complications with welding copper nickel

By dadeacon Date 08-25-2003 04:36

Hello all. I'm a welder of 26 yrs., and never have I had such a hard time at success as with the welding of copper nickel. I'm building a finned tube, made of copper nickel to a stainless steel block (300 series). We use inconel as the filler material. The (cn)tube is thin walled, approx. .040, and the block is approx. 1 1/2". It seems the problem is the different melting temperatures of each material. The stainless melts faster, and the copper nickel much slower; and of course the wire a bit slower.
I use a technique where I apply a "cold" pass of inconel onto the stainless, then another pass on top. After allowing to cool, I fuse the top pass to the copper nickel tube. In "every" case/job I get both indications (penatrent testing) and/or leaks in the hydro test. After several attempts - back and forth with the tests- I can usually seal it off; but by then I have a much larger weld bead than what is called for (.12 fillet). Also, it doesn't look so nice. Is it the heat, gas, (argon for both shielding and back purge), filler material, cleanliness or my technique???????
I'd bet there's a guru, well expert out there that has the solution. I hope. Please help.

By Niekie3 (***)

Date 08-25-2003 20:28

Hi dadeacon

Maybe we will be able to help more if you told us what types of defects you are getting. Porosity, pinholes or lack of fusion? Also, where do the defects lie? In the weld metal, on the tubesheet side, or on the tube side? Also, which process are you using? GTAW? Which grade of Inconel wire are you using?

Regards
Niekie Jooste
Fabristruct Solutions

By dadeacon Date 08-29-2003 04:01

Thanks for responding. The type of indications are small cracter cracks, at my stops. I get leaks at the start/stops when the part is hydro-tested. The indications are in the weld itself (start/stops) and at the toes of the weld. Where the fillet is joined to the stainless, and at the top of the fillet where the inconel is fused to the copper nickel tube. I am using GTAW. I use RNkk625, and also use RN82. Thanks.

By ssbn727 (*****)

Date 08-29-2003 21:07

Hello Dadeacon!!!
Cleanliness is JOB ONE!!! If your components are'nt clean or they have a thin film of residue on them then, that's where your problem begins!!! However, it does'nt end there...
You're saying that you're experiencing crater cracks? If this is the case then, you should start to lower your current when you're just short of the end of the weld but, not too much because, you want to deposit metal at the end, then come back into the weld from the end or the crater of the weld about 1/4" to 3/8", not depositing metal, and at the same time, lowering your current (Foot Pedal) until the arc extinguishes itself, and Keep the torch there until you're finished with the post flow of shielding gas!!!

DO NOT LIFT OR REMOVE THE TORCH BEFORE YOUR POST FLOW CYCLE OF GAS SHIELDING IS FINISHED!!! How do you have your shielding gas and backpurge configured?? Are you using post flow? Also, make sure that your tungsten is the right one and that it's not contaminated at ALL!!! What is your back purge flow rate or setting on?

ALSO!!! Do'nt change your travel angle on your torch when you do the crater fill because, if you do, then the crater will NOT be shielded when you're travelling back into the weld!!!

Always fill the crater during your restarts, and make sure you're watching how fluid the puddle is because, you DO'NT need to have so much current once the puddle has been established in order to deposit filler metal!!! The flipside of the coin is not to have enough current whereby there is'nt enough "wetting action" at the toes for good fusion there...

I also noticed that you're getting indications on the toes of the fillet?
Are you oscillating the weld pool? Please let me know when you get time!!!

Respectfully,

SSBN727 Run Silent... Run Deep!!!

By insp76

Date 08-29-2003 23:10

Dadeacon, I can feel your pain my freind, my guess is your a very experienced welder on a seemingly simple weld and it`s giving you trouble. SSBN727 has it pinned down preety good cleanliness and maybe pullout technique. I have seen contamination look like pull out problems but accually the problem is that the contamination surfaces when the puddle cools and causes pinholes and crack like defects when contamination mixes with the cooling of the molten puddle. The defects at the toes of the weld could be primarilly the same problem coupled with the fact that you might not be running enough heat. My guess also is that your welding on an exchanger of sorts "they are famous for the problems your talking about". What I recommend is,first grind with a new blade or stone then ss wire wheel the area ,then heat the area to about 250f and repeat the above cleaning. What your doing is first cleaning what contamination is on the surface, when you heat the area your openning up the pours of the steel and drawing out the inpregnated contaminants that are in the subsurface and your cleaning them away while in the heated condition. I was successful using this technique a couple of moths ago in a simillar situation. (one of those midnight phone calls when night shift needed me to wittness a hydro). Dye check the final weld making sure there are no relevant indications and hopfully get that hydro bought off.... Hope this helps you , have a good one!

By Niekie3 (***)

Date 09-01-2003 08:06

Hi dadeacon

I am sorry that it took me so long to respond, but I have been rather busy lately.

Welding the S/Steel with the 625 or 82 will result in a totally austenitic weld deposit. This means that hot cracking will be a problem, which means that you need to work very cleanly. Any contamination will result in hot cracking in this weld.

Welding the Cu/Ni with 625 or 82 can potentially result in cracking due to the formation of intermetallic phases. This will only become a problem when the amount of Cu reaches 15% or higher. As the Cu/Ni has around 70% Cu, (I believe!) a 40% dilution of the filler metal with the 625 will result in 28% Cu in the weld metal, that will result in cracking problems here.

I therefore believe that you potentially have a combination of problems. Hot cracking and hot-short cracking. My suggestion would be the following:

1) Butter the S/Steel side of the weld with 625.
2) Weld the buttered tubesheet to the tube with pure Ni filler. (Ni61)

You may be able to get away with welding the tube to the tubesheet with Ni61, without buttering the tubesheet. This will only work if you keep the dilution from the tubesheet very low. Must not have more than around 25% Fe in the weld metal.

Hope this helps.

Regards
Niekie Jooste
Fabristruct Solutions