American Welding Society Forum
By John P
I'm trying to put together a general procedure document to explain the general steps to repair ductwork for a client. The knowns are as follows:
-Duct is 18"x18", ASTM A480, Type 304, 18GA.
-Duct has leaks in the form of pin holes & cracks in the welds. These bad welds are in the longitudinal seams of the ductwork as well as the welds at the flanged connections.
-Duct has pin hole leaks in the actual field of the ductwork.
Since I am not a welder, I am asking how a welder would go about fixing these type of leaks.
I don't know what the duct work is ducting or the service conditions, but the first thing I would ask is can you use an epoxy sealer or similar to seal it? With thin gauge stainless, welded repairs will be tricky because of distortion. If the ducting is already installed you could have access problems and the possibility of catching things on fire. A sealer, if allowable and practical, would be the easiest. As far as welding it, maybe some of the others on this forum have some ideas?
By John P
The ductwork provides S/R 150 degree air to a process. The ductwork, over time, becomes contaminated with various monomers which are explosive. The ductwork that shows leakage (when monomer cools, it condenses and leaks out) will be dismantled and repaired (if possible). Welding can be done at the plant, but outside in a temporary facility. The ductwork is rated for a max pressure of 15" wg.
As stated already, you will have a major problem with:
I would recommend you use GMAW in dip transfer, as this allows you to give a very low heat input. This you can do without grinding out the crack. If you get the heat input just right, you could get a reasonably good penetration without burning through. The other alternative would be to use some sort of non-consumable backing such as a ceramic backing. This will assist with the burn-through problem, but you will still be left with the distortion problem.
Depending on the size of the pinholes, they can be either closed with a tig torch, or a small patch inserted.
In the case of welding being performed without grinding the cracks open, you will have to get rid of the inevitable contamination in these cracks. If not, you will end-up with hot cracking. You can do this by heating the metal up to approx 300°C in these areas to burn out most of the rubbish, but this will also contribute to distortion. Another alternative is to weld with a consumable that will result in a high percentage of Ferrite in the weld. e.g. 29/9 consumables. (Also called 312 consumables.)
Hope this helps.
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