Duplex 2205 and Super Duplex 2507 Stainless Steel

Both are duplex 2205. S31803 is the original UNS number with wider limits: chromium 21.0–23.0 %, molybdenum 2.5–3.5 % and nitrogen 0.08–0.20 %. Mills found that heats at the bottom of those ranges gave weaker pitting resistance and less stable phase balance after welding, so S32205 was added with tighter, higher minimums: chromium 22.0–23.0 %, molybdenum 3.0–3.5 % and nitrogen 0.14–0.20 %.

Almost all 2205 made today meets both, and plate and pipe are commonly certified as S31803/S32205. If your drawing says S31803, S32205 material satisfies it. If it says S32205, a heat that only meets S31803 does not. Check the certificate.

Pitting resistance equivalent number is calculated from the chemistry, most often as PREN = %Cr + 3.3 × %Mo + 16 × %N. It is a ranking tool, not a guarantee, but it tracks real seawater behaviour quite well.

316L comes out around 24, 2205 around 35, and 2507 above 40. That last figure is the usual working definition of super duplex. On a project specification you may see a minimum PREN written in, often calculated on the actual heat analysis from the certificate.

2507 has 24–26 % chromium, 6–8 % nickel, 3–5 % molybdenum and 0.24–0.32 % nitrogen. Minimum yield is 550 MPa against 450 MPa for 2205, and pitting and crevice resistance in warm seawater is markedly better. It is used for seawater cooling and firewater lines, desalination high-pressure piping, offshore process piping and pump parts.

For brackish cooling water, process water with moderate chlorides, or pulp and paper service, 2205 is normally enough. Paying for 2507 makes sense when the water is warm, salty and stagnant at times, or when a failure would be expensive to fix.

• Temperature: the ferrite phase embrittles with long exposure around 475 °C, so design codes generally restrict 2205 to roughly 300 °C maximum. Duplex is not a furnace or boiler material.
• Welding: use overalloyed filler such as ER2209 for 2205 and ER2594 for 2507, and control heat input and interpass temperature to keep the ferrite-austenite balance.
• Low temperature: toughness drops at sub-zero temperatures more than with austenitic grades; check impact requirements for cold service.
• It is magnetic. That is normal for duplex and not a sign of wrong material.

Fabricators used to 304 and 316L notice the difference on the first job. Duplex is stronger, so it needs more press force to bend and springs back more; allow for that on rolled shells and bent pipe. Plasma and laser cutting work well, but grind back any heat-affected edge that will see corrosive service.

Cleanliness matters as much as with any stainless. Use dedicated stainless wire brushes and grinding discs, keep carbon steel dust away, and pickle and passivate welds so the heat tint is removed. A weld that looks fine but carries heavy oxide tint will pit early in seawater, which then gets blamed on the material.

For acceptance testing, many oil, gas and desalination projects call for ferrite measurement and an ASTM A923 test on the base metal to show no harmful intermetallic phases. Mention it at enquiry if your specification needs it.

AISI 329 is an older duplex grade with higher carbon and no deliberate nitrogen addition. It is still traded for shafts and pump parts, and we list it with our magnetic stainless grades. For welded piping and plate, modern nitrogen-alloyed 2205 has replaced it because it welds without losing toughness or corrosion resistance.

Specifications to quote: plate, sheet and strip ASTM A240; seamless pipe and welded pipe A790; tube A789; bar A276 or A479; forged flanges and fittings A182 (F51 for S31803, F60 for S32205, F53 for S32750); butt-weld fittings A815. Each lot is backed by a test certificate issued through a Govt. recognised test house. If you are comparing against 316L stainless on cost, send us both sizes and we will quote both.