In white cast irons, carbon is present as

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Graphite flakes

Graphite nodules

Cementite

Carbon does not exist

answer is C.

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Detailed Solution

concept - Cementite, a form of carbon, can be found in white cast irons. It has a low concentration of carbon and silicon (a graphitizing agent), as well as a quicker cooling rate, which prevents the precipitation of carbon as graphite.
Instead, the metastable phase of cementite causes the carbon to precipitate out of the melt. The comparatively big particles of cementite develop when it precipitates out of the melt. The lower iron-carbon austenite phase is left over when cementite precipitates out because it sucks carbon from the original melt to form a eutectic mixture. Pearlite and a eutectic combination of cementite and pearlite make up the structure of white cast iron (converted from austenite).
Cementite, which is naturally hard and brittle, dominates the microstructure of white cast iron. Because white cast iron lacks graphite, it is also hard and brittle and shows white crystalline fractures.
Hence, the correct option is 3.

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