Effect of carbon state in iron: When carbon is present as combined carbon (primarily as iron carbide, cementite), how does it influence the hardness and grain character of iron?

Introduction / Context:
Carbon in iron can exist in free (graphitic) or combined form (cementite, Fe3C). The state of carbon strongly affects mechanical properties and microstructure. Understanding this distinction is essential for cast irons and steels alike.

Given Data / Assumptions:

• Combined carbon refers mainly to iron carbide formation not free graphite.
• Microstructure may include pearlite (lamellae of ferrite and cementite) and cementite networks.
• Cooling rate and composition can further refine the grain size.

Concept / Approach:
Cementite is a hard, brittle phase. Its presence increases hardness and strength. In steels, finer pearlite spacing and refined grains enhance strength and hardness further. Combined carbon tends to promote a harder, finer microstructure compared with graphitic irons that are softer and machinable.

Step-by-Step Solution:
Recognize that cementite hardens the matrix.Finer distribution (e.g., fine pearlite) leads to fine-grained appearance and increased hardness.Therefore, combined carbon generally makes iron harder and the structure finer.Select the option “hard and gives a fine-grained crystalline structure”.

Verification / Alternative check:
Hardness tests show higher values with increased combined carbon; microscopy reveals refined lamellar spacing with faster cooling.

Why Other Options Are Wrong:
Soft variants align with graphitic or annealed structures, not cementite-rich ones.

“Hard and coarse-grained” ignores typical refinement associated with increased pearlite and controlled cooling.

Common Pitfalls:
Assuming carbon always refines grains regardless of cooling history; overlooking that graphite formation softens the material.

Final Answer:
hard and gives a fine-grained crystalline structure