Effect of sulphur in cast iron: Which statement best summarises the influence of sulphur on the properties and structure of cast iron?

Introduction / Context:
Trace elements in cast iron can profoundly affect microstructure and properties. Sulphur is generally considered detrimental and must be controlled through charge selection and additions like manganese to tie up sulphur as MnS.

Given Data / Assumptions:

• Base iron: gray or ductile iron without special desulphurisation beyond normal foundry control.
• We are selecting the most generally correct effect of sulphur.
• Other elements (Si, P, Mn, Mg) are at customary levels unless noted.

Concept / Approach:
Sulphur forms low-melting iron sulphide films and promotes carbide formation. The result is increased hardness, increased brittleness, hot shortness, and a tendency toward “white” iron, especially at high cooling rates. The most universal and succinct descriptor across cast irons is that sulphur increases hardness and brittleness. Although sulphur can push the structure toward white and hard iron, that outcome depends on cooling and overall chemistry; hence the broader statement (hardness and brittleness increase) is preferred as the correct generalisation.

Step-by-Step Solution:
Identify sulphur’s metallurgical role: carbide promotion and FeS formation.Link to macroscopic properties: hardness ↑, brittleness ↑, hot shortness ↑.Select the option that is broadly correct independent of exact casting section/cooling rate.Therefore choose “increases hardness and brittleness”.

Verification / Alternative check:
Foundry texts recommend low S and adequate Mn to avoid hot shortness and to improve machinability and toughness.

Why Other Options Are Wrong:
Soft and easily machinable: opposite of sulphur’s effect in irons.

Makes iron white and hard: can occur but is process-dependent; statement is less general than the chosen answer.

Aids fusibility/fluidity: describes phosphorus, not sulphur.

Improves nodularity: Mg/Ce control nodularity; sulphur harms it.

Common Pitfalls:
Assuming a single outcome (white iron) in all cases; overlooking the role of cooling rate and Mn/S balance.

Final Answer:
increases hardness and brittleness