Duplex and super duplex: strength that costs you cycle time
Duplex stainless steels are roughly half ferrite and half austenite. That structure buys about twice the yield strength of 316 with markedly better chloride pitting resistance, at a fraction of what a nickel alloy costs.
The shop pays for it. Two phases with different hardness and different ductility mean the cutting edge is continually crossing a boundary, and each crossing is a small impact.
The grades that matter
| Grade | UNS | Cr / Ni / Mo | PREN | Yield | Character |
|---|---|---|---|---|---|
| 2304 | S32304 | 23 / 4 / 0.3 | ~26 | ~400 MPa | Lean duplex, 316 replacement |
| 2205 | S32205 | 22 / 5 / 3 | ~35 | ~450 MPa | The workhorse |
| 2507 | S32750 | 25 / 7 / 4 | ~42 | ~550 MPa | Super duplex, seawater |
| Zeron 100 | S32760 | 25 / 7 / 3.5 + W, Cu | ~41 | ~550 MPa | Super duplex, oilfield |
PREN — pitting resistance equivalent number, calculated as %Cr + 3.3 × %Mo + 16 × %N — is the useful shorthand. Above about 40 an alloy is generally considered suitable for seawater at ambient temperature. 316 sits around 25.
Why it is hard on tools
Two phases, alternating loads. Ferrite and austenite machine differently. The edge experiences a rapidly varying cutting force, which is fatigue loading on the carbide. Edge chipping rather than smooth flank wear is the typical failure.
Work hardening in the austenite. The austenitic half hardens ahead of the tool much as 316 does, with the same consequences — work hardening in superalloys applies directly.
High cutting forces. Yield strength roughly double 316 means proportionally higher forces, deflection and power draw.
Poor thermal conductivity. Around 15 W/m·K. Better than nickel alloys, far worse than carbon steel.
Expect roughly 50 to 60 % of the cutting speed you would use on 316, and around 20 to 30 % for super duplex.
Starting parameters
Coated carbide, rigid setup, generous coolant.
| Operation | 2205 | 2507 |
|---|---|---|
| Turning, roughing | 90–130 m/min | 60–90 m/min |
| Turning, finishing | 120–170 m/min | 80–120 m/min |
| Milling, roughing | 80–120 m/min | 55–85 m/min |
| Milling, finishing | 110–150 m/min | 75–110 m/min |
| Drilling | 20–35 m/min | 15–25 m/min |
Feeds: 0.15–0.35 mm/rev turning, 0.08–0.20 mm/tooth milling. As with any work-hardening alloy, keep the feed up and never dwell.
Tooling: tough PVD-coated carbide grades intended for stainless, positive rake, reinforced edge. The reinforcement matters more here than on austenitic stainless because the failure mode is chipping, not wear.
The heat you must not add
This is the constraint unique to duplex, and it belongs on the drawing.
Duplex derives its properties from a balanced phase ratio near 50/50. Hold it in the 650–950 °C range and brittle intermetallic phases — sigma, chi — precipitate. In super duplex, sigma can form in minutes. The result is a severe loss of toughness and corrosion resistance that no amount of dimensional inspection will catch.
Practically, this means:
- Welding must follow a qualified procedure with controlled heat input and interpass temperature. Not a job for improvisation.
- Do not stress relieve duplex the way you would carbon steel. A conventional stress relief cycle sits squarely in the damaging range. If distortion is a concern, control it by machining sequence instead — see distortion and stress relief.
- Do not let grinding or heavy machining scorch the surface. Visible blueing on a super duplex part is a question, not a cosmetic issue.
If the part is welded, ferrite content measurement is a reasonable acceptance requirement to write into the specification.
When duplex is the smart choice
Duplex is frequently the right answer where buyers reflexively reach for a nickel alloy.
For seawater, brine and chloride service below about 300 °C, 2507 delivers pitting resistance comparable to Inconel 625 with roughly twice the yield strength, at materially lower cost. Offshore hardware, desalination, seawater pumps and valves, marine shafting.
For sour service oil and gas, Zeron 100 and 2507 are standard where 316 would fail.
Where duplex does not work: above roughly 300 °C, because the prolonged exposure embrittles it; in reducing acids, which is nickel-alloy territory — see Hastelloy C-276; and in cryogenic service, because the ferrite phase goes brittle.
On the drawing
State the UNS number, not the trade name. Call out the standard — ASTM A479 for bar, A182 for forged fittings. If the part is welded, name the qualified procedure and say whether ferrite content will be measured. And say the service environment, so a supplier can tell you if you are paying for super duplex where 2205 would do.
Background: duplex stainless steel.
Send a duplex drawing and we will check the grade against the service before anyone quotes it.