Metallurgy — red-shortness in hot-working of iron/steel: which impurity causes cracking when the metal is bent at red heat? Choose the most appropriate cause.

Introduction / Context:
When iron or low-alloy steel is worked at red heat (hot-working temperatures), certain impurities can make the metal brittle. This phenomenon is called red shortness (hot shortness). Understanding which element causes this defect helps in specifying proper steelmaking controls and selecting grades for forging and rolling.

Given Data / Assumptions:

• Iron/steel is bent or forged at elevated (red heat) temperatures.
• Impurities considered: sulphur, carbon, phosphorus, silicon.
• We focus on hot-working brittleness specifically, not room-temperature effects.

Concept / Approach:
Sulphur in steel forms iron sulphide (FeS). FeS has a low melting point and concentrates at grain boundaries. At hot-working temperatures, a FeS-rich film becomes weak or partially molten, promoting intergranular tearing and cracking. Manganese is added in steelmaking to combine with sulphur and form MnS, which is less harmful during hot work.

Step-by-Step Solution:

Verification / Alternative check:
Metallurgy references consistently attribute hot shortness in steels to sulphur (FeS). Process control aims at S reduction and Mn/S balance.

Why Other Options Are Wrong:

• Carbon: Raises strength/hardness; not the primary cause of hot shortness.
• Phosphorus: Causes cold shortness (brittleness at low temperature), not chiefly hot shortness.
• Silicon: Deoxidizer; excess may embrittle, but not the classic red-short agent.

Common Pitfalls:
Confusing red shortness (hot) with cold shortness (phosphorus-dominated) and overlooking the Mn–S control strategy.

Final Answer:
Sulphur