Welding along the radius of a hvy wall sq tube

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I'd like to skip weld a 44" long square tube to a 44" long flat plate.For a cross-sectional view, picture the lower case letter " b "... where the " l " part of the b is a 3/4" thick by 4" tall flat plate, and the " o " part of the b is a 2"x2" by .313" square tube.The flat plate standing on edge is unstable, at only 3/4" wide.  Attaching a tube in parallel along the lower half of the plate turns the wobbly "l" into a more stable "b".  (For unrelated reasons beyond the scope of this question, an angle iron cannot be used instead of a tube, since the tube's horizontal surfaces serve other needs).Please note the wall thickness of the 2"x2" tubing... at .313".  This 5/16" wall thickness tubing is not commonly found.  With most tubing, the thicker the wall, the larger the radius.  The radius of this tubing is quite pronounced (round).Which leads back to the original question:   Is it a bad practice to fillet weld into the camfer created by this radius?  Unlike a 45 degree bevel, the radius profile tapers gradually into a diminishing valley until  plate surface meets the flat of the tube.  I want to marry the major flat surface of my 4" wide plate to one of the flats of the tube.   I am concerned about the root, however, where the flat of the tube meets the flat surface of the plate.  Since the natural bevel created by the radius is roundish instead of at a flat 45 degrees, I wondered what concerns I should look out for.Thank you in advance for your advice.Respectfully,Charles Brown
Reply:Don't see an issue with this as I've done this several times over the years and have never had a failure. Pictures and a description of the actual use of said project could shed light if this is enough to fix the "wobbly I " problem.Millermatic 200Hobart Handler 120Victor O/A & Ramco BandsawLincoln 225 ACSnapOn AD HoodMiller XMT304/22AHypertherm Powermax 1650 G3Lincoln Idealarc DC600 w/Extreme 12 VSMiller Digital Elite "Joker"
Reply:Two things.  A smaller rod for the root pass will ensure you get in there and lay a root. Be carefull to get to the bottom and not long arc or you will make a mess   (ask me how I know....)Having said that, you could grab your grinder and flatten the edge if it makes the root easier.  You will be replacing the metal with weld anyway.Pics of what you did and how it turned out!Dave J.Dave J.Beware of false knowledge; it is more dangerous than ignorance. ~George Bernard Shaw~ Syncro 350Invertec v250-sThermal Arc 161 and 300MM210DialarcTried being normal once, didn't take....I think it was a Tuesday.
Reply:I used to build agricultural machinery (slashers) many years ago and we did something almost identical to what you describe on every machine, only difference was it was 4x4 rhs and 8" wide plate. This was to extend the standard 15' slashers to up to 30', so a pretty high stress joint and it worked fine.You don't mention what process or machine you will use, we were using 500 amp migs, so had plenty of juice for the job.Cheers Andrew
Reply:FODFA, MinnesotaDave, and RaptorDuner...Thank you very much for your responses.  They were each helpful, in different ways, and provided a bit more confidence that the proposed joint design is a viable option. There are no pictures because the steel for the project isn't cut yet, as we are still exploring other options on how to put it together.  The two 44" parts described above were initially going to be bolted, and when the idea of welding them instead surfaced, I had become concerned about that steep radius, and it then occurred to me that I don't remember ever seeing a joint of this type.As for process or machine, most likely multi-pass stick. (7018)  Power supply is 300 amps.  Using a smaller electrode to get deep into the narrow valley sounds like a good idea, but can the smaller rod carry the current to burn into the 3/4" plate with enough nugget inside the plate for a good root?   I thought about preheating the 3/4" plate, as it is more than twice as thick as the 5/16" wall tubing.  Aiming the flame into the crack will still likely heat up the tube (although air inside the tube would dissipate it faster),  Preheating the plate prior to fit up will make it too hot to handle for alignment and clamping to the tube.   Preheating the backside of the plate after fit up and prior to welding occurred to me.  Seems obsessive though.I thought about MIG'ing it, by increasing the wire stick out and pulling the gas cup back a bit to clear the radius of the tube, and increasing the gas flow to try and make up for the cup being pulled back.  (.035 ER70S6, same 300 Amp power supply).Tubing is A500B. Not sure about the plate yet. The design might be changed from 4" to 4.5", and I don't know what is available locally at that size.  Most likely A36.Respectfully,Charles Brown
Reply:Problem with increasing stick out with mig is that you decrease the "heat" when you do so. You'll have to compensate by upping your voltage and wire speed to maintain the same "heat" you would have if you had a shorter stick out. Personally I'd be tempted to use mig for several reasons. One I'd be worried about burying slag in a deep joint like that, and two I tend to run mig better than I do stick. I'd probably try a test pieces with my my MM185 cranked up near max with .035 wire and back down if I had too much heat. I know it will bond in plenty for that application to the 3/4" with the voltages that high. The only real question is if I'd get enough depth, but I'd only need 5/16" on each leg.I wouldn't be super concerned about using the smaller rod. In reality all you are doing is filling up the internal "void" to help prevent a ton of slag getting trapped down there.  To avoid slag deep in the root with the 1st pass, I might choose 6010/11 in this case even though it's a closed root joint for the most part. 6010/11 tends to burn out any buried slag if run well and has less to clean up. When you go and run the top bead, that's really when you will gain you strength. Keep in mind with a fillet joint like this, all you really need is 5/16" on the legs since that is the thickness of the thinner material. Chances are you'll end up with more than this on the legs with a joint configuration like this. The thinner tube will most likely fail before the 3/4" does as long as you are running at least 1/8" rods for your top pass. 3/32" would still probably work, but I'd opt for the larger rods to gain the amps to get a better bond with the 3/4" plate..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:I weld this style of joint damn near every day on tubing ranging in size from 2" x 1/8" wall to 12" x 3/4" wall with plate thickness ranging from 1/4" to 2".  If you do end up using A36 hot rolled grind off the mill scale in the weld area. When you lay your root pass, favor the plate side of the joint. Pre heating the plate after its clamped together wouldn't hurt either.For reference the tube is 12" x 1/2" wall with 1" plates. Attached ImagesDarwinian evolution used to fix stupid. Now that humans have mostly removed themselves from the process of natural selection the survival rate of idiots has skyrocketed.