I can only comment on the few wall tube, pad welds I’ve had the opportunity to do or witnessed being done. GMAW-CS was the welding process of choice and a vertical down, modified (quickly oscillated) stringer was primarily used but I’ve seen a narrow weave used successfully also. Really, whatever it took to achieve fusion with the tube wall and successive passes. We usually did a PT or MT focusing on the pad weld’s peripheral edge once the weld was completed. Many of these so-called “temporary repairs” lasted through more then their share of turnarounds.
Time for my two cents....I do this sort of thing all the time. Most, if not all the time I use smaw VD for padwelds due to the fact that you won't have as much metal deposit, just enough to build the tube back to(or close to) original tube wall thickness. Yes there is the concern with trapped slag, however if the padding is done correctly the worry is very minimal. The best way to pad is to "box" the area in..1" above, below and on the sides if feasable. Make sure to make the horizontal tie-ins last on the vertical beads...that way you have a transition of weld metal on starts and stops to minimize potential undercutting, craters, etc. If a welder is worth his salt, he'll handbrush or powerbrush every pass to ensure no trapped slag.. And if you blow a hole, hit it with a 4lb. hammer to knock the slag out of the inside and run 6010 or 7010 to seal the hole...grind it smooth then put the cover pass on with 70,80,or 9018, or whatever electrode you're using....
Sorry for the in-depth, but you could end up with problems if padwelds aren't done correctly....
John
Bash-on,
How you pad weld the areas depends on a few different factors. The main thing to focus on is your heat input. The lower the better. This is best controlled with your travel speed.
Do the tubes have water in them? If not, then jump around a lot so as not to get any area too hot, there by minimizing your chances of a burn through.
I always perferred to weld the areas circumferentially, instead of verticle down, just to avoid the chances of trapping slag. The quality of deposited weld will make a difference on the longevity of the repairs. Of coarse, this depends on what process your using. A GMAW process will give you a lower heat input, depending how you manipulate it. But it requires a greater level of cleanliness. I personally would avoid weaves.
If the tube sample analysis shows copper on the ID of the tubes, then this is another reason to watch your heat input. The copper usually comes from the condenser. Copper melts at 2080 degrees F. Even if you don't burn through, you can(and probably will) still get the tube hot enough on the ID to suck copper into the microstructure, where it will sit and cause intergranular corrosion.
Another reason to avoid burning through is now you have a hole in the tube. The magnetite on the ID will interfere with getting a good weld.
The "weepers caused by micro-fissures" are generally caused by liquid-ash corrosion. Removal of the corrosion products is important,(preweld cleaning).
Pad welding is a "get you by" technique for tube repairs. Replacing the areas with new tube material is the way to go, but I've been on many boiler shutdowns where areas were found during the UT inspection that were below min. wall. Sometimes we do dutchmen(replacing the leaky areas with new tube stock), sometimes we pad weld the areas, and sometimes we pad weld and then metal-spray the padded areas with an Inconel 625 material, for corrosion resistance(corrosion is why the tubes are thin and leaking in the first place). How we repair the areas is usually determined by how quickly the plant wants to get back on-line. The owners usually want the plant running RIGHT NOW. In any case, all the welded areas are usually MT'd.
There are companies that specialize in overlay of large areas utilizing GMAW. Depending on how large an area is affected, this might be an option worth checking into.
What does the client want? Sometimes they will specify how the areas are fixed. Especially if they have an in-house metallurgist. I've seen times where GMAW was under no conditions allowed anywhere on their boiler. I've also seen the client specify verticle down E7018. In any event, I'd talk to the client before I did anything.
It has been my experience that more than 100 sq.in. of overlay requires the AI(Authorized Inspector) to make a determination as to whether replace or overlay.
Hope this helps.
Dale Simonds
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