Cementite (Iron Carbide, Fe3C) — Chemical Composition by Mass Cementite in steels and cast irons consists of which approximate mass percentages of carbon and iron?

Introduction / Context:
Cementite (Fe3C) is an intermetallic compound central to the microstructures of steels and cast irons. Its composition determines the carbon balance in phases such as pearlite and influences the hardness and brittleness of iron–carbon alloys.

Given Data / Assumptions:

• Fe3C stoichiometry is fixed: three iron atoms to one carbon atom.
• Atomic masses (approx.): Fe ≈ 55.85, C ≈ 12.01.
• Mass percentages are required, not atomic fractions.

Concept / Approach:
The mass fraction of carbon in Fe3C can be computed using the molecular mass: 355.85 + 12.01 ≈ 179.56. Carbon contribution ≈ 12.01/179.56 ≈ 0.0669 or 6.69%, commonly cited as 6.67%. Iron is the remainder, 93.33% by mass. This classic figure appears in phase-diagram discussions and is the anchor point for understanding eutectoid and eutectic compositions involving cementite.

Step-by-Step Solution:
Compute total mass: 355.85 + 12.01 ≈ 179.56.Compute carbon percent: 12.01 / 179.56 ≈ 0.0669 → 6.67% (rounded).Compute iron percent: 100% − 6.67% = 93.33%.Report composition: 6.67% C and 93.33% Fe.

Verification / Alternative check:
Standard metallurgy references list cementite as Fe3C ≈ 6.67% C by weight.

Why Other Options Are Wrong:
Options involving “ferrite” mix phase names with mass percentages; ferrite is a separate phase (α-Fe), not a component of cementite.13% carbon is far too high and inconsistent with Fe3C stoichiometry.

Common Pitfalls:
Confusing atomic percent with weight percent; Fe3C has only 1 in 4 atoms as carbon but still only 6.67 wt% C.

Final Answer:
6.67% carbon and 93.33% iron