2205 is a duplex stainless steel (about 22% chromium, 5–6% nickel, 3% molybdenum, 0.15% nitrogen), with an ideal microstructure of roughly half ferrite and half austenite — one half provides strength and stress corrosion resistance, the other provides toughness and weldability. Welding is about finding the balance between the two.
Quick Verdict
No preheat, control heat input, interpass temperature not exceeding 150℃, matching filler metal, nitrogen-added shielding gas. 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 Central
Heat input (in 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 (too-fast cooling): austenite doesn't have time to form, ferrite fraction too high, toughness deteriorates, cracking prone;
- The common executable range is 0.5–2.5 kJ/mm (affected by plate thickness, joint type, and filler diameter; must be set per WPS);
- Interpass temperature ≤150℃; if higher, stop and wait — this is the most easily overlooked item on site because it's invisible if not recorded.
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 (duplex matching); do not use 316L filler — austenitic filler gives an all-austenitic weld, and both strength and stress corrosion resistance drop;
- Add nitrogen to shielding gas: commonly Ar + 2–3% N₂. Nitrogen is an austenite former and is lost at welding temperatures; without compensation, ferrite will exceed limits;
- Backing protection (argon purge) must also include nitrogen or at least ensure adequate protection, otherwise the root oxidizes and turns black.
How to Verify Ferrite Content
Measure with a ferrite meter (Feritscope) on the weld and heat-affected zone; the 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, and heat-affected zone — measuring only the weld center misses the most critical location;
- Ferrite meter readings are indirect magnetic measurements with inherent error; take multiple readings at the same location and average;
- When disputed, verify by metallography (rate per standard charts) — more convincing than the instrument.
Common Pitfalls
- Preheat: duplex steel does not require preheat; preheating equals increasing heat input and only causes ferrite to exceed limits;
- Using carbon steel fixtures and carbon steel wire brushes for cleaning — iron contamination makes the weld rust first;
- No post-weld pickling and passivation: oxidation color on duplex welds must be removed, otherwise the passive film cannot re-form;
- Multi-layer multi-pass welding without interpass temperature control — it gets hotter as you weld, and the last few passes accumulate excessive heat input;
- Treating "post-weld solution annealing" as a routine step: full solution annealing is only for extremely demanding applications; usually control heat input to solve it.
For your duplex steel project, tell us the plate thickness, joint type, and governing standard. We can provide matching filler and test pieces per your WPS, and supply material certificates with each batch — the cost of duplex steel isn't in the steel price, it's in a single rework.
