Welding on Vibrating Structures

I don't think it would hurt to try it. You may want to try to fashion some sort of external brace to dampen the vibration while welding. That's my guess.

he could weld practice piece on top of the cover so he could get a feel for how much it moves

Excellent point! Not enough info in the OP to tell if the air to the preheater is filtered or what the setup is.

Not as long as the welder synchronizes his motions with the vibrations.

It could actually give you a better weld, if it is vibrating.

The issue of the flashfire is another matter and should be taken into consideration.

What you wish to consider is that you want to use a technique that creates more ductility. A rule of thumb is that the stronger the metal, the harder and less ductile it is. For this reason you would be better to use a weaker rod like 6010 as opposed to 8018, even though the latter is stronger it is also more likely to work harden and fail. If it is excessive vibration you need to make sure that there is a backup for safety in case your weld should fail. All metal will harden as it is "worked" or stressed, and this decreases the ductility, while adding to the hardness. A simple example of this "work hardening" is when you bend a wire back and forth to break it. What you really are doing is hardening it until it fails and breaks.

Rich Hurd

Maybe we should consider the thickness of metal to be welded and length of repairs. Backstepping on long seams gives a great advantage allowing the weld area to cool while continuing the work. What is the length of welded area and how much vibration do you have to contend with? Mechanical vibration of a few thoundanths or a few sixteenths of an inch can be compensated by a good welder. As I said earlier vibration can be advantageous in reducing stress built up in the weld area. I would not think that you would be trying to repair something comparable to an earthquake situation. I would say go on and do it. if it looks like crap simply grind it out and weld it again. This is common practice in welding contaminated material to cook out the contaminanted material and grind it out and do it again. In a bag house situation I would imagine it is already quite dirty. Be sure to power brush or sand/grind the weld area. Chicken feather sanding disks do a great job with minimal material removal. these can be found at almost any welding supplier. Thin parting disks will get you down to a good root area. Same as the chichen feather disks or flap disks used in a side grinder. Dont be scared. jump on it and get it done.

Getting started is half done. Been there and done that.

Chicken feather sanding disks?

Sand paper flap disks.