The Five Stainless Steel Families

One definition, five structures

A steel counts as stainless when it contains at least about 10.5% chromium, enough to form the thin chromium-oxide film that protects the surface. Beyond that common root, the crystal structure — set mainly by nickel and chromium balance — divides stainless steels into five families.

Family	Structure	Magnetic?	Headline grades	Hallmark
Austenitic	Face-centred cubic	No (when annealed)	304, 316, 321	Best general corrosion resistance and formability
Ferritic	Body-centred cubic	Yes	430, 409	Nickel-free, lower cost
Martensitic	Hardenable, tetragonal when quenched	Yes	410, 420, 440C	Hardness by heat treatment
Duplex	Roughly half austenite, half ferrite	Yes	2205, 2507	High strength, chloride resistance
Precipitation-hardening	Martensitic or austenitic base + ageing	Usually yes	17-4PH (630)	Very high strength from a low-temperature age

Austenitic — the default family

Grades 304 and 316 account for the majority of all stainless produced. They form and weld readily, stay tough to cryogenic temperatures, and 316's molybdenum adds pitting resistance for marine and chemical duty. They cannot be hardened by heat treatment — only by cold work. Weaknesses: moderate strength, susceptibility to chloride stress-corrosion cracking at elevated temperatures, and a price that follows nickel and molybdenum markets. They are non-magnetic when annealed, though heavy cold work can make them slightly magnetic.

Ferritic — the economical family

With chromium but essentially no nickel, ferritics such as 430 (decorative, appliances) and 409 (exhaust systems) cost less and resist stress-corrosion cracking well. Their limits are toughness and weldability: grain growth in weld zones and lower ductility confine them mostly to thinner sections and moderate corrosion duty.

Martensitic — the hardenable family

Grades like 410, 420 and 440C trade corrosion resistance for hardness: quenched and tempered, they serve in cutlery, valve parts, shafts and surgical instruments. Their corrosion resistance is the lowest of the families — they are "stainless" relative to carbon steel, not relative to 304.

Duplex — strength plus chlorides

Duplex grades such as 2205 offer roughly twice the yield strength of 304 together with strong resistance to chloride pitting and stress-corrosion cracking — the combination that made them standard in oil and gas, desalination and chemical tankers. Fabrication needs more care (heat input control), and service temperature is typically limited to below about 300 °C by embrittlement mechanisms.

Precipitation-hardening — the strength specialists

17-4PH and its relatives are machined in a soft condition, then aged at a few hundred degrees to precipitate a strengthening phase, reaching very high strengths for shafts, gears, aerospace and marine hardware.

A note on the magnet test

"Stainless is non-magnetic" is a myth that misleads incoming inspection every week. Magnetism follows crystal structure, not quality: three and a half of the five families are magnetic, and even austenitic grades can turn slightly magnetic after cold work. Use PMI testing, not a magnet, to verify grade.