Question

In white cast irons, carbon is present as:
A.Graphite flakes
B.Graphite nodules
C.Cementite
D.Carbon does not exist

Answer 1

Hint: Cast iron is a class of alloys containing iron and carbon with the carbon content being more than 2 percent. In white cast irons, carbon is present in the form of an iron carbide. This iron carbide in white cast iron has the formula ${\text{F}}{{\text{e}}_{\text{3}}}{\text{C}}$ .

Complete step by step answer:
In white cast irons, carbon is present as cementite which has the formula ${\text{F}}{{\text{e}}_{\text{3}}}{\text{C}}$ . It has a low carbon and silicon content (graphitizing agent) and also a faster cooling rate due to which the precipitation of carbon as graphite is prevented. Instead, the carbon is precipitated out of the melt in the metastable phase cementite. This cementite which is precipitated out from the melt, is formed as relatively large particles. When the cementite precipitates out, it withdraws the carbon from the original melt to form a eutectic mixture and the left out phase is the lower iron - carbon austenite. Thus, white cast iron has a structure which consists of pearlite and a eutectic mixture of cementite and pearlite (converted from austenite).
The microstructure of white cast iron is dominated by cementite which is hard and brittle in nature. Hence, white cast iron is also hard and brittle and displays white crystalline fractures as it is free from graphite.

Thus, option C is the correct answer.

Note:
White cast irons have an excellent resistance against abrasion and wear due to the large masses of carbides. Hence, they are commonly used as wear resistant surfaces. They are commonly used for rolling mill rolls, pulverizers, crushers, shot blasting nozzles etc.
The high chromium white cast irons have a very good abrasion resistance. This class of white cast irons is effectively used in coal grinding mills, hard rock mining, slurry pump, brick moulds etc. This group of white cast irons is considered the best combination of abrasion resistance and toughness among the white cast irons.