Help with part distortion after welding

[freedom_in_4low]

Working on this bumper project modifying a Barnes bumper to be more like a NV, added a receiver and boxing in the bottom.

This is current state. I've finished the ends and the receiver, but have only tacked the center bottom edge (a section of angle iron) and haven't closed in from the angle iron to the lower mounting bolt tabs.

The bumper currently has about 1/8" of arch, that is the center of the bumper is about 1/8" higher than the ends. It still installs ok but it's just enough to see in the gap below the tub.

My concern is that it gets worse when I weld that bottom edge on because all the heat, once cooled, would shrink smaller than it started with and pull it into more of an arch. I thought about alternating side by side in short segments to minimize the heat, and then cutting a slot or two in the top edge and welding them up, so the shrinkage in the top might pull the arch out of it, but I had a buddy suggest that if I weld my bottom angle on from the outsides and work my way in, it'll pull the arch out....i haven't really worked that out in my head but he's more qualified than I am so i tend to trust his guidance, though it did come with a disclaimer that he's super tired with their new baby and would think about it for a while.

So, expert fabricators, how to deal with this?

 

[freedom_in_4low]

my buddy got back in touch today and changed his advice to preheat it a bunch, weld a few inches at a time, alternating sides, starting from the middle, to limit any further warpage as much as I can, and if I'm not happy with the final result, my only option is flame straightening. I might also get a welding blanket to wrap it up in after I finish to slow down the cooling as much as I can.

Flame straightening is along the lines of what I was thinking before, but since I don't have an oxy-fuel torch, was thinking I could put some arc heat in the area I want shrunk and use that to somewhat simulate frame straightening. Usually, flame straightening involves quenching which I don't think I want to do on a weld, but since a weld arc is WAY hotter and more focused than a flame heated section of steel, I was thinking I might get some of the same shrinkage effect as a quenched, flame heated area because it will cool relatively quickly. I don't have an oxy fuel setup and not a lot of use for one, so I don't really want to spend hundreds on that to fix a slight bend in a bumper I have like, $200 in.

I think though, with the concentrated heat of a weld instead of the broad, spread out heat of a flame, I'll end up warping the top edge and pulling the bumper into a horizontal bow and be out of whack in 2 dimensions. So probably a bad idea.

 

[bucky]

I saw a great video on youtube that showed the techniques of using heat to straighten out metal with a torch. Unfortunately I do not have a link.

 

[freedom_in_4low]

I saw a great video on youtube that showed the techniques of using heat to straighten out metal with a torch. Unfortunately I do not have a link.

There's a few of them on YouTube. All I've found so far use an oxy-fuel torch. I know it needs to get into the 1000F range to work... I know the flame of my propane torch will get hotter than that but I don't know if i can get 3/16" that hot with it. No way to find out until I try.

 

[Mike_H]

Do you have a press?

 

[freedom_in_4low]

Do you have a press?

negative. Working with a pretty minimal metalworking setup as long as I'm in this rental. I'll be more willing to buy stuff when I get into the new place where the garage isn't doubling as storage and I won't have an imminent move.

I've thought about rigging something up with a bottle jack under a vehicle, but vehicle weight limits how much load I can put on something. Also thought about grabbing a 5-6' length of heavy square tube or I-beam to clamp it to while I weld, since the bumper is beefier than my welding table.

 

[NashvilleTJ]

I used the weld bead technique to straighten my 14 bolt. It works. My bet is that if you ran a few beads along both top edges of the tube it would flatten out. Grind them down and you will never know it happenned.

I posted up how I straightened the 14 bolt over in my build thread.

Edit: Post #349 in my build thread.

 

[freedom_in_4low]

I used the weld bead technique to straighten my 14 bolt. It works. My bet is that if you ran a few beads along both top edges of the tube it would flatten out. Grind them down and you will never know it happenned.

I posted up how I straightened the 14 bolt over in my build thread.

Edit: Post #349 in my build thread.

dude. YES. Thanks for confirming I'm not completely nuts or stupid for thinking of this. and thanks for sending me straight to the post...it drives me nuts when people say "look in my build thread" so I can spend 2 hours reading through the whole damn thing to find the one detail I'm looking for.

 

[Grant Lasson]

I've thought about rigging something up with a bottle jack under a vehicle, but vehicle weight limits how much load I can put on something. Also thought about grabbing a 5-6' length of heavy square tube or I-beam to clamp it to while I weld, since the bumper is beefier than my welding table.

I think you're onto a good idea here. I've done a lot of metalwork and welding. When there was a weld that had the chance to twist or mis-shape the product, we often used c-clamps, stronger tubes, hydraulic jacks and other leverage to pre-bend the metal in the opposite direction. When welded, the shrinkage would pull it back straight. It's easier to do beforehand but I think you can still do it now. Bend the bumper until it is drawn beyond the shape you want. Use spacers to get an arc. Weld it and let it cool. See where you're at. If it wasn't enough, reapply leverage and then heat with a powerful torch/rosebud. (I don't know about a Barnes bumper--if it's really thick material this may be impractical but your picture doesn't suggest that.) Heat the edge that needs to stretch. Good Luck.

