Improving machinability of steels: The machinability (ease of cutting, chip breaking, and surface finish at given tool life) of plain carbon steels is most effectively improved by adding which elements?

Introduction / Context:
Free-machining steels are engineered to break chips easily, reduce cutting forces, and improve surface finish. This is achieved by adding specific elements that form inclusions or low-melting constituents at the tool–chip interface.

Given Data / Assumptions:

• Base material: plain carbon steel for general machining.
• Objective: improved chip control and lower tool wear.
• Additions must not primarily target high-temperature strength or corrosion resistance.

Concept / Approach:
Sulphur forms manganese sulphide (MnS) inclusions which act as chip breakers and lubricants. Lead in small amounts smears at the cutting interface, improving lubricity and finish. Phosphorus can increase strength and machinability at the cost of toughness. Together, these form the basis of many free-cutting grades (e.g., 12L14). Nickel and chromium, while beneficial for strength and corrosion resistance, generally do not improve machinability and can make cutting more difficult.

Step-by-Step Solution:
Relate machinability to inclusion engineering: MnS and Pb improve chip breakage and lubrication.Assess candidate elements: Ni and Cr raise strength/hardness without enhancing chip control.Select the trio most associated with free-machining behavior: S, Pb, P.Confirm that these additions are standard in free-cutting steels.

Verification / Alternative check:
Machining handbooks list leaded and resulfurized steels with substantially higher machinability ratings than Ni/Cr-alloyed counterparts.

Why Other Options Are Wrong:
Nickel and chromium (alone or together) are not the primary additions for machinability; they target mechanical properties and corrosion resistance.

Boron/niobium are microalloying elements for hardenability and grain control, not machinability.

Common Pitfalls:
Assuming that stronger alloys cut better; in reality, controlled inclusions and lubricious phases drive machinability.

Final Answer:
sulphur, lead and phosphorus