Material group · ASME B16.5 / EN 1092-1
Stainless Steel Flanges — 304/316L, Duplex 2205, 2507
Stainless steel flanges resist corrosion in chemical, food, pharmaceutical and marine service. The grade choice is driven by the chloride level, temperature and the risk of stress-corrosion cracking.
304/304L is the general-purpose austenitic grade for mild corrosives, water and food service. 316/316L adds molybdenum for improved chloride resistance and is the standard for chemical processing, pulp and paper, and coastal atmospheres. 321 and 347 are stabilised grades for elevated-temperature service where intergranular corrosion is a risk.
Duplex 2205 has roughly twice the yield strength of austenitic grades and much better chloride stress-corrosion resistance, which makes it the economical choice for marine, offshore and desalination piping. Super duplex 2507 is used where 2205 is not sufficient, and 904L for severe sulphuric and phosphoric acid service. All stainless flanges are machined from forged rings, pickled and passivated, and supplied with PMI verification and 3.1 certificates.
Grades we machine and forge
| Grade | Specification | Service temperature | Typical use |
|---|---|---|---|
| 304 / 304L | ASTM A182 F304/F304L | −196 to 400 °C | Water, food, mild chemicals, general industry |
| 316 / 316L | ASTM A182 F316/F316L | −196 to 400 °C | Chemical processing, marine atmosphere, pulp and paper |
| 321 | ASTM A182 F321 | −196 to 600 °C | Elevated-temperature service, heat exchangers |
| 347 | ASTM A182 F347 | −196 to 600 °C | High-temperature and nuclear-adjacent service |
| Duplex 2205 | ASTM A182 F51 | −50 to 300 °C | Offshore, desalination, chloride-bearing process lines |
| Super duplex 2507 | ASTM A182 F53 | −50 to 300 °C | Severe marine and sour service |
Selection notes
- Specify L grades (304L/316L) for welded assemblies to avoid sensitisation at the weld.
- Duplex grades are not suitable for service above about 300 °C because of sigma-phase embrittlement.
- All stainless parts are pickled and passivated; PMI is performed on 100% of heats before shipment.
Welding and fabrication
- Austenitic grades weld well but have higher thermal expansion than carbon steel; allow for distortion and use a matching filler.
- Use 308L/316L consumables for 304L/316L joints, and control heat input on duplex to preserve the ferrite–austenite balance.
- Duplex welds may need a nitrogen purge and a controlled cooling rate; do not post-weld anneal unless the procedure requires it.
- Pickling and passivation after welding restores the passive layer and removes heat tint, which is where pitting starts.
How to order
- Specify the grade and the product form (forged ring, bar or plate) if the specification requires it.
- State whether the parts will be welded, so the L grade and consumables can be confirmed.
- For marine or offshore orders, state the class approval and any additional impact-testing requirement.
- Confirm the surface condition: machined, pickled and passivated, or polished.
Common specification mistakes
- Ordering a grade without stating the certificate level — 3.1 is the baseline, not an upgrade.
- Using 304/316 instead of the L grades on welded assemblies, then finding sensitisation at the weld.
- Treating a PN rating and an ASME class as interchangeable; the pressure may match and the bolt pattern will not.
- Specifying a material for sour service without checking the NACE hardness limit and heat-treatment condition.