He Leak Test with Expl Clad Tubesheets

[MrBTU]

We fabricated some heat exchangers with 90/10 CUNI tubes strength welded to 70/30 overlayed tubesheet. The units passed hydro fine, but they will not pass helium leak test - we are getting leakage through the strength weld (which we determined after bringing a guy in who could sniff each tube) The He leak test is a post-hydro, customer requirement.
Some more info - we had two other units, welded by the same welder, with solid 90/10 tubesheets, that passed the helium leak test just fine.
Has anyone had similar experience, that can shed some light on the metallurgy or the process of expl clad that can explain this?
Thanks

 

[rmw]

Give us some more detailed description please. Yes, I understand the meaning of strength weld, but that said, did you roll the tube into the base metal? What base metal? What thickness overlay? Did you test (e.g. UT) the integrity of the cladding? If you rolled, did you roll at all in the overlay? Or did you leave a gap between the weld and the beginning of a roll if you rolled? If you rolled did you weld first, then roll, or vice versa? Weld process? Weld material?

rmw

 

[MrBTU]

We used a 1/4" after machining expl clad on 516 GR70 (not overlay as I wrote above). The welds were made with 70/30 wire, and lpt'd after the strength weld. The strength weld is in the cladding only, a groove only weld, .05" per UW-20. After the welding, we did a light contact roll from 1/8" behind the weld to within 1/8" of the back of the tubesheet. The cladding was UT'd by the vendor.

 

[rmw]

I need to look at some stuff that I have at the office before I comment farther on parts of what you have replied. The starting of the roll 1/8" back from the weld concerns me. That is mighty close to the weld itself. Any slip up and you might be rolling the weld or the heat affected area as well. Was there any change in rolling procedure between rolling into the 90/10 tube sheet and the 516GR 70 clad tubesheet?

rmw