (IS, ASTM, BS, ISO etc), Laboratory Equipment Suppliers,
      This answer was edited.
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    5. During slow cooling BCC iron is the stable state. In cast iron the carbon content is high. When molten metal is poured inside the moulding cavity, the outer layer gets quenched so martensitic structure forms with high carbon content, but with depth the degree of super cooling reduces and there formaRead more

      During slow cooling BCC iron is the stable state. In cast iron the carbon content is high. When molten metal is poured inside the moulding cavity, the outer layer gets quenched so martensitic structure forms with high carbon content, but with depth the degree of super cooling reduces and there formation of BCC iron is most probable and for this reason BCC lattice  pushes away the free carbon. This free carbon then forms either graphite if silicon is present or remains in free state.

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    6. 1. Because of presence of sufficient alloying element (mainly Si) in cast iron graphitization takes place . 2. Cast iron has more carbon than steel. Those carbons are found in free form.

      1. Because of presence of sufficient alloying element (mainly Si) in cast iron graphitization takes place .

      1. 2. Cast iron has more carbon than steel. Those carbons are found in free form.
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    7. Platinum

      Cementite is a metastable phase and it decomposes into iron and carbon during slow cooling. In  cast iron carbon is present in the form of graphite. It contains carbn 3-4%ans silicon 2-3%. But in white cast iron carbon present in the form of cementite. Generally the presence of carbon in graphite foRead more

      • Cementite is a metastable phase and it decomposes into iron and carbon during slow cooling.
      • In  cast iron carbon is present in the form of graphite. It contains carbn 3-4%ans silicon 2-3%.
      • But in white cast iron carbon present in the form of cementite.
      • Generally the presence of carbon in graphite forms or in cementite depends only upon the cooling rate.
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