Stainless steels are commonly grouped into five families by room-temperature microstructure — the first step in grade selection:

1) Austenitic (mostly 300 series): 304/316L/321 etc.; good corrosion resistance and formability, non-magnetic (may become slightly magnetic after cold working), weldable, largest volume. 2) Ferritic (400 series): 430/409/439 etc.; magnetic, lower cost, moderate corrosion resistance; used in decoration, appliances and automotive exhaust. 3) Martensitic: 410/420/440C etc.; hardenable by quenching and tempering, lower corrosion resistance; used for cutlery, bearings and valves. 4) Duplex: 2205/2507 etc.; austenite + ferrite, high strength, good chloride and stress-corrosion resistance; used in seawater, chemical and pressure service. 5) Precipitation hardening (PH): 17-4PH/15-5PH etc.; strengthened by aging, tensile strength above 1000 MPa; used in aerospace and precision parts.

Quick guide: corrosion + welding → austenitic; cheap + magnetic → ferritic; hardness → martensitic; high strength + chloride → duplex; high strength + precision → PH.

Note that grades within a family still differ significantly (e.g. 304 vs 316L with molybdenum); the family is only the first filter.