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 ensures toughness and weldability. Welding is about finding the balance between the two.
One-sentence judgment
No preheating, control heat input, interpass temperature not exceeding 150℃, filler metal must match, shielding gas with nitrogen addition. If any one of these five is violated, the corrosion resistance of the weld will be noticeably lower than the base metal — while the appearance looks perfectly normal.
Why heat input is the core
Heat input (unit kJ/mm) determines weld cooling rate, and cooling rate determines the two-phase ratio:
- Heat input too high (slow cooling): excessive ferrite, and chromium nitrides precipitate in ferrite, corrosion resistance and toughness both decrease;
- Heat input too low (too fast cooling): austenite has no time to form, ferrite fraction too high, toughness deteriorates, easy cracking;
- Common executable range is 0.5–2.5 kJ/mm (affected by plate thickness, joint type, filler diameter — must be set per WPS);
- Interpass temperature ≤150℃, if higher, stop and wait — this is the most easily overlooked on site because if 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 (matches duplex), do not use 316L filler — austenitic filler gives all-austenitic weld, strength and stress corrosion resistance both drop;
- Add nitrogen to shielding gas: commonly Ar + 2–3% N₂. Nitrogen is an austenite former; at high welding temperature nitrogen is lost, if not replenished ferrite will exceed spec;
- Back purging (argon fill) also needs nitrogen addition or at least sufficient protection, otherwise root oxidizes and turns black.
How to verify ferrite content
Use a ferrite meter (Feritscope) to measure at weld and heat-affected zone, common acceptance range is 35–65% (varies by project/standard, set per technical specification). Note three points:
- Measurement points must include weld center, fusion line, heat-affected zone — measuring only weld center misses the most dangerous location;
- Ferrite meter reading is an indirect magnetic method with inherent error, take multiple readings at same location and average;
- When disputed, use metallographic method for verification (rate per standard chart), more convincing than instrument.
Common pitfalls
- Preheating: duplex steel does not need preheating; preheating equals increasing heat input, only causes ferrite to exceed spec;
- Using carbon steel fixtures, carbon steel wire brushes for cleaning — iron contamination makes weld rust first;
- No pickling and passivation after welding: oxidation color on duplex weld must be removed, otherwise passive film cannot regrow;
- Multi-layer multi-pass welding without interpass temperature control, gets hotter and hotter, last passes accumulate excessive heat input;
- Treating "post-weld solution annealing" as routine: overall solution treatment only for extremely demanding occasions, usually solved by controlling heat input.
For duplex steel projects, tell us plate thickness, joint type and applicable standard. We can provide matching filler and test pieces per your WPS, and supply material certificates with each batch — the cost of duplex steel is not in the steel price, but in one rework.
