GUIDE
Stainless steel families: austenitic, ferritic, duplex, martensitic
Every stainless grade belongs to one of four metallurgical families. Knowing a grade's family tells you more about how it behaves than its commercial designation does — including what it will never be able to do.
What defines a family
Pure iron changes crystal structure with temperature. Alloying elements shift those transformations, and some lock a structure in place at room temperature.
Nickel stabilises austenite. Chromium, molybdenum and silicon stabilise ferrite. The proportion of the two groups decides the family — and from that follow ductility, magnetism, weldability, low-temperature behaviour and failure mode.
Which is why the family is more informative than the grade: it predicts the limits.
Austenitics
The dominant family, roughly three quarters of stainless produced. Austenitic structure stabilised by at least 8% nickel.
What they offer — Remarkable ductility, allowing bending, deep drawing and severe forming. Excellent weldability. Low-temperature performance with no brittle transition, down to cryogenic service. No magnetism in the annealed condition. A wide temperature range, from cryogenic to several hundred degrees for the stabilised grades.
Their weakness — Chloride stress corrosion cracking. Under the combined effect of tensile stress, chlorides and temperature they crack with no loss of metal, no visible sign, up to failure. It is the most treacherous failure mode in stainless, and it belongs to this family.
Reference grades — 304L, 316L, 316Ti, 321, 317L, 347H, and by extension the super austenitics 904L and 254 SMO.
Ferritics
Ferritic structure, with little or no nickel. Magnetic.
What they offer — A material cost disconnected from the nickel market. And above all practical immunity to chloride stress corrosion cracking, which makes them the answer to a problem austenitics cannot solve: a chlorinated domestic hot water cylinder is exactly that case.
Their weakness — A ductile-to-brittle transition. Below a certain temperature, fracture becomes brittle. They are therefore excluded from cold service, unlike austenitics. Their ductility is also lower, and grain grows in the heat-affected zone on heavy sections.
Reference grades — 430, without molybdenum, for appearance and indoor structure. 444, stabilised and at 2% molybdenum, whose PREN matches a 316L without a gram of nickel.
Duplex
Mixed structure, roughly half austenite and half ferrite. The most recent family in routine industrial use.
What they offer — Roughly double the yield strength of austenitics, allowing reduced wall thickness. Good chloride resistance, from the PREN 25 of the lean grades to the 42.5 of 2507. And the robustness of ferritics against stress corrosion cracking.
Their weakness — A hard limit at around 300 °C in continuous service: beyond that, sigma phase precipitates and embrittles the alloy irreversibly. And welding that requires a qualified procedure, since phase balance is set on cooling.
Reference grades — 2101 and 2304 as lean, 2205 as standard, 2507 as super duplex.
Martensitics
Martensitic structure obtained by quenching, like a tool steel. Magnetic, hardenable by heat treatment.
What they offer — Hardness and wear resistance no other family reaches.
Their weakness — Low corrosion resistance and difficult weldability. They belong to mechanical engineering, cutlery, shafts and wear parts, not to piping.
Reference grades — 410, 420.
Summary
| Criterion | Austenitic | Ferritic | Duplex | Martensitic |
|---|---|---|---|---|
| Nickel | 8% and above | none or trace | 1.5 to 8% | low |
| Magnetic | no | yes | yes | yes |
| Ductility | very good | moderate | moderate | low |
| Weldability | very good | acceptable | procedure required | difficult |
| Cold service | to cryogenic | no | limited | no |
| Stress corrosion cracking | susceptible | immune | highly resistant | — |
| Upper limit | high | moderate | around 300 °C | — |
| Piping use | dominant | targeted | growing | no |
The two families outside the classification
Two groups complete the catalogue without being stainless steels.
Nickel alloys — 600, 625, 800, 825, C-276 — go beyond stainless territory: concentrated acids, wet chlorine, very high temperatures. Their logic is no longer chromium but nickel and molybdenum, and PREN becomes a frequently misleading indicator.
Titanium and copper alloys belong to entirely different chemistry. Titanium is indifferent to chlorides; copper-nickels bring an antifouling effect no stainless possesses. On these families PREN does not apply — the formula is built on the chromium, molybdenum and nitrogen of stainless steels.
Frequently asked questions
- How can I identify a grade's family without documentation?
- A magnet separates austenitics, non-magnetic when annealed, from ferritics, duplex and martensitics, which are not. That is a first sort, not an identification: only the material certificate identifies a grade.
- Can a grade change family?
- No, but its structure can change locally. A cold-worked austenitic becomes slightly magnetic; a poorly welded duplex becomes too ferritic in the weld. In both cases the grade is unchanged — its local properties are not.
- Which family for process piping?
- Austenitic in the large majority of cases. Duplex where chlorides, mechanical loading or stress corrosion cracking justify it. Ferritic in targeted heat exchanger and hot water cases. Martensitic never.