Categories of Stainless Steels
I. Austenitic - A family of alloys containing chromium and nickel (and manganese and nitrogen when nickel levels are reduced), generally built around the type 302 chemistry of 18% Cr, 8% Ni, and balance mostly Fe. These alloys are not hardenable by heat treatment.
II. Ferritic - This group of alloys generally containing only chromium, with the balance mostly Fe, are based upon the type 430 composition of 17% Cr. These alloys are somewhat less ductile than the austenitic types and again are not hardenable by heat treatment.
III. Martensitic - The members of this family of stainless steels may be hardened and tempered just like alloy steels. Their basic building block is type 410 which consists of 12% Cr, 0.12% C, and balance mostly Fe.
IV. Precipitation-Hardening - These alloys generally contain Cr and less than 8% Ni, with other elements in small amounts. As the name implies, they are hardenable by heat treatment.
V. Duplex - This is a stainless steel alloy group, or family, with two distinct microstructure phases -- ferrite and austenite. The Duplex alloys have greater resistance to chloride stress corrosion cracking and higher strength than the other austenitic or ferritic grades.
VI. Cast - The cast stainless steels, in general, are similar to the equivalent wrought alloys. Most of the cast alloys are direct derivatives of one of the wrought grades, as C-8 is the cast equivalent of wrought type 304. The C preceding a designation means that the alloy is primarily used for resistance to liquid corrosion. An H designation indicates high temperature applications.