Details
Material hierarchy: Metal - Ferrous metal - Steel
Stainless steels are a group of highly alloyed steels designed to provide high corrosion resistance. The principal alloying element in stainless steel is chromium, usually above 15%. The chromium in the alloy forms a thin, impervious oxide film in an oxidizing atmosphere, which protects the surface from corrosion. Nickel is another alloying ingredient used in certain stainless steels to increase corrosion protection. Carbon is used to strengthen and harden the metal; however, increasing the carbon content has the effect of reducing corrosion protection because chromium carbide forms to reduce the amount of free Cr available in the alloy.
In addition to corrosion resistance, stainless steels are noted for the combination of strength and ductility. Although these properties are desirable in many applications, they generally make these alloys difficult to work in manufacturing. Also, stainless steels are significantly more expensive than plain C or low alloy steels.
Stainless steels are traditionally divided into three groups, named for the predominant phase present in the alloy at ambient temperature:
Most stainless steels are designated by a three-digit AISI numbering scheme. The first digit indicates the general type, and the last two digits give the specific grade within the type.
The traditional stainless steels were developed in the early 1900s. Since then, several additional high alloy steels hae been developed that have tood corrosion resistance and other desirable properties. These are also classified as stainless steels: