Thread: Flattening a steel plate

Originally Posted by Graziano

I used welds to straighten a 4.2m length of 5mm thick 100x50mm RHS that I'd welded eleven purlin brackets onto. I'd welded the brackets on one side and the bow was pretty noticeable when I'd finished, so I turned it over and used the stick welder to make a hot wide arc with a large air gap, this used very little filler rod and ran it side to side to give about a 10mm wide line opposite the bracket welds.

In other words it deposited little filler rod but did heat a 10mm wide strip but the same length as the welds on the other side to the welds. I started opposite the middle bracket then went along and did opposite every second bracket working towards both ends out from the middle. By the time I reached the ends and let it cool between welds it had been pulled very straight. Also it was hardly noticeable after painting.

Originally Posted by WelderMick

Good to know! I was assuming you'd need to deposit a fair bit of weld on the theory that more molten metal = more contraction, or at least get real deep penetration - so is it more the heat input that makes the difference? Makes sense in your case that you would balance the welds. Cheers - Mick

Working on the same principle as Graziano, you can also use oxy/acet to heat up rather than welding to the same effect, I quite often use the oxy/acet set to straighten beams and frames that I've welded.

Originally Posted by WelderMick

Good to know! I was assuming you'd need to deposit a fair bit of weld on the theory that more molten metal = more contraction, or at least get real deep penetration - so is it more the heat input that makes the difference? Makes sense in your case that you would balance the welds. Cheers - Mick

It's definitely the heat that causes shrinkage of the steel, they use oxy all the time when fabricating steel to straighten stuff.