MIG Open Root 3g FAIL

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Hey everyone, here are some pictures of my 3rd attempt at mig 3g.22.5° bevel, 3/8" mild steel, no land, 1/8" gap, downhill root (is this how MIG open root is always done?), rest uphillI believe machine was millermatic 252, set at ~19.5 V, 270 ipmRoot side:Face side:Here are what problems I think there are:  I should have run the root more like 17.5 V and 275ipm to push more reinforcement through, as I'm only flush or very slightly under throughout.Ran the other passes, or at least the cap at more like 19v, 250ipm so it wouldn't be so rounded up (although its below the 1/8" limit).Keep my cap weave more consistent.  Near the end, its looks different than at the bottom.  I think I was either holding one side longer or move up with the weave more so on one side.  Do you agree with my observations?  Any other advice?Thanks.Last edited by LeviC; 11-11-2015 at 03:08 PM.
Reply:I'm not sure if it's just me and my phone but the pictures aren't showing up.
Reply:Alright, I think I've fixed it.Oh!  And the gas used was 90/10 (seems an unusual mixture) at ~35cfhLast edited by LeviC; 11-11-2015 at 03:17 PM.
Reply:Not too bad. Travel across the center quicker with your weave. You don't ever really need to worry about reinforcement in the center of the cap, it takes care of itself. I think you may have gotten back in the puddle which causes a lack of penetration in the root. It's important to stay on the leading edge.
Reply:Originally Posted by burnsNot too bad. Travel across the center quicker with your weave. You don't ever really need to worry about reinforcement in the center of the cap, it takes care of itself. I think you may have gotten back in the puddle which causes a lack of penetration in the root. It's important to stay on the leading edge.
Reply:I'm not sure on the allowable limits for concavity in the root but will look it up this weekend. Hopefully someone will answer beforehand. Also, I tend to keep a fairly low angle when running the root vertical down. A 45* angle is a starting point but I would suspect that I run somewhere around 30* off the plate vertical down. For tips on the cap watch the video Jody posted on weldingtipsandtricks.com. Between his explanations and his videos it's hard to explain it any better.
Reply:It looks like your root is not achieving full penetration. Your cap has quite abit of undercut on it. Get those issues resolved.JasonLincoln Idealarc 250 stick/tigThermal Dynamics Cutmaster 52Miller Bobcat 250Torchmate CNC tableThermal Arc Hefty 2Ironworkers Local 720
Reply:@ LeviC - why are you MIG'n the root downhill?  Working IAW a WPS, or in training mode?  Recommend you go Up for all passes.  Also, believe a 3G (non-backed) joint should have a land vice a knife-edge.  Knife-edged if using a backing plate.  Try adding a nickel land to your edges.  Dial in your root technique first & foremost.  As snoepro mentioned, your cap is "crowny" (bell-curve) and you have uncut on sides.  Your cap should not exceed 1/8" height.  Tight arc, pauseause on each side while weaving, and fast across the middle.  Prior to applying the cap, shoot for a flush, or slightly below flush, geometry on your plates. 90/10 in short-circuit mode? Why not C25?  Welding filler type/size and parameters would be helpful for others to assist in troubleshooting."Discovery is to see what everybody else has seen, and to think what nobody else has thought" - Albert Szent-Gyorgyi
Reply:I would also recommend a landing on the bevel for that. It gives support for a thick puddle. Like burns said, you need to stay at the leading edge of the puddle, with a slight upward angle. With the landing, you'll have sort of a 3D puddle, and you want to stay on the bottom face of it, to the point that it almost looks like you are going to miss the puddle completely. I don't think you need to do any machine adjustments at this point. For the cap, the information already given is correct, but not really specific enough to get you fixed up. More like giving you a general direction for more trial and error. It's hard to determine how fast to go across the middle by just saying go faster. Just like all welding, your bead size and shape is determined by watching and manipulating your puddle. When you are doing your weave and you drag your puddle up and over, you want to look at the puddle following you. If your heat/wire speed are good, you will see the puddle form what looks like a comet tail of molten metal as you drag the puddle. You want to pause on the side long enough to see the comet tail catch up to the puddle. Then once it catches up, you want to see your puddle fill in any undercut you just carved out on the bevel edge, and watch it tie into the previously laid part of your weave so that you are making a straight edge as you are going up, and not a bunch of semi-circles. If you are doing your weave and you see  your puddle not completely leaving the center, you're going to end up with what you have in the picture. The perfect setting for your wire speed would have you able to see the comet tail I was taking about disappearing at the same time the puddle fills in enough to make a good tie in. Don't be afraid to pause at the sides a little longer. When I was learning, my caps looked a lot like that. I don't know why, but it seemed counter intuitive to pause so long on the sides. I had to get over that, and once I did, I was making nice even fish scales on 3G mig.
