Question

Cast iron with high-silicon content is a

• A. White iron
• B. Grey iron
• C. Malleable iron
• D. Pig iron

Hint:

High silicon promotes graphite formation, leading to grey cast iron.

Answer 1

The Correct Option is B

Step 1: Role of silicon in cast iron.
Silicon is a graphitizing element in cast iron. Higher silicon content promotes the formation of free graphite rather than cementite.
Step 2: Characteristics of grey cast iron.
Grey iron contains graphite flakes in its microstructure, which are formed due to the presence of higher silicon content. This gives the fracture surface a grey appearance.
Step 3: Eliminating other options.
(A) White iron: Forms when silicon content is low and carbon exists as cementite.
(C) Malleable iron: Produced by heat treatment of white iron.
(D) Pig iron: A raw material, not a final cast iron type.
Step 4: Conclusion.
Therefore, cast iron with high silicon content is grey iron.

Answer 2

Silicon's role in cast iron is to influence whether carbon ends up as free graphite or as iron carbide, so tracing that effect through to each option's typical composition and appearance is the way to settle this.

1. White iron: This type forms when silicon content is kept low, allowing carbon to remain combined as hard, brittle cementite rather than separating out as graphite, which gives white iron its pale, silvery fracture surface and is the opposite of what high silicon content produces.
2. Grey iron: Silicon acts as a graphitizing agent, encouraging carbon to precipitate out as flakes of free graphite during solidification rather than staying combined as cementite; a high silicon content promotes exactly this graphite formation, and the presence of graphite flakes is also what gives the fractured surface its characteristic grey colour, matching the description in the question.
3. Malleable iron: This is produced by taking low-silicon white iron and subjecting it to a prolonged heat treatment that converts the cementite into graphite nodules afterward, so it is defined by its heat treatment history rather than by having a high silicon content from the start.
4. Pig iron: This is the crude, unrefined iron obtained directly from a blast furnace, a raw material rather than a finished cast iron classified by its silicon-driven graphite structure.

Since high silicon content specifically promotes the formation of free graphite flakes, which is the defining feature of grey iron, that is the correct classification.

Therefore, the correct answer is Grey iron.