How to weld repair 304H flange that has SCC due to polythionic acid

[BMMS]

I have a large diameter SS304H flange that has several cracks in the process exposed face. Flange has been in 1300F service for 17 years. Cracking seems to have developed after shutdown and exposure to atmosphere. Suspect polythionic acid attack. Has anyone developed a recommended cleaning, excavation and weld repair procedure?

 

[Jmoore1]

Is there anything else in the line? several other corrosion/cracking mechanisms could be present in stainless steel over 1000 F

 

[BMMS]

Process flow is hot gas (CO, SOx, NOx,O2 and fluidized ceramic catalyst).
Thanks for you question and reply

 

[EdStainless]

The way to fix SCC is replacement.
You can look at polishing spots and looking at the microstructure.
It is probably sensitized (chrome carbides in the grain boundaries. This makes it much more susceptible to SCC (chlorides or pta).

If the micro isn't too bad then maybe clean, re-anneal, inspect, grind out cracks, weld repair, inspect, chase new cracks, and so on.

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Plymouth Tube

 

[Jmoore1]

Is this off of a catalytic cracker? this sounds like the burnoff from a regenerator...I would not recommend in kind replacement or repair...you need to refractory line this nozzle or replace witha superalloy (preferrable Hastelloy or Inconel) to prevent corrosion from the flue gas, as well as the acids that form, other option is to weld overlay the nozzle with a hastelloy or inconel welding electrode..... is the failure at the weld?

 

[EdStainless]

I should have been more clear.
It is foolish to try a repair.
Replace it with the correct metallurgy.

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Plymouth Tube

 

[BMMS]

Thank you both for this insightful information and recommendations. Yes this is a hot wall design location in a FCCU regen. cat line. The cold wall design and superalloy approach will both be looked at closely. I appreciate your responses.

 

[Guest102023]

EdStainless is correct; you need to replace it.

You can repair, but will still have suspect metallurgy in remaining, uncracked material, as well as a dog's breakfast of different metallurgies and PASCC susceptibilities.

 

[blacksmith37]

As has been pointed out; it must be replaced if you have polythionic SCC. However, because 304 gave good service otherwise; probably 321 or 347 should be satisfactory.

 

[Jmoore1]

I doubt a 300 series SS is a good as the acids in flue gas attack the austenitic stainless as well, as the cheapest option I would chose refractory lined CS or SS.....other options are, weld overlayed SS with hastelloy or inconel, or straight hastelloy or inconel.

As hastelloy or inconcel are exorbitantly expensive, I would chose the refractory line nozzle if your nozzle is >= 12". (and you can probably even go to a carbon steel nozzle)