Alloying in steels — Which alloying element typically improves hardness/strength while maintaining or improving toughness (i.e., does not sacrifice ductility) compared with the others listed?

Introduction:
Alloying elements modify steel microstructure and properties. Many strong carbide formers (e.g., V, Mo, Cr, W) raise hardness and wear resistance, often at the expense of ductility. Nickel is notable for enhancing toughness and low-temperature ductility while contributing to strength, making it an exception among common hardenability promoters.

Given Data / Assumptions:

• Plain-carbon steel base; additions are within common ranges.
• Heat treatment provides appropriate microstructures.
• Comparison focuses on ductility/toughness trade-offs.

Concept / Approach:

Nickel is an austenite stabilizer that refines microstructure and improves toughness and impact resistance, particularly at low temperatures, without the degree of ductility loss associated with heavy carbide-forming additions. Vanadium, molybdenum, chromium, and tungsten strongly form carbides, increasing hardness, creep resistance, or wear resistance but often reducing ductility if not carefully balanced.

Step-by-Step Solution:

Verification / Alternative check:

Steels such as 9Ni for cryogenic service and Ni-Cr-Mo alloys demonstrate nickel’s toughness benefits; data show higher Charpy impact energy with Ni additions.

Why Other Options Are Wrong:

B/C/D/E typically increase hardness/strength via carbide formation or solid-solution/precipitation effects, with a tendency to reduce ductility if overused or improperly heat treated.

Common Pitfalls:

Assuming all strengtheners reduce ductility equally; nickel is a notable counterexample that enhances toughness and maintains ductility.

Final Answer:

nickel