 

[freedom_in_4low]

I think you're onto a good idea here. I've done a lot of metalwork and welding. When there was a weld that had the chance to twist or mis-shape the product, we often used c-clamps, stronger tubes, hydraulic jacks and other leverage to pre-bend the metal in the opposite direction. When welded, the shrinkage would pull it back straight. It's easier to do beforehand but I think you can still do it now. Bend the bumper until it is drawn beyond the shape you want. Use spacers to get an arc. Weld it and let it cool. See where you're at. If it wasn't enough, reapply leverage and then heat with a powerful torch/rosebud. (I don't know about a Barnes bumper--if it's really thick material this may be impractical but your picture doesn't suggest that.) Heat the edge that needs to stretch. Good Luck.

it's 3/16...thicker than I've worked with before but it's all relative.

This is the torch I'm working with.

 

[NashvilleTJ]

I think you're onto a good idea here. I've done a lot of metalwork and welding. When there was a weld that had the chance to twist or mis-shape the product, we often used c-clamps, stronger tubes, hydraulic jacks and other leverage to pre-bend the metal in the opposite direction. When welded, the shrinkage would pull it back straight. It's easier to do beforehand but I think you can still do it now. Bend the bumper until it is drawn beyond the shape you want. Use spacers to get an arc. Weld it and let it cool. See where you're at. If it wasn't enough, reapply leverage and then heat with a powerful torch/rosebud. (I don't know about a Barnes bumper--if it's really thick material this may be impractical but your picture doesn't suggest that.) Heat the edge that needs to stretch. Good Luck.

I had the same problem when folks asked me about something in my build - especially over on JF because it was huge. One thing I did the helps me now is I maintain a table of contents on both. It took me about a minute to find that post.

 

[Grant Lasson]

it's 3/16...thicker than I've worked with before but it's all relative.

This is the torch I'm working with.

View attachment 315011

That's not going to be ideal. I'd put more effort into the remaining pre-welding preparation. 3/16" has already demonstrated to you that it can be bent over a distance. If you get a pretty good size tube (e.g., 2"x4"x1/4") and use the 4" side to resist the deflection, I think you can do it. Unfortunately, a 2 x 4 x 1/4" x 4' tube may be a $150 dollar purchase these days.

BTW, your torch is not hopeless--especially if you use it in conjunction with the leverage. Work from the middle to the outside and work methodically. It doesn't have to glow red to release some tension. More heat in the inner 1/3rd of the arc. Apply more pressure when heated.

As mentioned previously, you can apply shrinking pressure by welding beads on the opposite side/edge. It would be interesting to try that in conjunction with the leverage in the absence of a bigger torch.

 

[someguysjeep]

set it on the ground on some blocks under your frame rail, put a bottle jack on the high spot and (carefully) use the jeeps weight to push the bend out?

 

[freedom_in_4low]

Well here goes nothing.

Without a heavy welding table or the desire to spend a ton of money on a piece of rectangular tubing I used the best option I had on hand which was the hitch receiver that came on the LJ with a piece of scrap between them as a shim. I'm honestly not sure whether the receiver or the bumper is flexing more here but it's worth a shot. It did break my tacks on one side so it's doing something.

I did about 2" at a time, starting from the middle, alternating sides, working my way out, preheating to 350F before welding and waiting for it to be cool to the touch before doing the next bead. I figure that's about the best I can do; I'll let it sit overnight and have just a couple of stretches left to box in on the bottom edge before I unclamp it.

 

[someguysjeep]

the bar on top can be used to force the crown out, but may not keep once relieved. you'd have to push past and actually bend the material some to remove a crown.
the bar on top won't hold the piece well for welding down the sides of that receiver. thats gonna wanna curl it inward on the ends and should be clamped down flat or the backing bar clamped to the backside to hold this action to a minimum.

once it's cool enough to handle it's done moving.

 

[NskLJ]

Just think out loud here, could you mount it upside down on your Jeep (with spacers for clearance) then use a floor Jack in the middle. Then weld it?

 

[freedom_in_4low]

the bar on top can be used to force the crown out, but may not keep once relieved. you'd have to push past and actually bend the material some to remove a crown.
the bar on top won't hold the piece well for welding down the sides of that receiver. thats gonna wanna curl it inward on the ends and should be clamped down flat or the backing bar clamped to the backside to hold this action to a minimum.

once it's cool enough to handle it's done moving.

Receiver is already welded in. I'm not looking for this to remove any crown, just trying to keep it from getting any worse while I put a flange on the bottom edge of the bumper. For permanent straightening I expect to end up heating the top. I'll give the propane torch a try, just because it didn't seem to take that long to get an area to 400F so maybe I can get it to 1000F with a little patience. But if that fails I'll lay some beads on it.

 

[pagrey]

I don't think a propane torch has a chance of getting an entire bumper glowing red, it's not even easy with a small section.

 

[ColoJeep]

For what it's worth you can use a MAPP gas , (yellow ) cylinder on your propane torch and get a little more heat.

 

[freedom_in_4low]

I don't think a propane torch has a chance of getting an entire bumper glowing red, it's not even easy with a small section.

definitely not the whole bumper at the same time. I'm more going for a half dozen spots, done one at a time. And I still have a plan B because my expectations are low even for that.