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Key takeaways
- Duplex steels have a mixed ferrite-austenite structure, which gives about twice the yield strength of 316 stainless and much better resistance to chloride stress corrosion cracking.
- 22Cr duplex and 25Cr super duplex are separated by their pitting resistance (PREN); super duplex is used for seawater and the most aggressive services.
- Welding must keep the ferrite-austenite balance and avoid sigma phase, so heat input, interpass temperature and ferrite content are controlled and tested.
What makes duplex different
A duplex stainless steel has roughly equal amounts of ferrite and austenite in its microstructure. That gives:
- high strength, often about twice the yield strength of 316L, so walls can be thinner,
- good resistance to chloride stress corrosion cracking, which limits austenitic grades in warm, salty conditions,
- good pitting and crevice corrosion resistance.
22Cr vs 25Cr
22Cr duplex (UNS S31803 / S32205) is used for process hydrocarbon lines, produced water and many utility services.
25Cr super duplex (UNS S32750, S32760) has more chromium, molybdenum and nitrogen. It is used for seawater, firewater and the most corrosive process conditions.
The difference is expressed by the pitting resistance equivalent number:
PREN = %Cr + 3.3 × %Mo + 16 × %N
Super duplex grades are typically specified with a PREN of at least 40.
Temperature limits
Duplex steels lose toughness if held at high temperature, so they are normally limited to around 250 °C for pressure parts. At low temperature, impact testing is needed for cold service, typically down to around −46 °C.
Welding is where it goes wrong
The balance between ferrite and austenite in the weld and heat-affected zone depends on heat input and cooling rate.
- Too fast a cooling rate gives too much ferrite, which lowers toughness and corrosion resistance.
- Too much heat, or too many passes at high interpass temperature, can form sigma phase, which makes the steel brittle and less corrosion resistant.
Typical controls in a welding procedure are:
- a heat input range,
- a maximum interpass temperature (often 100 to 150 °C),
- ferrite measurement, commonly accepting roughly 30 to 70% in weld metal and HAZ, depending on the specification,
- corrosion testing (ASTM G48) and impact testing of the procedure qualification.
Hydrogen and cathodic protection
Duplex steels under cathodic protection subsea can absorb hydrogen and crack under high local stress. DNV-RP-F112 gives design rules against this hydrogen-induced stress cracking (HISC). Topside piping is not normally affected, but subsea spools and flanges are.
Code reference
ASTM A790 / A928 Duplex stainless steel pipe (seamless and welded).
NORSOK M-601 Welding and inspection of piping, including duplex requirements.
ASTM G48 Pitting and crevice corrosion tests.
DNV-RP-F112 Duplex stainless steel: design against hydrogen-induced stress cracking.
What to take back to your desk
When reviewing a duplex welding procedure, check four things: heat input range, interpass temperature, ferrite results and G48 results. If any is missing, the procedure isn’t ready.
Educational content. Always design to your project specification.
