Cast iron impurities — which element makes cast iron hard and brittle? Among common impurities in cast iron, which one most strongly promotes hardness and brittleness (hot shortness) when present in excess?

Introduction / Context:
Trace elements in cast iron significantly influence microstructure and properties. Foundry control of impurities is therefore critical for achieving the desired balance of strength, machinability, and toughness.

Given Data / Assumptions:

• Gray or ductile iron base composition with residual impurities.
• Effect of impurity content considered qualitatively.

Concept / Approach:
Sulphur promotes the formation of iron sulfide (FeS), which has a low melting temperature and segregates at grain boundaries. This causes hot shortness (brittleness at elevated temperatures) and, in cast irons, increases hardness and brittleness. Manganese is often added to tie up sulphur as manganese sulfide (MnS), mitigating these effects. Silicon generally promotes graphitisation, softening gray iron and improving machinability. Phosphorus increases fluidity and can form steadite (Fe–Fe3P eutectic), which can embrittle if excessive, but at typical impurity levels sulphur is the primary culprit for hard, brittle behavior. Copper at small additions tends to strengthen pearlite without the severe brittleness associated with sulphur.

Step-by-Step Solution:

Verification / Alternative check:
Foundry specifications often cap S at very low levels and ensure Mn:S ratios large enough to neutralize sulphur’s adverse effects.

Why Other Options Are Wrong:

• Silicon: tends to make iron softer by graphitization (within normal ranges).
• Manganese: counteracts sulphur; excessive Mn can harden but is not the primary impurity problem.
• Phosphorus: increases fluidity; excessive P can embrittle, but the classic “hard and brittle” tag is most consistently associated with S.
• Copper: pearlite promoter, not a primary embrittling impurity.

Common Pitfalls:
Confusing phosphorus-induced steadite embrittlement with sulphur’s hot shortness; ignoring Mn’s protective role.

Final Answer: