steel to stainless

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Howdy!  (must have spent too much time in the sun today)I have a new project that I need some (a lot) of advice on.  I have a 400 gallon stainless steel water tank that has a plate steel piece welded to it with a bushing to attach regular pipe to.  The steel that is welded to the tank is about 4 inch square with a bushing welded to the middle of it.  The whole thing is at least 20 years old.  The stainless part is in good shape, but the plate steel has rusted out badly enough to leak considerably.  It doesn't just leak in one spot, or I would just fill it with epoxy.  There are a number of leaks across the whole plate.  I haven't got around to taking the tank off the trailer and turning it upside down yet.  That will happen this afternoon and I will take a wire wheel to it and see how much more rust comes off.  Looks pretty bad.I have a plasma cutter, O/A torch, SA-200 stick welder and will have a crackerbox TIG machined delivered this afternoon (which I know nothing of).  This is not exactly how I wanted to learn TIG, but I am willing to try.What would be anyone's suggestion on how to go about making this repair?  I have never done any type of welding with stainless, if that matters.  I have an assortment of rods for the stick welder, but no supplies at all for the TIG, yet.
Reply:With what you have listed, I would use the plasma to cut around the steel plate to remove it. Then cut another piece to cover the hole left from that. The new plate can be stick welded using the SA200 using stainless rods. They run close to the same as rods for carbon steel. No need to learn how to TIG on this project right off. Get some SS rods and git-er done. If you can make a leak free seal with steel rods you should be able to do the same with SS rods.
Reply:x2. (which is apparently too short a reply.  Hence the meaningless extra words.) Originally Posted by BistineauWith what you have listed, I would use the plasma to cut around the steel plate to remove it. Then cut another piece to cover the hole left from that. The new plate can be stick welded using the SA200 using stainless rods. They run close to the same as rods for carbon steel. No need to learn how to TIG on this project right off. Get some SS rods and git-er done. If you can make a leak free seal with steel rods you should be able to do the same with SS rods.
Reply:Do whatever it takes to weld in the flat position.  Many guys have trouble running stainless rods vertical/overhead.  Also, stick is not good for anything less than 1/8 in thick.  Thinner is doable, but very hard. Unless your pretty good with the stick whip, id learn to use your tig before you attempt it.  Tig is easy once you do it a couple times.  309 rod is best for steel to stainless(stick or tig)  Any steel or stainless rods could be used in a pinch, but it might crack at some point if any considerable force or much vibration is on it220V AC/DC Stick MachineLincoln 125HDChop Saw, Grinders, Etc.Air CompressorTons of Hand ToolsFab Table, Half Bar Grate, Half Solid w/ Sears Vice
Reply:I have a home built stainless steel water cooler tank that I use with my Miller Syncrowave 200. When I got it, the tank was sealed and had no inside access. It also had a small leak on one side at the bottom. I could have welded it from the outside but I wanted to gain access for cleaning and maintenance anyway. I cut a nice square access opening in the top of the tank and made a cover for it. The motor and Procon pump sit on that part. I used stainless steel rod with my stick machine from the inside to repair the tank. I discovered that the stainless steel rod nuns very smoothly and makes a beautiful bead. I fixed the leak with one pass and that was fifteen years ago. The stainless rod is a dragger like 7018.The excellent advice given by the posters above is the way to go. Get the tank flat and remove the regular steel and cover with a stainless patch using stainless rod. You'll be glad you did!Miller Millermatic 252Miller Syncrowave 200Liincoln AC-DC 225Victor O-A Set
Reply:If you replace the carbon steel plate with another you gonna have the same problem down the road. I would redo do it in all stainless.  What is the size of the plate and pipe bushing ?
Reply:Thanks for the advice.  I'm a whole lot more at ease about this now.Also, after I took a wire brush and grinder to the work I revealed how the original was put together and it turns out to be in a whole lot better shape than I thought it would.  These pics show a hole cut in the stainless that is about an inch and a half across and the stainless bead remaining after I chipped out the rusted plate steel.  There still is a little rusty plate left under the inside edges of the stainless bead and I'm not too sure how far I should go to get rid of it.  I could just grind the whole bead off down to the tank, or take a RotoZip with a carbide blade and chip out the rest of the rust, leaving the bead.  Would a little bit of rust go badly with stainless rods?  Or, would they perform like 6010/6011 rods?The stainless tank steel is less than 1/8 of an inch and it would be nice if I could leave something on the tank to weld to.  I would like to cut a piece of plate to fit inside the bead and just weld it to the existing bead.  I wouldn't have to worry about blowing a hole in the tank (or at least, less likely).  This, of course, would leave a place to trap water between the plate steel and the stainless, which would rust out again over time.  However, this work has lasted 20 years, so I'm not real worried about that.Now that I have revealed what was under all the rust, I would appreciate any more advice you can give me. Attached Images
Reply:Like BD1 said above, why would you replace the rusted plate with another carbon steel one? Get a piece of stainless plate, weld a new threaded bushing to it, and weld it into your tank. You definitely want to remove any traces of carbon steel, rust, or slag from the tank before you weld the new plate in. Use 3/32" stainless rods and you shouldn't have any problems with burning through.JohnA few weldersA lot of hammersA whole lot of C-clamps
Reply:????Those pictures show whats left -after- you removed the remains of the plain/regular rusty steel piece?  Just the stainless steel tank and the stainless steel bead (with some probable plain steel mixed in with the bead metal)?I'd grind out the old stainless bead and weld your new piece/fitting to the tank, using stainless for the piece/fitting and stainless filler.  Use whatever process/machine you have and feel comfortable with.Or just plasma cut the section out and then weld your repair/replacement plate into place.  No double-layer trap point that way.Why remove the old bead?  Because it has a mix of plain rusty steel in it or 'under' it.  And as you mentioned, if you weld a new plate on top of the existing bead and tank material, you have made a built-in 'trap' point for water and whatever to really get corrosion a starting point.  The best laid schemes ... Gang oft agley ...
Reply:Thank you, I'll see if I can find a piece of stainless laying around.  This may explain why there are square holes cut in the top of the side of the tank.  Maybe someone took the steel they needed from there and figured that not much water would slosh out.I don't know anything about what kind of rod to use.  309L was recommended above, but it is hard to find locally.  Would 312 work for me?  Would anything else work?  Maybe better still, what should I stay away from?It rained this weekend and left me with a pretty rusty mess after it dried.  What does this mean about the stainless? Attached Images
Reply:It means that there is still residue from the carbon steel plate that was there. You need to grind it down to the original metal of the tank and weld in a new stainless plate.309 and 312 are for welding carbon steel to stainless. If you are (hopefully) welding in a new piece of stainless, 308 is the correct rod but 309 or 316 can be used.If you can't find a piece of stainless "laying around", you can order one from onlinemetals.com or several other suppliers.JohnA few weldersA lot of hammersA whole lot of C-clamps
Reply:Thank you John,  That was very helpful.  I'll be passing by a pipe yard today and I'll stop and see if I can get a little piece of stainless to go back with.  I've got some 312 rods at TSC (it's all they had).  I'll pick up a 3# box of 309L toady at AirGas.  That should be enough to get me by.
Reply:Originally Posted by Silicon-basedIt means that there is still residue from the carbon steel plate that was there.