How do you stop distortion?

Distortion is created because metal expands when heated and contracts when cooled. The weld puddle is hot so most of the distortion takes place when the weld cools. Adjusting the fit-up, usually by leaving a gap between pieces to be welded, can help. Fixturing the piece so that it cannot expand or contract is another way. Pre heat and post heat can prevent some distortion. When installing lifting pads we would weld three or four spots on the bottom of the pad. This held it off the surface. The cooling weld deposit pressed the spots into the metal and allowed the joint to contract.