Hi Sachin,
Usually from the very first principles there are a few ways to avoid formation of chromium carbide at grain boundaries in austenitic stainless steels during welding :-
- Use "L"grade typically C-0.03 or below
- Use stabilized grades e.g 316L(Mo stabilized), 321(Ti stabilized) or 347(Nb stabilized).
Both routes are acceptable from good welding and materials design perspectives. I am not sure what is the Ferrite Level (FN)asked at the fabrication stage. As per API-582, Minim Ferrite No (FN) for 347S.S is 5 FN while for the rest it could be as low 3FN. If you start off with higher ferrite no during fabrication for 321 or 347 clad, there may be chances of forming sigma phase after prolonged exposure.
You may forward your query to UOP on the rationale behind choosing 304L clad for FCC columns.
One Note Here:-NACE RP-170-2004-Talks about Polythionic Acid SCC in Refinery Equipments during maintenence shutdowns, which includes the FCCU. Resistance to above SCC can be enhanced by certain heat treatments such as stabilising anneal during fabrication, which is more required for stablized grades e.g 321 S.S,347S.S
Appreciate your feedback.
Thanks.
Pradip Goswami,P.Eng
Welding and Meatllurgical Engr/Specialist
Ontario Power Generation Inc.
Date: Sat, 28 Nov 2009 15:47:25 +0000
To: materials-welding@googlegroups.com
Subject: [MW:3758] Material selection
From: sachin.sankhe@rediffmail.com
Dear Freinds,
I have query on below subject
We have got main colum section of FCC unit wherein base metal is LAS cladded with SS304L.This requirement in specified by UOP Process Licensor
However design temp of Main column is around 55o deg.C
Under this circumstances,I feel SS304L is not suitable at this temp since it will result in formation of chromium carbide at grain boundries.
I feel UOP shall have specifed SS347.
Kindly share your knowledeg
Regards
Sachin
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