Problems welding 2205 tubes to carbon TS with inconel625 6

[Texgrande]

We are having problems welding duplex 2205 tubes to a carbon steel tube sheet using inconel 625 rods with the GTAW process. We keep getting small indications after PT testing and after grinding out the 1ndications and repair welding the PT test shows no indications. Upon making the 2nd pass of the strength welds the PT tests are revealing more indications some of which go down to the 1st pass and (1) had what appeared to be a crack across the face of the weld. We are at a loss to determine the problem. Has anyone had this problem before or any suggestions.

 

[boilerone]

I am not a welding guy at all but Inconel 625 (58% Nickel Min.) seems not a good candidate for welding 2205 (6.5% Nickel max.) to carbon. Both of the two electrode selecting guides that I found point to use AWS E309MoL (13% Nickel) for 2205 to carbon GTAW welding. Here is the link to a very detailed 2205 welding guide:

http://www.outokumpu.com/35058.epibrw

 

[metengr]

You need to use a 309Mo filler metal as mentioned above for dissimilar metal welds with duplex to carbon steel. The problem with using a Ni-base filler is that the duplex structure in the weld deposit is critical. In this case, you have so much Ni that the primary weld solidification mode is austenitic and results in hot cracks.

 

[MikeMet]

Inconel 625 is not the best choice for dissimilar metal welds due to the Nb additions. The Nb in the filler metal can form intermetallic compounds, such has hard nitrides, that can cause cracking in the weld as it cools and degrade the corrosion resistance Better choices would be ERNiCrMo-10 (Inco 622, Hast C22), ERNiCrMo-14 (Inco 686) or ER309LMo. Tube to tube sheet welds are subject to very high cooling rates due to the heat sink of the thick tube sheet. You should also consider adding 1-3% nitrogen to the shielding gas. Nitrogen is an austenite former and help ensure you get a proper austenite/ferrite balance in the weld nugget.

 

[Texgrande]

Just wanted to thank you guys for the quick response and valuable information. It appears that the choice of filler metal is a bad selection, however that is what the customer is requesting so we are attemting to make the best of a bad situation.

 

[jmiles]

Its been my experience that alot of customers can toss around terms like inconel and duplex cause they ahve ehard somethign or other about it, not always because they know alot about it, in this case id suggest its worth challenging the customer ina polite way about why they are insisting on using inconel for the filler material as you would prefer to use another amterial for better welding properties.

You would be surprised how often someone telsl you they want one specific thing because they cant think of other options but they are really happy with whatever work.

 

[fegenbush]

If the customer is insisting on Inconel 625 filler material, it may be for corrosion resistance reasons. If you have an appropriate welding procedure, it may be most economical to overlay the 625 onto the 309Mo. This, of course, depends greatly on the diameter of the tubes and thickness both of the tube and tubesheet.

Fegenbush

 

[stanweld]

Texgrande,
How are you cleaning? Are you welding after pressure rolling the tubes? Cleanliness is mandatory when welding making the weld described.

I'm not sure why your customer is requiring 625 alloy for the involved weld. If the tube sheet/channel side only requires carbon steel, the shell side should be the corrosive side requiring the 2205 alloy, assuming U-Tube design. If that is the case ER309Mo would be recommended. Mikemet's recommendationas would be superior to 625 if a nickel base, up-alloyed, weld filler metal is required.

 

[Texgrande]

Just wanted to thank all you guys for the quick and concise responses. I will be passing this info on to the powers that be in hopes that they will take a better look at the situation next time. Oh, they did manage to get the tube ends all welded and PT tests revealed no rejectable indications but I agree with you guys that 309 would be the answer to problems with these welds. They did do a light roll prior to the first pass on the welding so it is possible that the tube ends were not cleaned as well as they should have been. However I am in agreement with you guys that due to the dissimilar metals that 309 would be the choice for filler material in this application.Lets hope the hydro test goes ok. Thanks again, Tex Grande