Stainless Steel Classification.

Austenitic stainless steels:

include the 200 and 300 series of which type 304 is the most common. The primary alloying additions are chromium and nickel.

Ferritic stainless steels :

are non-hardenable Fe-Cr alloys. Types 405, 409, 430,422, and 446 are representative of this group.

Martensitic stainless steels :

are similar in composition to the ferritic group but contain higher carbon and lower chromium to permit hardening by heat treatment. Types 403, 410, 416, and 420 are representative of this group.

Duplex stainless steels :

are supplied with a microstructure of approximately equal amounts of ferrite and austenite. They contain roughly 24% chromium and 5% nickel. Their numbering system is not included in the 200,300 or 400 groups.

Precipitation hardening stainless steels :

contain alloying additions such as aluminum which allow them to be hardened by a solution and aging heat treatment. They are further classified into subgroups as martensitic, semi austenitic, and austenitic precipitation hardening stainless steels. They are identified as the 600-series of stainless steels (e.g., 630, 631, 660)

Source : bohler welding