We are currently at the gathering stage for a code pressure vessel for dilute Sulfuric acid service, fabricated from 2 inch SA516-70, a portion of which is overlayed with Alloy 20 Cb3 (UNS 08020) 5/16" thick, and the remainder with lead. The nature of the vessel makes it very difficult to overlay certain areas after Post weld heat treatment at 1100 deg. F. for 135 miniutes. The fabrication team wants my blessing on overlaying small portions before PWHT and the remainder after, but I am reluctant without more knowledgable opinions. I am concerned about the alloy (20% Cr., 32-26% Ni., 2-3% Mo., 1.5-2% Mn., 3-4% Cu., and 0.025% C) going into sigma phase, carbide precipitation, and subsequent inter-granular or even crevice corrosion.
Can one of you shed some light on this please?
Tim
That might be quite a soup at those temps for that long for Alloy 20.
You need to contact a manufacturer for answers to that one.
I'm guessing they say no.
A whole lotta C available from the substrate, a whole lotta MC carbide possibilities (Cr, Mo, Cb), and a whole lotta segregation of elements.
Keep us posted. I'd like to know too.
Since the overlay will be fully austenitic, I am guessing that you may be all right with the 1100F for 2.25 hours since it isn't that hot, and isn't that long with regards to intermetallic formation. However, I couldn't find any data on PWHT of 320LR or 20Cb3 except for solution annealing. I assume that you are using 320LR filler? It is low carbon so carbide precipitation shouldn't be a problem except with the possibility of carbon migration from the base metal as JS55 mentions. As usual, the only way to know for sure is to test it. A quick and dirty test might be to subject a sample to PWHT then bend it and see if ductility is lost as compared to the as-welded sample. The best way would be to qualify the procedure with both heat treat condions, and then also subject both methods to corrosion testing as a comparison. A good lab can suggest a test that would be applicable to sulfuric acid service. The only one I've done much of is ASTM G48 corrosion testing in ferric chloride.
Greg,
Yeah I looked for PWHT stuff at those temps and couldn't find anything either. Which doesn't surprise me. Alloy 20 is seldom used for dissimilars on materials requiring stress relief. So you end up with solution anneal data more than anything.
You're right, just gonna have to test it and see.
I wanna ask to keep us updated but nobody ever does.
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