Families of Stainless Steel
There are five families of stainless steel: ferritic, martensitic, austenitic, duplex, and precipitation hardening. These names are derived from the crystal structure of the steels, which determines their metallurgical behavior.Ferritic
Ferritic stainless steels are plain chromium stainless steels with a chromium content varying between 12% and 18% and low carbon content. They are magnetic and not hardenable by heat treatment.
Martensitic
Martensitic stainless steels were the first stainless steels that were commercially developed (as cutlery) and have a relatively high carbon content (0.1% - 1.2%) compared to other stainless steels. They are plain chromium steels containing between 12% and 18% chromium. Alloy 410 is the basic, general purpose, and magnetic grade that is hardenable by quenching and tempering. These stainless steels can be heat treated to obtain high strength with good ductility.
300 Series Austenitic
300 series austenitic stainless steels are non-magnetic. When nickel is added to stainless steel in sufficient amounts, the crystal structure changes to "austenite." The basic composition of 300 series austenitic stainless steels is 18% chromium and 8% nickel. This enhances their corrosion resistance and modifies the structure from ferritic to austenitic. Austenitic grades are the most commonly used stainless steels, accounting for more than 70% of production. (Alloy 304/304L followed by Alloy 316L are the most commonly specified grades by far.) They are not hardenable by heat treatment.
Duplex
These are stainless steels containing relatively high chromium levels (between 18% and 28%) and moderate amounts of nickel (between 4.5% and 8%). The high corrosion resistance and the excellent mechanical properties of duplex stainless steels can be explained by their chemical composition and balanced ("duplex") microstructure of approximately equivalent volume fractions of ferrite and austenite. Alloys 2304 and 2205 are the most common grades.
"Super" duplex grades have enhanced pitting and crevice corrosion resistance compared with the ordinary austenitic or duplex types. This is due to the further additions of chromium, molybdenum, and nitrogen to these grades. Alloy 2507 is the most common grade.
Precipitation Hardening
Precipitation hardening stainless steels, like the martensitic types, can be strengthened (i.e., hardened) by heat treatment. The mechanism is metallurgically different from the process in the martensitic types. This means that either martensitic or austenitic precipitation hardening structures can be produced. These stainless steels combine high strength and hardness with corrosion resistance which is superior to that of the martensitic chromium stainless steels.
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