Nickel (2-5% in alloy round steel) improves toughness, especially at low temperatures, by stabilizing the austenitic structure. In grades like 4340 (1.65-2.00% Ni), it enhances hardenability, allowing deeper heat treatment penetration for large-diameter bars. Nickel also boosts tensile strength (4340 reaches 1,034 MPa) while maintaining ductility, making it suitable for aircraft landing gear and structural components. Its cost limits use to high-performance applications, unlike manganese, which is a cheaper strengthener.