Welding Stainless Steel - in need of an unbiased opinion

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I am in need of an unbiased opinion...I have been told that welding stainless to stainless can damage a concrete substrate due to the heat of the welding process.  Is there any truth to this?Here are the specifics:There is an existing stainless steel trough pan in a recessed area of a concrete floor slab.  We are adding on to the trough pan, and I specified a continuous weld between the overlap of the new trough pan and the existing one.  The installer has proposed doing 3 tack welds (one at each edge, and one in the middle), and infilling the remaining area with silicone sealant.  The width of the trough is 12".  This is in a commercial kitchen, so the continuous weld is preferred to avoid future issues when the sealant breaks down over time.  The claim is that a continuous weld will damage the concrete substrate.Please help...Any unbiased insight would be greatly appreciated.Thanks,Christine
Reply:Well, depends on a lot of things, like how thick is the 'crete? how long has it been there? has it been kept dry, or does it get wet often? can the 'crete be grooved out behind the weld so the weld won't be made in direct contact?My OPINION is that the concern would be more with the metal itself pulling out of the concrete it it was poured around, the metal being chilled by the concrete, or water from the concrete getting to the back side of the weld. The concrete itself wouldn't be a major worry for me IF the concrete has been kept dry so that there is no free water soaked in, it is at last moderately thick, and a groove can be cut behind the weld so that the heat of welding isn't directly on the concrete. This presumes thin material, which I would expect in food service, and that the weld will be TIGged, which I would also expect. If the material is lapped, it will be more difficult than is it is butted (due to the difficulty of getting behind the weld area), but the prep is much easier if you don't worry about what is in back.Heat, such as from welding, can damage concrete, as set concrete is held together by a hydrated (water is a structural component, chemically bonded in) cement, and enough heat will cause the water to come out of the structure, permanantly reducing strength--it can't be put back. Even a small air gap, preferably purged with inert gas, greatly reduced the heat that will get into the 'crete, reducing risk of damage, and proper welding technique will reduce the heat even more.Without seeing the setting, I can't give any firmer answer. Undoubtedly, there will be other opinions.
Reply:I've welded a few things on the floor and never had it crack.  I've had a few chips come off but that was only a surface deal. And they were not the whole length of the welds.Common sense in an uncommon degree is what the world calls wisdom.
Reply:If you can get a bit of airspace underneath, the weldor might be able to slip an insulator between the sheet metal and slab to complete the weld.Get it hot and hit it hard...
Reply:"There is an existing stainless steel trough pan in a recessed area of a concrete floor slab. We are adding on to the trough pan, and I specified a continuous weld between the overlap of the new trough pan and the existing one. The installer has proposed doing 3 tack welds (one at each edge, and one in the middle), and infilling the remaining area with silicone sealant. The width of the trough is 12". This is in a commercial kitchen, so the continuous weld is preferred to avoid future issues when the sealant breaks down over time. The claim is that a continuous weld will damage the concrete substrate."If the installer isn't going to come back and re-do the silicone every two or three years, tell to do the job like it was bid. Brainfarth is right, concrete does 'pot' out and it doesn't take much heat to do it. It's a matter of quick heat in a small area in respects to surrounding area that causes the surface to pop loose. That may happen some but I'm assuming that the stainless trough is what is to be the 'carrier' and the concrete trough nothing more than a recessed area for the stainless.  You and I know darn well that once that silicone goes on it'll recieve absolute minimum attention from that day on.  Sometimes you have to chose the better of the bad options. A continuous weld is the best method. Concentrate your efforts on HOW to accomplish that and not how to get around it.Like Enlpck said tho, check out that moisture situation. It will be a big player in this issue if it is wet all the time."The things that will destroy America are prosperity at any price, peace at any price, safety first instead of duty first, the love of soft living and the get rich quick theory of life." -Theodore Roosevelt
Reply:Without knowing the thickness of the S.S. & what type of joint fit you will have, my choice would be to back-step the weld with mig.You shouldn't have as much heat directed on the cement that way & then grind the weld smooth to the finish you need.
Reply:If it is ok to tack at a few points without ruining it.. tell the guy to tack it a whole bunch of times... bottom line is, if they guy can make a few hot spots without ruining everything, he can weld the whole thing without ruining it... Sounds like he just doesn't want to take the time to do it right, since he may have to be a little more careful and take a little longer so he can control the heat. find yourself a REAL weldor to do the job.
Reply:Additional information:The concrete thickness is 5" reinforced with 6x6-W2.9xW2.9 W.W.F. or #3's @ 12" O.C. each way.The slab was poured about a year and a half ago, but they are just now getting to installing the interior finishes. (It's a fairly large project - 477,000 SF, thus the time delay.)To my knowledge it has always been kept dry.The trough extension will be installed by demolishing the existing slab on grade, over excavating, and infilling with grout.  This should give them an opportunity to allow space for a groove behind the weld.The thickness of the stainless is 12 gauge.Please let me know if this information changes any of your opinions.The original design was for an overlapped joint.  On Friday I suggested a butt joint.  They didn't seem to think that would solve the problem.  Today I will suggest leaving the groove behind the joint.  Can't wait to see what they say about that!Thanks for all of your input!Christine~
Reply:If they're infilling with grout, then there mayn't be much problem with doing the weld before filling the area around the weld. Theis solves all questions.As others have said, for a good welder, I wouldn't see much problem keping the heat down enough not to damage the concrete to any extent even without keeping the back side clear IF the concrete is set and dry. Again, dry is key. ANY free moisture trapped in the area being welded and there will at minimum be problems with the weld, and at worst can be fairly substantial stem flash, spalling concrete and dislodging the stainless. As long as there is a way for moisture to get out, the heat od welding should cook it off ahead of the weld.
Reply:Please let me know if this information changes any of your opinions.==============================================Nah, actually reinforces it a little. Of course it sounds like you will have some new 'green' concrete in the area of the weld which you may want to allow for as in curing time. You don't want steaming moisture to contaminate the weld. Good case for a lap joint.John and Jose both mentioned small welds and/or back stepping. That's exactly what could be done if there are any great concerns about this slab cracking loose and falling off into a black hole. Lapping the two sheets will actually help absorb and even reflect some of the heat back from the concrete. You could even pre-slip a 3 inch wide, thin piece of polished in under it and leave it as a sacrifice if someone is that concerned. Small pots or pits out of a slab under a stainless trough is NOT going to destroy anything. That's IF it they even occur. Next time a contractor drops soomething and puts a ding in the floor ask them if the "substrate" is destroyed.  You can't let a weld on two pieces of twelve gauge consume too much attention on a job this size."The things that will destroy America are prosperity at any price, peace at any price, safety first instead of duty first, the love of soft living and the get rich quick theory of life." -Theodore Roosevelt
Reply:Side issue:After spending 15 years in the dairy business (as a certified pasteurizer operator) and 10 years in the restaurant business, the reason that the full length weld was specified was becasue sealant would not pass inspection.  All stainless must be solid and continous in food areas (even floor drains) so that bacteria cannot accumulate in sealed areas.  That joint will have to be welded (and polished to match the suface area) in order to pass any type of inspection.I can't help with the concrete/heat/welding aspects of this issue, but I know about food safety laws and they are strict!  BTW, I have seen a LOT of stainless welded onto plates in concrete with no adverse affects.  Just stitch it together instead of trying to run a continuous bead.  Do short strokes, move locations, cool between passes and it will likely be fine.Hey! Bring the camera...Is it still hot?   $%#^%^#  Guess so...