Free-machining additives in low-carbon steels: What is the combined effect of adding lead, sulphur, and phosphorus?

Introduction / Context:
Free-machining steels incorporate elements that enhance chip formation and reduce friction at the tool–chip interface. Recognizing how lead, sulphur, and phosphorus act helps explain why certain grades machine faster and produce better finishes with longer tool life.

Given Data / Assumptions:

• Low-carbon steels with free-machining additives.
• Conventional turning/drilling with HSS or carbide tools.
• Normal coolant application.

Concept / Approach:
Sulphur combines with manganese to form manganese sulphides, which act as built-in chip breakers and lubricants, reducing the tendency for built-up edge (BUE). Phosphorus can increase machinability by improving chip brittleness and adding strength at room temperature. Lead, present as soft inclusions, provides lubrication at the tool–chip interface. Collectively these changes promote small, broken chips, reduce BUE, lower cutting forces, and improve surface finish—classic signs of improved machinability.

Step-by-Step Solution:

Verification / Alternative check:
Machinability ratings (relative to AISI 1212) document higher cutting speeds and lower tool wear for leaded/sulfurized grades compared with plain low-carbon steels.

Why Other Options Are Wrong:
Each individual effect is correct; the comprehensive answer is “all of these.” The claim of “no effect” contradicts extensive industrial experience and data.

Common Pitfalls:
Assuming improved machinability always means better mechanical properties (it may reduce ductility or weldability); overlooking environmental and health considerations of leaded steels.

Final Answer:
all of these