Grey Cast Iron — Key Characteristics Which statement best describes the nature of carbon and mechanical behavior in grey cast iron?

Introduction / Context:
Cast irons are classified by how carbon is present in the microstructure. Grey cast iron is prevalent in machine bases, housings, and engine blocks. Recognizing its signature graphite morphology and the resulting mechanical properties is fundamental in materials selection.

Given Data / Assumptions:

• Grey cast iron typically contains 2.5–4.0% carbon and 1–3% silicon.
• Slow cooling and silicon promote graphitization.
• Graphite appears as interconnected flakes in a pearlitic/ferritic matrix.

Concept / Approach:
In grey cast iron, much of the carbon precipitates as free graphite flakes rather than remaining combined as iron carbide. The flake morphology produces stress concentration, giving relatively low tensile strength and poor impact resistance, but high compressive strength and excellent damping and machinability.

Step-by-Step Solution:
Identify carbon form: free graphite flakes distributed through the matrix.Relate to properties: graphite flakes lower tensile and impact strength but do not severely reduce compressive capacity.Hence the most accurate statement is: carbon exists as free graphite in grey cast iron.

Verification / Alternative check:
Fracture surfaces of grey iron show a grey, dull appearance due to graphite; tensile test data confirm lower ultimate tensile strength but relatively high compressive strength.

Why Other Options Are Wrong:

• High tensile strength: incorrect; grey iron is weak in tension.
• Low compressive strength: incorrect; grey iron has good compressive strength.
• All of these: cannot be true since (b) and (c) conflict with known behavior.
• Carbon entirely as cementite: describes white cast iron, not grey iron.

Common Pitfalls:
Confusing grey and white iron; white iron forms hard cementite network with high wear but poor machinability.

Final Answer:
carbon in the form of free graphite