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I-625 Filler Metal for AL-6XN Superaustenitic SS

I-625 Filler Metal for AL-6XN Superaustenitic SS

I-625 Filler Metal for AL-6XN Superaustenitic SS

(OP)
I'm currently working on an failure analysis of some 3" welded pipe nozzle with a WN flange on the outside and the inside of the vessel. Equipment (4 units) was installed ~20 years ago, and failures are becoming chronic. Leaking was due to penetration in the HAZ along one of the circ seams. Environment is hot chlorides (too hot), clearly too much for the AL-6XN to handle. There was a pronounced galvanic effect along the I-625 welds, which were over-welded, and showed indications of runaway heat input and interpass temperature. The factory long seam was also affected, but much less. I tried an (informal) oxalic acid etch test and saw no indications of sensitization (admittedly unlikely). The lab did not etch for sigma phase so that question remains open (and is getting beyond the scope of the investigation, since the materials will be upgraded anyway).

My question is, what is the current thinking on the use of I-625 (9% Mo) filler metal on AL-6XN (and the other superaustenitic grades)? I have found a few mentions about metallurgical issues. Alloy C-22 (~13.5% Mo) has come up more than once as a recommendation. https://www.eng-tips.com/viewthread.cfm?qid=334280

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

RE: I-625 Filler Metal for AL-6XN Superaustenitic SS

I would prefer to see 22, 622, 686, or 59 used as filler.
If you most severe attack is HAZ I believe that sigma (or other) phase is responsible.
With 20yr and minimal seam weld corrosion it sounds like the material selection wasn't off by much.
I presume that you went to one of the alloys listed above for this service?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: I-625 Filler Metal for AL-6XN Superaustenitic SS

(OP)
I wasn't involved in the fabrication but I remember seeing the drums arrive on the client's site in 1998 and being shocked by red rust stains on the shell that were visible from the road. They were built by some back-alley carbon steel shop - just one of the many joys of the EPC business model. nosmiley
This nozzle was basically a victim of process peculiarities that drove the temperature up beyond the limit of the material; otherwise it would have survived much longer.
In this investigation there was also severe corrosion attack on the ID directly opposite the external shell and repad welds 0.300" away, which was my clue that the welding was not under good control. I may ask the lab to go back with a different etch and look for sigma, the first and most likely bad actor that should appear under excessive weld thermal conditions.
In any event I will violate my usual guideline of no silver bullets and recommend an upgrade to one of the C alloys, probably 276. That should fix it forever.

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