2205 is a duplex stainless steel (about 22% chromium, 5–6% nickel, 3% molybdenum, 0.15% nitrogen), and the ideal microstructure is half ferrite and half austenite — one half ensures strength and stress corrosion resistance, the other half ensures toughness and weldability. Welding is about finding the balance between the two.
One-sentence judgement
No preheating, control heat input, interpass temperature not exceeding 150℃, filler metal must match, shielding gas supplemented with nitrogen. If any one of these five is broken, the corrosion resistance of the weld will be noticeably lower than that of the base metal — while the appearance looks completely normal.
Why heat input is the core
Heat input (unit kJ/mm) determines the weld cooling rate, and the cooling rate determines the two-phase ratio:
- Heat input too high (slow cooling): too much ferrite, and chromium nitrides precipitate in the ferrite, corrosion resistance and toughness both decrease;
- Heat input too low (cooling too fast): austenite does not have time to form, the ferrite proportion is too high, toughness deteriorates and cracking is likely;
- The common executable range is 0.5–2.5 kJ/mm (plate thickness, joint type and filler metal diameter all affect it, and it must be set per the WPS);
- Interpass temperature ≤150℃; if it is higher, stop and wait — this is the item most easily overlooked on site, because if it is not recorded it is invisible.
Heat input = voltage × current × 60 ÷ welding speed (mm/min) ÷ 1000. Calculate it and write it into the WPS, rather than "going by feel".
Filler metal and shielding gas
- Use ER2209 filler metal (matched to duplex), 316L filler metal must not be used — using austenitic filler metal produces an all-austenitic weld, and both strength and stress corrosion resistance drop;
- Add nitrogen to the shielding gas: commonly Ar + 2–3% N₂. Nitrogen is an austenite-forming element, and nitrogen is lost at high welding temperatures; if it is not supplemented, ferrite will exceed the limit;
- Backing protection (argon purge) must likewise add nitrogen or at least ensure sufficient protection, otherwise the root oxidizes and turns black.
How to verify ferrite content
Measure in the weld and heat-affected zone with a ferrite meter (Feritscope); the common acceptance range is 35–65% (different projects/standards vary, set per the technical specification). Note three points:
- The measurement points must include weld centre, fusion line and heat-affected zone; measuring only the weld centre will miss the most dangerous location;
- The ferrite meter reading is an indirect magnetic measurement and has its own error, so average multiple readings at the same location;
- When there is a dispute, recheck with metallography (rate per the standard chart), which is more convincing than the instrument.
Common pitfalls
- Preheating: duplex steel does not need preheating; preheating is equivalent to increasing heat input and will only make ferrite exceed the limit;
- Using carbon steel fixtures and carbon steel wire brushes for cleaning — iron contamination will make the weld rust first;
- No pickling and passivation after welding: the oxidation colour on duplex steel welds must be removed, otherwise the passive film cannot grow back;
- Multi-layer multi-pass welding without interpass temperature control: it gets hotter as welding proceeds, and the heat input of the last few passes accumulates beyond the limit;
- Treating "post-weld solution annealing" as a routine process: overall solution annealing is only used for extremely demanding applications, and normally the problem is solved by controlling heat input.
For a duplex steel project, tell us the plate thickness, joint type and applicable standard. We can supply matching filler metal and test pieces per your WPS, and provide material certificates with each batch — the cost of duplex steel is not in the steel price, but in a single rework.
