Which statement about sulfur in free-machining steels is true?

• A. 0.10-0.15% sulfur improves machinability but worsens weldability
• B. 0.10-0.20% sulfur improves weldability
• C. Sulfur does not affect machinability
• D. Sulfur increases magnetism

Answer: (C)

Sulfur is added to free‑machining steels to boost machinability. It forms manganese sulfide inclusions that act as tiny lubricants and convenient fracture initiation points in the chip, which lowers cutting forces and helps chips break more predictably. That makes machining faster and tools last longer. But those sulfide inclusions also weaken welds: they reduce ductility and can promote cracking in the heat-affected zone, making welding more difficult. So the idea that sulfur doesn’t affect machinability isn’t correct, and the typical, true statement is that a modest sulfur level improves machinability but worsens weldability. Magnetism isn’t meaningfully changed by sulfur.