Tube/Tubesheet orbital

[enginerd74]

I had posted this question in another forum, but maybe this is the best location for the question (with some added history).
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I am digging up some information on a proposed repair to fix leaks on one of our waste heat boilers, and am wondering what procedure and filler metal should be used to repair leaking orbital welds (very small pinholes) between the SA240 TP405 tubesheet (3/4") and the SA268 TP439 tubes (1-1/4" x 12 BWG).

I understand the past repairs on this boiler tubesheet/tube interface has been completed with 308, 309 &/or 310 (with limited sucess). The original drawing shows that Inconel 82 or 182 was to be used for the orbital welds.

Any specific help on this issue or direction to reading material that addresses these types of questions (I have similar questions for a proposed weld between 304H & Avesta 253MA) would be much appreciated.

As well, is there a book/website that does a good job as a primer to understanding this field?

 

[metengr]

enginerd74;
Did you perform a liquid pentrant test or confirm if these pin holes you see are just really pin holes or they could be root cracks that are breaking through to the OD surface. If these are indeed small pin holes, this sounds like a porosity problem associated with the GTAW process. Either way, you need to remove (machine) all of the existing seal weld deposit from each tube and re-weld using Inconel filler metal.

 

[EdStainless]

The real problem with re-welding a unit that has been in service is cleaning. You need very clean and dry conditions to get sound welds.
I agree with MET, make sure that are truly dealing with pinholes and not cracks.
You could try simply re-flowing some welds with TIG, but if there are any underlying problems this may only make them worse.
Machine flush and re-weld with the 82/182. Be careful, the HAZ in your materials will be rather brittle so try to minimize mechanical and thermal shocks.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[stanweld]

Porosity, including pinhole porosity, at the tie-in location was a common problem with automatic tube to tubesheet welds when the the tubes were rolled into the tube sheet prior to welding. Thermal cycling would lead to leakage through the thin remaining membrane.

My advice for the repair would be to machine away the existing stainless steel weld metal, thoroughly clean the joint area and reweld using the 82/182 nickel base filler metal with two weld passes.

 

[enginerd74]

Thanks so much for the help fellas.

 

[Tomtation]

The pin holes sound like they're the result of contaminated welds. Cleanliness cannot be over emphasized for any type of welding. probably caused by lubricant left on tubes after rolling.

 

[EdStainless]

The other major cause of pin holes is that the tubes are rolled to tight prior to welding. This traps gases behind the weld that will cause pinholes when they blow out. You should just roll enough to center the tube, then weld, and afterward go back and hard roll.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net