Problem with welds distoring the steel

I'm working on a plasma table (it's under the custom shop tool category). This is my first big welding project. I've done a cart and a bunch of little things, but nothing on this scale.

I used 2" 14 ga square tube for the frame. I MIG welded 2" x 1/8 flat bar around the bottom of the square tube. One edge was centered on the square tube. This gave me 1" protruding on the inside. I used this for the bottom on the water table.

The problem I had was that after I welded the 2" bar the 2" square tube got a pretty serious arc to it. It was about 3/4" higher in the middle than the ends on a 6' 6" run.

For the future...... What can you do to avoid this?

Thanks

How did you weld it... continuous from one end to the other or stitch ?

Continuous from one end to the other. (Not all at one time.)

I was working from one end to the other and doing pieces between the braces on both sides of the table.

Welding causes contraction, there is no way around it. If you weld less (like skip welding instead of welding continuous weld seams, or by welding thinner, or narrower weld beads, perhaps using colder temperature settings for less penetration) it would reduce the amount of contraction. You only need so much weld to get the job done. In some cases, more than that is counterproductive for causing needless unnecessary distortion.

There are a lot of other tricks you can do to reduce the shrinkage/distortion to an acceptable amount. If you pre-heated the material, it would have reduced the contraction/distortion somewhat. If you fixtured the weldment to help keep it's shape as the weld bead cools, it can reduce it. Or you can even fixture it to pre-bend it the other direction, planning that it will contract into perfect alignment after the welding is completed. Using lower temperature joining methods, like brazing instead of welding, it another trick to reduce the contraction from the joining. Changing the sequence of the welds can make a big difference as well.

But anyhow, none of that really helps you now, that you have already joined the materials, and they're distorted. How can you straighten them, you asked?

Well one way is to apply lots of force to just bend it. Not too practical for larger weldments. I like another method...

If heat caused the problem, heat can also often fix it. Namely, add some nice, thick, deeply penetrating, wide weld on the back side of the joint, where it's relatively too long. If all you have is MIG, well you could just grind it off afterwards. Then repeat until it is bent back to straight. If you had TIG or oxy-acetyelene, you can cause shrinkage by just heating the part alone. "flame shrinking" or "flame bending" it's called (typically done with O/A) With TIG, I like to add some filler rod, and just grind it off afterward. (You have control over how much you decide to add, or not.) Might not be what you were planning on originally...

As Jakeru noted, the issue is with shrinkage.
The neutral axis of a part is the key to weld distortion. In lay terms, the neutral axis is the line of symmetry of the part (center line of a square or round item, not so much for an angle or a tee).

Sometimes the neutral axis of an assembly is not intuitive and parts bow in a direction opposite to what an initial prediction would estimate.

The thermal straightening (shrinking) that was mentioned works by heating a spot on a part hot enough so that the thermal expansion of the part actually yields the metal (permanently deforms) in compression around the spot. When the part cools, the compressed metal stays compressed, but the thermal expansion due to the heat goes away and causes overall shrinkage. It is sort of an "art" to get it to work.

So you would heat the side you want it to move to, correct ? Attachment 7602

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Originally Posted by zoama585

So you would heat the side you want it to move to, correct ? Attachment 7602

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Yes, usually the side that you shrink will bow the part towards the shrink. This is not always intuitive if it is not a standard, simple, symmetric shape. If you shrink the side that the arrow is point to, it will move as shown.

To be clear, it is not just heating, it is literally making the part glow in spots and then letting it cool. The temperature needs to be high enough so that the compressive yield strength of the material is lower that the force exerted by the thermal expansion of the hot spot.

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Originally Posted by sportbike

It is sort of an "art" to get it to work.

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When I've seen it done correctly it appears like a black art. I knew an old time welder who had it down. He did a lot of structural steel work and would often pre bend a beam, then as he welded it in place, it would magically straighten and mate up to where it was supposed to go, sometimes moving as much as a foot over a 20 foot span.

Much like the guys who can shrink and stretch flat sheet metal into the shape of an automobile with nothing but hammers and skill.

Black arts, I tell you! ;)

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Originally Posted by Rambozo

When I've seen it done correctly it appears like a black art. I knew an old time welder who had it down. He did a lot of structural steel work and would often pre bend a beam, then as he welded it in place, it would magically straighten and mate up to where it was supposed to go, sometimes moving as much as a foot over a 20 foot span.

