Thin-on-thick lap weld

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I'm going to be doing a few lap welds on a project I'm working on. This will be thin (1/8") pieces welded onto a tank with 3/8" walls. The purpose of the thin pieces is to cover holes so that the tank can hold water.  The holes are where some old valves were removed, and are about 2" in diameter.What rod would you use for this? I'm thinking 1/8" 7014. This will be with a 220V AC stick welder, by the way. Can you give me any tips about rod angle, amps, and anything else? I tried doing this on some practice pieces, and I had a pretty difficult time keeping the edge of the thin metal from melting back away from the bead. Probably too much current.Last edited by Ruark; 03-06-2012 at 05:29 PM.-RuarkLincoln 3200HDHobart Stickmate LX235TWECO Fabricator 211i
Reply:I would use a 3/32 rod at around 75 amps. Keep the rod aimed more towards the base metal and just watch the very edge of the puddle at the top and advance fast enough to avoid melting the corner away. HthSent using Tapatalk on AndroidSupport Bacteria -  It's the only culture some people have!
Reply:What position are you welding in?IW Local 580 NY, NY
Reply:First off, I'm wondering why you would not match the thickness of the patches to the thickness of the tank shell. Is this a coded vessel? As you have stated, the holes in the tank are about two inches in diameter, so why not make round plugs to fit these holes. Put a 35 degree bevel on each side, leaving a 3/32 " land, and weld from both sides, using1/8" - 6010 for the root pass and 3/32" - 7018 to weld it out. Having worked at building many tanks, and without getting into too much more detail, it's the method to use on this.Bgbkwndo.
Reply:Not all that easy to do without some practice.Keep the arc on the thick piece and then 'wash' the puddle to the edge of the thin piece.  Immediately get the arc back onto the thick piece, and repeat the process.  If you dwell or pause on the thin piece, you'll probably melt it badly.I'd also go for a smaller diameter rod and lower amps to slow down the deposit speed and maybe a 6013 instead of a 7014 rod. Personal preference there and not doing it in a 'production setting' where fast-n-furious may be preferred because time=money.  Someone with more experience with SMAW and 'thin' stuff may prefer differently, which is fine.Maybe you can overlap the cover pieces more and then do a 'burn-through' plug/rosette-type weld into the base metal.  You'd have to be real careful to properly and fully overlap all the welds so you don't get leakage though.Or if there were previously valves there, maybe just use the existing (?) mounting holes and just bolt the cover plates in place with some gasket or sealant.Also, if the 'tank' has 3/8 thick walls, are you sure 1/8 inch cover plates are adequate for the job (strength and pressure)?Related question, is this 'code' work or more like a farm trough?Last edited by MoonRise; 03-06-2012 at 05:58 PM.  The best laid schemes ... Gang oft agley ...
Reply:Heh, good questions.  It's just a flat position.  It's not code; it's a "backyard farm project." This is an old 250 gallon propane tank; the project is to make it into a water-filled roller to use on a couple of my pastures.  The old valve holes have walls that stick out about an inch. I'm going to cut those off and grind them down flush. Then cover them up with pieces of 1/8" plate. The water fill hole will be on the end of the tank; not an issue. I do have some 1/8 and 3/8 - 1/2 scrap plate in the shop, so I'll be practicing.  I need to use at least 1/8" plate so it won't get bent up when it rolls over a rock. This will weight about 2600 pounds when it's full of water; thinner plate would be damaged. I'd use 3/16 or even 1/4 plate if I could, but I don't think I could get it bent into a curve to fit the tank surface.The valve holes have to be flush because the frame needs a "scraper bar" which is a piece of angle iron going across the roller, to knock off any dirt that builds up on it.  That obviously wouldn't work with those holes sticking up, so they have to go.  Here's a pic:Last edited by Ruark; 03-06-2012 at 06:23 PM.-RuarkLincoln 3200HDHobart Stickmate LX235TWECO Fabricator 211i
Reply:7024 3/32 would be a great rod for this. Aim at the thick and let it wash over the thin. Easy as pie.
Reply:Propane is denser than air and will sink to the bottom of the tank.  How long since the tank held propane?
Reply:So will the scraper bar catch on the lap joint covers, or will they be close enough to flush to not matter?If it were my project, I'd probably grind those bungs flush, then grind a 45 degree bevel on their edges, cut a piece of scrap a little oversized to fit the round hole, hammer it to match the curvature of the tank, and bevel its underside slightly as needed to get it to sit flat (or just under the surface) of the hole (so it is resting on the hole's bevel).  Then hold that with a magnet as I tack the opposite side.
Reply:Don't worry, it's empty... (-;  It sat outside full of soapy water for about 2 weeks.The scraper bar won't actually scrape against the tank, it'll be maybe 1/4" off of it.  Not a problem. rlitman - I don't really have the tools to cut shaped pieces out of 1/8" plate.  My main concern here was in the OP - just how to weld it.  For sure, it looks like I'll need to go to 3/32 and at a lower amps, e.g. around 75.-RuarkLincoln 3200HDHobart Stickmate LX235TWECO Fabricator 211i
Reply:Focus the heat onto the heavier steel, then let it wash down onto the thinner stuff.You'll really put the heat almost entirely on the thicker section, while watching it flow out to the thinner section.  Watch your puddle closely.Your amps need to be set to penetrate the thicker material.  Don't set amps for the thinner stuff because it won't adequately weld the thick stuff.This is a standard lap weld.  I always set my amps at around 135amps, then let it flow onto, in this case, the 11ga floorplate.  The heat of the puddle is ample to fuse the thinner material into the weld without blowing it out.  This is 1/8 7018AC.You can move the rod side to side gently to facilitate the wash in, or to apply a little bit of heat to the thinner material.  With good puddle control you can get a nice straight line on the edge of the puddle with the wash motion.  The same holds true when washing down onto thin material."Any day above ground is a good day"http://www.farmersamm.com/
Reply:I agree with everyone on the "weld on the 3/8 and wash into the 1/8" approach. But I really don't think it's necessary to go crazy with low amperage and small rod. 99.9% of my welding has been with 7018, so I would use that, in 1/8".IW Local 580 NY, NY