Duplex 2205 root ferrite and pitting after hydrotest
A chemical plant fabricates 168.3 mm × 7.11 mm UNS S32205 duplex stainless steel pipe spools. The root is GTAW with ER2209 filler, argon shielding and an argon purge; fill passes are GMAW-P with Ar/2%CO2. The qualified procedure limits heat input to 0.4–0.6 kJ/mm and interpass temperature to 150°C. After hydrotest, pitting is found in the root weld metal and adjacent HAZ. Ferrite numbers in the root are 70–80, compared with 35–60 in the fill. The purge oxygen level was 100 ppm. No nitrogen was added to the purge or shielding gas. The client requires a CPT of at least 25°C, but the failed area gives 20°C. Production includes 40 similar spools, some already installed.
1. Explain why the root has 70–80 FN and pitting while the fill has 35–60 FN. What roles do purge gas, nitrogen, heat input and interpass play?
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Nitrogen is a strong austenite stabiliser in duplex stainless steel. An argon-only purge removes nitrogen from the root weld pool, and the low root heat input (0.4–0.6 kJ/mm) gives fast cooling, both of which push the ferrite–austenite balance toward ferrite. A root ferrite number of 70–80 is well above the target range and indicates insufficient austenite. The fill passes have more heat input and slower cooling, and may also pick up some nitrogen from the base metal and filler, so their ferrite number remains in the 35–60 range.
A ferrite-rich root with nitrogen loss has lower toughness and lower pitting resistance. The purge oxygen level of 100 ppm is acceptable, so oxygen contamination is not the main cause; the absence of nitrogen is. The interpass limit of 150°C is appropriate and is not the cause. The CPT of 20°C versus the required 25°C confirms that the root microstructure is not corrosion-resistant enough.
2. Recommend corrective actions for the procedure and for the already-fabricated spools.
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For the procedure, add nitrogen to the root purge, typically 2–5% N2 in argon, and consider a small nitrogen addition to the shielding gas where qualified. Increase root heat input within the duplex range, for example toward 0.8–1.2 kJ/mm, while keeping interpass at or below 150°C. Confirm the filler composition and ferrite target, and qualify the revised WPS with ferrite measurements in the root and fill. Do not simply increase heat input without checking toughness and sigma formation.
For the 40 spools, inspect all accessible roots. Spools with confirmed pitting or ferrite outside the qualified range should be quarantined. Local grinding and repair welding with the nitrogen-containing procedure may be acceptable if the wall thickness allows, but full replacement may be required for installed spools where the root is inaccessible. The repair procedure must be qualified and the repaired areas re-tested for ferrite and corrosion resistance.
3. Design the re-qualification and production control plan, including the tests you would use to demonstrate CPT ≥25°C.
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Qualify a test coupon representing the production joint, position and thickness. Use the revised purge and shielding gases, record heat input and interpass temperature, and measure ferrite in the root, fill and cap by a qualified method. Perform metallography to confirm the austenite–ferrite balance and check for sigma or nitrides. Perform corrosion testing by a recognised critical pitting temperature method, such as ASTM G150 or an agreed equivalent, and require CPT ≥25°C in the root and HAZ.
For production, monitor purge oxygen and nitrogen, gas flow, heat input and interpass temperature for each spool. Check ferrite on the root of the first spool and on a sampling basis thereafter. Retain the test coupons and records. If any spool falls outside the qualified window, quarantine it and evaluate by ferrite measurement and corrosion testing before acceptance.