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

Silicon is what metallurgists call a graphitizing element in cast iron, meaning it encourages carbon to separate out as flakes of free graphite rather than remaining chemically combined with iron as hard cementite. When silicon content is high, this graphite formation dominates, and the resulting flakes give the fractured surface a dull grey colour, which is why this type is called grey iron. Low-silicon compositions instead retain the carbon as cementite, producing the pale, glassy fracture of white iron, and malleable iron is simply white iron that has been heat-treated afterward to convert that cementite into graphite nodules, rather than a naturally high-silicon composition. Pig iron, meanwhile, is just the unrefined output of the blast furnace and is not classified this way at all.

Therefore, the correct answer is Grey iron.

Answer 2

Looking at the fracture appearance and processing history of each option offers another way to reach the same conclusion.

1. White iron: Produced with lower silicon content, its carbon stays locked as cementite, giving a hard, brittle, pale fracture surface, which is the reverse of what a high silicon content produces.
2. Grey iron: A high proportion of silicon pushes carbon out of solution as graphite flakes during cooling, and it is these graphite flakes scattered through the metal matrix that both give the fracture its grey colour and account for the higher silicon content described in the question.
3. Malleable iron: This starts life as a low-silicon white iron casting and only later, through an extended annealing treatment, develops graphite nodules, so its final graphite structure comes from heat treatment, not from a naturally high silicon content.
4. Pig iron: This is simply the first, unrefined form of iron tapped from a blast furnace before any further processing into a defined cast iron grade, so it is not classified by silicon-driven graphite formation at all.

Tracing each type back to its silicon content and graphite-formation history confirms that a high-silicon cast iron corresponds to grey iron.

Therefore, the correct answer is Grey iron.