Distortion help please

[Noods]

Hi All

I was welding a rail made from 3mm steel the other day.. It's a metre long by 6cmsx4cms.. Ran a butt weld of approx a metre, came out luvly.. But, the rail bent like a banana... Is there anything I could have done to have stopped it.. I'd tacked it first every 10 cms or so.. Should I have clamped it to my bench?
Thanks...

 

[the snooper]

probably done it in stages leaving to cool inbetween

 

[Tinbasherdan]

Weldin stages, one end then the other and vice versa till you meet in the middle. Depending on how distorted it is you might be able to pull it back with a bit of heating and cooling

 

[hotrodder]

Things expand when they get hot and contract as they cool. If a piece of metal is layed on a table it will grow in all directions when heated and shrink evenly as it cools. Clamp it in a vice and when heated it can't expand in all directions so it expands more in the directions it can. As it cools it shrinks evenly in all directions and changes shape as a result. This can also be used both to straighten stuff (as said) and also to shape things for example https://books.google.co.uk/books?id...wgaCh1BGgpY#v=onepage&q=flame bending&f=false

Welds shrink

Skipping about and/or letting things cool can reduce distortion a chunk* but won't necessarily reduce it to acceptable levels. Clamping things down can further reduce distortion but won't guarantee it'll stay straight when the clamps are removed, even if you let it cool completely first. A lot depends on the shape of the part and where the welds are... sometimes you have to 'preset' stuff before welding i.e. bend it slightly in the opposite direction so it ends up straight. Various examples in the first five hits from googling twi job knowledge distortion Searching the internet for welding neutral axis will also get a bunch

* Strength drops as temp goes up so the part is less able to resist the shrinkage forces

 

[Richard.]

As said above you can control distortion to a point but not guaranteed to stop it completely. Half the problem is you've chosen the worst possible process for the job. Tig is a slow process which generates a lot of localised heat. The way I see it you've got 2 good options to help with trying to reduce it. Mechanical restraint (clamp it down) and reduce local heat input. (Pick up the current and weld at a decent travel speed.) don't reduce your current thinking that will reduce heat input as it will only force your travel speed down and make the problem worse.

 

[Kent]

Hi All

I was welding a rail made from 3mm steel the other day.. It's a metre long by 6cmsx4cms.. Ran a butt weld of approx a metre, came out luvly.. But, the rail bent like a banana... Is there anything I could have done to have stopped it.. I'd tacked it first every 10 cms or so.. Should I have clamped it to my bench?
Thanks...

Confused you mean 3mm wall rhs? But then I can't see why a 1mtr butt weld?
Everything can be negated but depends on what your trying to do and that's were I am stuck. I just don't understand the task at hand
Limit heat input by using a faster process? Hold rigid in a fixture, backstep the weld. Chain stitch or similar. Weld order
After all that we have flame straightening and big hammers
Don't laugh at the last one I have gone to great big two handed sledgehammer in the past as part of a procedure (demount chassis for gpo)

 

[Mr shiney]

I would have used MIG on something like that. MIG is a fabrication tool ideally suited to what you are doing. TIG is a beautiful art form and suited to small accurate jobs.