Question

Cementite is:
A. Predominant content of cement
B. A mineral of calcium
C. A carbide of iron in steel
D. None of these

Answer 1

Hint: Cementite is an iron carbide with chemical formula $F{e_3}C$ and is the mixture of carbon and iron. Cementite is present in most steels and cast irons. It increases corrosion resistance significantly when immersed in the solution of sodium chloride.

Complete step by step answer: Cementite is also known as iron carbide. It is a compound of iron and carbon with the chemical formula $F{e_3}C$. It contains $6.67\% $ carbon and $93.3\% $ iron by weight. It has an orthorhombic crystal structure with the lattice parameters as \[{\text{a = 0}}{\text{.508 nm, b = 0}}{\text{.675 nm,c = 0}}{\text{.451 nm}}\] . In the structure of cementite there are $12$ atoms of iron in the unit cell and $4$ of carbon. Out of these twelve iron atoms, four iron atoms are present in the mirror plane and the remaining eight iron atoms are present on the corners of a unit cell. Cementite is a brittle and hard material. It is an important constituent in Ferrous metallurgy. Cementite is present in steel and cast iron. It is also present in a huge amount in meteoroids. Cementite is also called iron-carbon system (i.e. plain-carbon steel and cost). Hence, we got to know that cementite is a carbide of iron in steel.
>b>Hence option (C) is the correct answer.

Additional Information: Cementite changes from ferromagnetic to paramagnetic at its Curie temperature (approx. $480K$). Cementite is thermodynamically unstable and firstly converted to austenite (low carbon level) and graphite (high carbon level) at higher temperatures. It does not decompose at temperature below $723^{o}C$.

Note: Cementite forms directly from the melt in the case of white cast iron. In carbon steel, cementite precipitates from austenite or from martensite during temperature. The pure form of cementite can be found frequently in Earth’s surface.