Metallurgy — understanding cementite Cementite in ferrous alloys is best described by which of the following statements?

Introduction / Context:
Cementite is a fundamental phase in the iron–carbon system, strongly influencing hardness, brittleness, and microstructure in steels and cast irons. Recognizing its composition and crystal structure is basic to materials engineering.

Given Data / Assumptions:

• Cementite is typically denoted as iron carbide.
• It appears in pearlite, ledeburite, and other microstructures depending on composition and cooling history.

Concept / Approach:
Cementite is iron carbide with nominal composition Fe3C, a stoichiometric intermetallic compound. It has an orthorhombic crystal structure and is hard and brittle compared with ferrite. Its fraction and morphology affect mechanical properties.

Step-by-Step Solution:
Identify chemical formula: Fe3C → option (a) correct.It is indeed a compound of Fe and C → option (b) correct.Crystal structure is orthorhombic → option (c) correct.Therefore, the comprehensive answer is (d) all of the above.

Verification / Alternative check:
Fe–C phase diagrams and crystallographic data confirm Fe3C and its orthorhombic lattice parameters.

Why Other Options Are Wrong:

• Each single choice misses other true attributes; only the combined statement captures all correct features.

Common Pitfalls:

• Confusing cementite (Fe3C) with graphite (C) in cast irons; graphite appears in gray irons, not as Fe3C.

Final Answer:
All (a), (b) and (c)