Duplex 2205 spool: interpass失控 and low toughness
A fabrication shop is producing 12.7 mm wall 2205 duplex stainless steel spools to EN 10216-5. The WPS specifies GTAW root with ER2209 and FCAW fill with E2209T1-4, no preheat, maximum interpass 150 °C. The shop supervisor, worried about cold cracking after a previous carbon steel job, orders 120 °C preheat and keeps it on all day. During filling, the interpass temperature is measured at 285 °C on the outside of the pipe because the preheat is left on and the spool is small. After completion, the ferrite number is 28 FN (procedure requires 35–65 FN), Charpy V-notch weld metal at -40 °C gives 16, 18, 20 J (required 27 J average), and a metallographic replica shows grain-boundary precipitates. Transverse tensile and bend tests pass. The client asks whether the spool can be accepted, and the shop proposes a post-weld heat treatment at 600 °C for 2 hours.
1. Explain the likely cause of the low ferrite number and low Charpy toughness, and evaluate the proposed 600 °C post-weld heat treatment.
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The low ferrite and low toughness point to excessive interpass temperature, not to a need for preheat. Duplex solidifies primarily as ferrite and transforms partly to austenite on cooling. At 285 °C the transformation is driven too far, giving 28 FN, and chromium-rich nitrides or sigma phase can form at grain boundaries, embrittling the weld. The preheat was unnecessary: 2205 does not require preheat to avoid hydrogen cracking, and 120 °C preheat plus a small spool removed the heat sink, so interpass could not be controlled. The proposed 600 °C PWHT is the wrong remedy. For 2205, 600 °C lies in the sigma-forming range and would further reduce toughness; it is not a solution anneal. A true solution anneal at about 1020–1100 °C followed by water quenching could restore the structure, but it is impractical for a completed spool and risks distortion. Since the Charpy requirement is not met and the ferrite is outside the WPS range, the spool should be rejected. The passing tensile and bend tests only show soundness and strength, not toughness. The correct action is to cut out the affected weld and re-weld with controlled interpass.
2. State the immediate changes needed in shop practice and the tests you would require on the reworked spool.
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Immediate changes: remove the preheat entirely; measure interpass at the weld bead with a contact thermometer or thermal crayon, not on the outside of the pipe; cap interpass at 150 °C; use a written checklist and a second person to monitor; reduce heat input if the WPS permits; and use an argon backing gas with low oxygen for the root. The existing WPS already specified no preheat and maximum 150 °C, so the event is a deviation from a qualified procedure, not necessarily a need for a new PQR. If the shop wants to change consumable, shielding gas or heat input beyond essential variables, a new PQR is required. For rework, cut out the affected weld, re-weld under the corrected practice, and verify ferrite number on the production weld. Because the original weld failed toughness, I would require a production test coupon or a new PQR test with Charpy testing at -40 °C, plus metallography. If the service requires corrosion resistance, add a corrosion test such as ASTM A923. Do not accept the spool on the basis of tensile and bend tests alone.