2205 is a duplex stainless steel (about 22% chromium, 5–6% nickel, 3% molybdenum, 0.15% nitrogen). 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 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): excessive ferrite, and chromium nitrides precipitate in the ferrite, corrosion resistance and toughness both decrease;
- Heat input too low (cooling too fast): austenite has no time to form, ferrite proportion is too high, toughness deteriorates, easy to crack;
- The common executable range is 0.5–2.5 kJ/mm (affected by plate thickness, joint type, filler metal diameter, must be set per WPS);
- Interpass temperature ≤150℃, if higher, stop and wait — this is the 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, not "by feel".
Filler Metal and Shielding Gas
- Use ER2209 filler metal (matches duplex), do not use 316L filler metal — using austenitic filler metal will give a fully austenitic weld, strength and stress corrosion resistance both drop;
- Add nitrogen to shielding gas: commonly Ar + 2–3% N₂. Nitrogen is an austenite-forming element, at welding high temperature nitrogen is lost, if not supplemented ferrite will exceed the limit;
- Backing protection (argon purge) also needs nitrogen or at least sufficient protection, otherwise the root oxidizes and turns black.
How to Verify Ferrite Content
Use a ferrite meter (Feritscope) to measure at the weld and heat-affected zone, the common acceptance range is 35–65% (different projects/standards will vary, set per technical conditions). Note three points:
- Measurement points must include weld center, fusion line, heat-affected zone three locations, measuring only the weld center will miss the most dangerous position;
- Ferrite meter readings are indirect magnetic measurements, inherently have errors, take multiple readings at the same location and average;
- When disputed, use metallographic method for verification (rate per standard atlas), more convincing than the instrument.
Common Pitfalls
- Preheating: duplex steel does not need preheating, preheating equals increasing heat input, only makes ferrite exceed the limit;
- Using carbon steel fixtures, carbon steel wire brushes for cleaning — iron contamination will make the weld rust first;
- No pickling and passivation after welding: the oxidation color of duplex steel welds must be removed, otherwise the passive film cannot grow back;
- Multi-layer multi-pass welding without interpass temperature control, the more you weld the hotter it gets, the last few passes accumulate excessive heat input;
- Treating "post-weld solution treatment" as a routine process: overall solution treatment is only used for extremely high requirements, usually solved by controlling heat input.
For a duplex steel project, tell us the plate thickness, joint type and applicable standard. We can provide matching filler metal and test pieces per your WPS, and supply material certificates with each batch — the cost of duplex steel is not in the steel price, it is in one rework.