Much like the guys who can shrink and stretch flat sheet metal into the shape of an automobile with nothing but hammers and skill.

Black arts, I tell you! ;)

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Pre bowing a part is more exact and can be designed in fairly readily. The usual issue is more with getting the weld size to be accurate when doing a pre bow.

Hot spot shrinking is more of an art though.

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Originally Posted by sportbike

Yes, usually the side that you shrink will bow the part towards the shrink. This is not always intuitive if it is not a standard, simple, symmetric shape. If you shrink the side that the arrow is point to, it will move as shown.

To be clear, it is not just heating, it is literally making the part glow in spots and then letting it cool. The temperature needs to be high enough so that the compressive yield strength of the material is lower that the force exerted by the thermal expansion of the hot spot.

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I'm a litle confused. I welded the underside of a square tube and it bowed up. To correct it, wouldn't I heat the opposite side?

What's the best way to heat it?

Yes it will bow up, We used to bow are trailer frame I Beams ( sizes 10" to 16" ) upward sometimes as much as 6" in 30-40'. The heating causes the steel to shrink when it cools down the only way to avoid it is to preload the steel with pressure in the other direction. You may also try to heat the other side and bow it back using a light mist of air which will cause the steel to shrink in that area.The other thing that will help is running a higher penetrating weld at a faster speed. We used to bend I beams into a radius using only heat and water.

you can pull it back flat with a chain binder and some heat from a rose bud . I strap a chain hooked at each end with the chain binder in the center with a 4x4 block of wood under binder and heat the other side with with heat .

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Originally Posted by EmptyNester

I'm a litle confused. I welded the underside of a square tube and it bowed up. To correct it, wouldn't I heat the opposite side?

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Yes, because the length of the square tube got shorter on the side you welded. So it is trying to form an arc to relieve the stress. You can fix it with heat, force, or a combination of both. You would have to put in about as much heat as you did welding it, and then the square tube will still have two short sides and two long sides, so it will not be very square anymore, but that might not be a problem in your design. The trick is to properly design things to account for the welding and/or to use techniques to minimize distortion. Fixtures and clamps can help, too. For example if you know you are going to have things welded to both sides of a beam like that, you weld both of them at the same time, rather than one complete, then the other. On a lot of things it helps to get the entire thing tacked up before doing any welding. I know that is not always possible, and it requires a little planning, but it can really hep.

They continoulsly weld a bead to the ibeam top side in mobile home plants to prebow the beams before they build the top to make sure everything is level when they are finished. Welding down the length of the tubing or ibeam will bow it. Welding across the tubing at a right angle to it, doesn't cause as much of an issue. When you build a trailer, that's something to keep in mind, when you hang your axles. Only weld the front and rear portion of the hanger. Welding down the side is not permitted in many places. The hanger manufacturers will tell you not to weld the hangers down the side as well as it is not necessary if it is properly welded.

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Originally Posted by performance

They continuoulsly weld a bead to the ibeam top side in mobile home plants to prebow the beams before they build the top to make sure everything is level when they are finished. Welding down the length of the tubing or ibeam will bow it. Welding across the tubing at a right angle to it, doesn't cause as much of an issue. When you build a trailer, that's something to keep in mind, when you hang your axles. Only weld the front and rear portion of the hanger. Welding down the side is not permitted in many places. The hanger manufacturers will tell you not to weld the hangers down the side as well as it is not necessary if it is properly welded.

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That's just the opposite of what we practiced.Our policy was no welding across the flange only weld with it's length, The understanding from a non engineering point was that in simple terms, The molecules in the steel rush from one end of the steel fabricated channel or tube to the other, to avoid a structural failure avoid adding extra material in there path as they will hit the spot where you have welded. Thus causing a possible fracture. This is the standard that we had in our plant. Most of our trailers were a minimum of 40,000 pound GVW and up to 80,000 pounds.Most of our Frame Rails were I Beam or Fabricated Channels Mfg by AO Smith 110,000 tensile strength. Taking for granted that small loaded pieces of structural steel will handle the weld across the flange. Not knocking you just adding what i learned years ago.