Welding square tubing - Warping!

Update : so today the temperature was a bit more bearable so I went ahead on the second bench, tacked the whole thing together and then started welding it up...took some of the advice given here and took my time and paused between welds, staggered the welds and took special care on the direction of weld to predict warping...to be honest I was not convinced the bench will end up straight, and at one point during welding it started to distort a bit.  But in the end after aĺl welds were finished it came out straight, so I'm a happy man now! Hope I can replicate this for the remaining final bench 

Just five minutes after J.S. had posted the pictures, only the top half inch was visible, the rest was green & yellow horizontal stripes, then a few minutes later just "'X's".

I've seen them today, nice one John.

Regards, Bear..

When I did my welding courses we were shown that when welding two flat plate edges together the joint progressively closes, so tack both end and work inwards!

"... but 50*50*2 is roughly twice as flexible as 40*40*3"

Is this observation relating to flexibility in bending? Or some other loading type (e.g. torsion)? Or localised loading on the thinner 2mm plate?

I ask since resistance to bending is a function of the flexural rigidity EI. For the same material, the comparison is formed by the ratio of the second moments of area of the section - (bo^4 - bi^4)/12 for a square tube. For the ratio of SHS second moments of area for the dimensions quoted, this becomes (50^4 - 46^4)/(40^4 - 34^4) = 1.45. It appears that the 50x50x2 will be 45% stiffer in bending than the 40x40x3 section.

Oh please, enough is enough, not another load of formulas.

Tony

So far as I understand, not that I do much welding, local expansion occurs when the weld is taking place and freezing locks the material into that configuration. The weld area then cools and contracts which introduces the stresses that cause the distortion. I've seen fitters remove large bearing outer journals from printing machines by running a weld around inside of the journal. Subsequent freezing, and cooling down to room temp contracts the bearing sufficiantly for it to tap out.

regards Martin

Well after reading through advice on here and other welding forums etc what I gathered is that distortion is always going to happen no matter what material dimensions or thickness you use. You can reduce it or make it work for you by work methods such as clamping direction of weld etc. I'm not going into formulas and equations - that was ages ago since I last had to use any and if it comes to have to use any for doing the job then I'd rather pass and do nothing!

All I can say is that my second attempt in welding 50x50x2 section tubing resulted in no distortion at all - or to be more accurate I made distortion work for me and ended with a straight bench...

Go on then Chris, please tell us what you found to work for you.

Pete

When I weld trolleys for my Stationary Engines I clamp my box section, usually 4 x 2, to my steel topped work bench and I'm sure this acts as a large heat sink because I've never had any distortion from welding. I usually use a stick welder and sometimes a Mig welder if the section is smaller.

Martin P

Hey Pete, nothing that you guys didn't tell me before...took your leads and applied them with a good result.

I started out with side frames of the bench - tacked them and made sure the angles were right then clamped them as tight as I could and did the full welds - I only did the vertical welds (butt welds) so each tube has two opposing sides welded (omitted the inside corner welds)

I found that the most important thing to limit distortion is where you start the weld ie. the direction of the weld, so I start from the edge working my way in, then at the diagonally opposite joint I do the same and so on. The distortion forces will all want to bend the tube inwards as the welds are all in the same direction and that balances out...at least that's what I think happens!

After I finished the sides I tacked the rest of the bench together and after I was satisfied that all was true I stared the full welds, going alternate opposite welds in a zig zag pattern - here the direction of welding was always towards the edges , so if I'm welding the sections from the bench centre line to the left I did them from right to left and those from the center line to the right I did them from left to right.

Does it make sense? I mean it's a bit difficult to explain through typing! and btw I'm a green welder so I'm not pretending I invented or discovered anything, just saying what worked for me, infact I could be saying a lot of nonsense! Tomorrow I intend to assemble the third and final bench, if it comes straight than I can confirm I'm doing it the right way and not a coincidence

Very good explanations there Chris. I'm sure that any who search for the info in future should find it useful too.

Pete

It's not the heat that causes warping, it is the weld contracting. It took me years to understand that and be able to use it.

Gordon has it right. If you have a weld pool freezing at 1000°C for example then all the way down to room temperature that cooling metal is contracting. If you have a large weld pool or a material with a large coefficient of linear expansion the contraction is larger and the movement due to welding increases. Stainless steel moves a lot more than mild steel as its thermal expansion is close to twice that of the mild steel. Clamping will not hold back the distortion since the forces can be in tonnes of force. Its like clamping a spring in a stressed position, when the clamps are released the part moves to the lowest stress position it can take up. The order of welding, the size of the weld pool and the interpass cooling that takes place with multi pass welds all have an effect.

Putting large welds on one side of a plate will cause it to bend no matter what clamping is applied beforehand.

The gap between the two items above is purely due to weld distortion. The drawing is being changed to make the plate thicker and machine flat after welding.

Martin C

Clamping I think is useful for thin walled material, say a 2mm thick sheet, if it's clamped to a table with a 1/2" steel plate surface, it will keep its shape after cooling or at least warping will be slight. What you have there in your photo is a fairly thick plate Martin, so I don't think clamping would be an option....In my case clamping 2mm thick tubing worked fine with minimal warping. From my limited experience all can say is for thin walled tubing best way to go is tack everything together and adjust until all angles are square, then weld diagonally opposite joints taking care of the weld direction.