Reply:@ TheBFA - you run your 3G root Down and not Up?  WPS-centric?"Discovery is to see what everybody else has seen, and to think what nobody else has thought" - Albert Szent-Gyorgyi
Reply:Originally Posted by ManoKai@ TheBFA - you run your 3G root Down and not Up?  WPS-centric?
Reply:Originally Posted by ManoKai@ LeviC - why are you MIG'n the root downhill?  Working IAW a WPS, or in training mode?  Recommend you go Up for all passes.  Also, believe a 3G (non-backed) joint should have a land vice a knife-edge.  Knife-edged if using a backing plate.  Try adding a nickel land to your edges.  Dial in your root technique first & foremost.  As snoepro mentioned, your cap is "crowny" (bell-curve) and you have uncut on sides.  Your cap should not exceed 1/8" height.  Tight arc, pauseause on each side while weaving, and fast across the middle.  Prior to applying the cap, shoot for a flush, or slightly below flush, geometry on your plates. 90/10 in short-circuit mode? Why not C25?  Welding filler type/size and parameters would be helpful for others to assist in troubleshooting.
Reply:With any open root weld test, you must get your root pass all the way through (full pen) and ideally have a small amount of reinforcement on the back side. This will be easier achieved by welding the root pass vertical up which give you more penetration with your puddle.JasonLincoln Idealarc 250 stick/tigThermal Dynamics Cutmaster 52Miller Bobcat 250Torchmate CNC tableThermal Arc Hefty 2Ironworkers Local 720
Reply:Under cut is when you melt the base metal away without your filler metal completely filling it back up. You'll typically see it on the edge of the bevel, where portions of the weld are below flush with the base metal. If you see it at the end of a weld bead it's usually called a crater or underfill.
Reply:We run all our Mig roots downhill, You need more gap, no land. 5/32 gap minimum. Keep your wire as far forward as you can get it.
Reply:Usually on a test your instructed on how the test is do be done. I've never done a weld test where I had the option to weld the root up or down.JasonLincoln Idealarc 250 stick/tigThermal Dynamics Cutmaster 52Miller Bobcat 250Torchmate CNC tableThermal Arc Hefty 2Ironworkers Local 720
Reply:The only time I see mig run downhill is for open root, short circuit on pipe. In fact, I've never even see anyone attempt vertical up, open root, short circuit on pipe. Not saying it can't be done but I've never seen it. Any other time I see people running down hill mig I just expect that something bad is happening. You can do it but you have to run way hotter than you think. You need a big gap, like Timmy Tig mentioned, so set a gap that you think looks too big and then open it up a little. Stay on the front of the puddle almost pointed like you are going to shoot through the backside of the joint.
Reply:MIG root is always downhill. If it's a real critical job TIG root will be specified. You were running too hot and the 90/10 gas likely didn't help. I did a bunch of pipe spooling and vessel roots up to 2 3/4" thick. 3/8" plate could use a 5/32" -3 /16" gap. Normally pipe over 2 1/2" uses a 3/16" gap unless it's thin wall. A big vessel could have a 1/4" gap and it wouldn't be a big problem. I don't know the wire speed but you generally a run a root 17-18 volts and about 110 amps give or take. It's actually colder than you'd use for general welding but experienced guys can run hotter and faster. Generally for a test guys will put about a 1/16" land on the bevel but on most production work they don't bother with a bevel. You want the puddle easily controllable and you just twist your wrist slowly back and forth with the arc on the leading edge of the puddle making sure you melt the edges and it gives you a really smooth root. Try to watch the puddle from underneath so you can see what it's doing. Using a tapered pipe nozzle for the root makes it easier to see what's going on. On pipe anyway, you usually run the nozzle right on the bevel and just use 1 hand on the gun. Often your other hand is used to turn the pipe or operate a thumb switch on powered rolls. A foot pedal is even better. The gun is pointed slightly up and what you're basically doing is pulling the puddle down while making sure you have fusion on the sides and the middle takes care of itself. I don't know if you're just learning or not but your root is not even remotely close to passing a test. Turn your heat down, go slow and once you figure it out, it's one of the easiest welds to do. Oh your bevel should be 37 1/2 degrees.Last edited by Welder Dave; 11-13-2015 at 02:14 AM.
Reply:@ TimmyTIG & Welder Dave - aw'right.  MIG roots are conmonly burn'd Down 3G.  Will have to check out my Lincoln Procedure Hndbk and D1.1.  Thanks!"Discovery is to see what everybody else has seen, and to think what nobody else has thought" - Albert Szent-Gyorgyi
Reply:I may be mistaken, but I don't think MIG roots are a D1.1 approved procedure. I think it's more of a pipe thing.
Reply:Originally Posted by TimmyTIGI may be mistaken, but I don't think MIG roots are a D1.1 approved procedure. I think it's more of a pipe thing.
Reply:I've always though short circuit MIG isn't qualified for structural as well but I suspect if it was used for a root pass/first pass and followed by stick or flux-core could then be qualified.