Stainless steel families: austenitic, ferritic, martensitic, duplex and PH
Every stainless steel has at least 10.5 % chromium. What separates the families is the structure of the metal, which defines whether it is magnetic, whether it hardens by heat treatment and where it resists corrosion.
Comparison of the families
| Family | Structure | Magnetic | Hardens by | Examples | Typical use |
|---|---|---|---|---|---|
| Austenitic | Austenite | No (slightly after cold work) | Cold work only | 304L, 316L, 321, 904L, 254 SMO | Chemicals, food, pharmaceuticals, cryogenics |
| Ferritic | Ferrite | Yes | Does not harden | 430, 409, 446 | Exhausts, high-temperature oxidation |
| Martensitic | Martensite | Yes | Quenching and tempering | 410, 420, 431, F6NM | Shafts, valves, pumps, hydraulics |
| Duplex and super duplex | Austenite + ferrite (~50/50) | Yes | Cold work only | 2205 / F60, F51, 2507 / F53, Zeron 100 / F55 | Seawater, oil and gas, chlorides |
| Precipitation hardening (PH) | Martensite or semi-austenitic | Yes | Aging | 17-4PH, 15-5PH, 17-7PH | Shafts, aerospace, high-strength parts |
What changes in the choice
- "L" in the name (304L, 316L): carbon up to 0.03 %, avoids intergranular corrosion after welding.
- Molybdenum (316L, 904L, duplex): increases pitting resistance in chlorides. Compare by PREN.
- Duplex: twice the yield strength of 316L and resistance to stress corrosion cracking; requires heat input control in welding.
- Martensitic and PH: when the part needs hardness and strength, with moderate corrosion.
- Aggressive medium: see the steel by corrosive medium table.
Frequently asked questions
Is stainless that attracts a magnet fake?
No. Ferritics, martensitics, duplex and PH grades are magnetic by nature. Even 304 and 316 become slightly magnetic after bending or machining. To confirm the alloy, the right way is PMI or chemical analysis.
What is the difference between duplex and super duplex?
Super duplex (2507, F53, F55) has more chromium, molybdenum and nitrogen than duplex 2205, with PREN above 40. It resists pitting and crevice corrosion better in seawater and chlorides.
Stainless as bar, plate, pipe and forgingAustenitic, martensitic, duplex, super duplex and PH in stock.
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