| Stainless
steels are classified by their
crystalline structure:
Austenitic stainless steels comprise over 70% of total stainless
steel production. They contain a maximum of 0.15% carbon, a minimum of
16% chromium and sufficient nickel and/or manganese to retain an
austenitic structure at all temperatures from the cryogenic region to
the melting point of the alloy. A typical composition is 18% chromium
and 8% nickel, commonly known as 18/8 stainless.
Ferritic stainless steels are highly corrosion resistant, but far
less durable than austenitic grades and cannot be hardened by heat
treatment. They contain between 10.5% and 27% chromium and very little
nickel, if any. Most recipes include
molybdenum; some, aluminium or
titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo,
29Cr-4Mo, and 29Cr-4Mo-2Ni.
Martensitic stainless steels are not as corrosion resistant as the
other two classes, but are extremely strong and tough as well as
highly
machineable, and can be hardened by heat treatment. They contain
11.5 to 18% chromium and significant amounts of carbon. Some grades
include additional alloying elements in small quantities.
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