Sulphur and phosphorus impurities exist in steel at what percentages, and what are their low-percent properties?

• A. <0.05% total; Sulphur at 0.10-0.15% in free machining steels improves machinability but with poor weldability; Phosphorus improves strength, corrosion resistance and machinability
• B. Both are above 1% elements that increase weld strength
• C. S and P exceed 0.05% but do not affect properties
• D. S and P are noble gases

Answer: (A)

Tiny amounts of sulfur and phosphorus are kept in steel because they have outsized effects on machinability, strength, corrosion resistance, and weldability. In free-machining steels, a small sulfur addition—about 0.10–0.15%—creates manganese sulfide inclusions that act as internal lubricants and chip-breakers, which makes cutting easier and reduces tool wear. The trade-off is a decline in weldability and ductility, especially under high heat or stress, so this benefit comes with a sacrifice in weld performance.

Phosphorus, even at low levels, can increase strength through solid-solution effects and can influence machinability in a way that makes forming and finishing easier. In some cases it can also affect corrosion resistance positively, though too much phosphorus tends to embrittle steel and hurt weldability. Because of these pronounced effects, the total impurity content is kept very small, and the described ranges reflect how these elements are managed to balance machinability with other properties.