straightening 2" hollow square section steel

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I was gifted some 2" square hollow section steel lengths about 8ft long.they all are bent in the center, like a .5-1.5" belly from ends in one plane only.Any idea how to straighten them ? not looking for perfectly straight.I tried using a hydraulic jack and welding some brackets to hold a pair belly to belly on the ends and jack in the middle(belly) but they simply sprung backthey were used in shipping containers carrying liquids in a giant bag which fills the container, they are placed across the door end so the bag sloshing bends them.thanksMM 180
Reply:Repeat your sandwich method with heat
Reply:How straight do you really want or need them to become? In other words,  does a slight bend really matter?Sent from my SM-G996U using TapatalkMillermatic 252 MIGMiller Dynasty 200DX TIGMiller Spectrum 625 PlasmaAltas 12x36 Metal LatheBridgeport Milling Machinewww.psacustomcreations.com
Reply:

Originally Posted by lennyk

I tried using a hydraulic jack and welding some brackets to hold a pair belly to belly on the ends and jack in the middle(belly) but they simply sprung backthanks
Reply:

Originally Posted by Lis2323

when bending with the jack you will need to bend "BEYOND" straight to allow for spring back.
Reply:Rosebud.Heat the long side. It'll take a few round of cherry red. All metal shrinks when it cools.An optimist is usually wrong, and when the unexpected happens is unprepared. A pessimist is usually right, when wrong, is delighted, and well prepared.
Reply:I loves heat shrinking.........................makes me all warm and fuzzy.  Learn to heat shrink, and distortion will never rule your life again as long as you live.