How Alloys Affect Physical Properties

Nickel is an important alloying element. In concentrations of less than 5%, nickel will raise the toughness and ductility of steel without raising the hardness. It will not raise the hardness when added in these small quantities because it does not form carbides, solid compounds with carbon.

Chromium in steel forms a carbide that hardens the metal. The chromium atoms may also occupy locations in the crystal lattice, which will have the effect of increasing hardness without affecting ductility. The addition of nickel intensifies the effects of chromium, producing a steel with increased hardness and ductility.

Copper is quite similar to nickel in its effects on steel. Copper does not form a carbide, but increases hardness by retarding dislocation movement.

Molybdenum forms a complex carbide when added to steel. Because of the structure of the carbide, it hardens steel substantially, but also minimizes grain enlargement. Molybdenum tends to augment the desirable properties of both nickel and chromium.

Stainless steels are alloy steels containing at least 12% chromium. An important characteristic of these steels is their resistance to many corrosive conditions.