from Dick Jaffe, Chaparral Steel:
Ni ( Nickel )
When present in solid solution, Ni "stiffens" Ferrite and decreases ductility.
Reduces distortion in heat treating. Permits use of milder quenching media.
Permits ability to achieve strength & toughness levels at lower carbon contents.
Improves weldability, plasticity & fatique properties.
Improves toughness, especially at low temperatures.
Improves ability to case harden.
Improves corrosion resistance.
Like Cu, Ni cannot be removed from molten steel. Ni content can only be lowered by dilution.
Ni is an Austenite stabilizer in Stainless Steels.