A flange is a flat plate welded to the end of a tube, and welding puts a great deal of heat into a small area. The metal around the weld expands while the surrounding plate stays cooler, and when everything returns to room temperature the contraction does not reverse evenly. The plate bows.
Thin flanges are worse. With little material to absorb the strain, the bowing shows up as a gap at the bolt holes or a face that no longer sits flat against its partner. On an exhaust pipe, that gap is a leak before the engine has even warmed up.
The most common pattern is a flange that curls away from the weld side, so the face tilts. Bolting it to its partner pulls the face back and loads the tube instead. The joint may seal cold and then leak hot, or it may hold at the bolts and leak between them as the plate returns to its warped shape.
A second pattern is bowing across the whole face in one direction, like a shallow dish. It is less obvious on the bench and easier to find with a straight edge laid across the face. If you can see light under the edge near the bolt holes, the flange has already moved.
Sequence matters more than anything else. Welding all the way around in one pass concentrates heat on one side and guarantees a pull in that direction. Stitching the weld in short sections on opposite sides, letting each cool, distributes the strain and keeps the plate flatter.
Heat input matters just as much. On thin material the risk is burning through and adding distortion at the same time. This is one reason TIG stainless steel welding suits flange work: the operator controls the current and the travel speed closely, and can back off before the plate is over-heated.
Holding the flange flat while it is welded is the simplest control there is. Bolting the flange to a heavy flat plate, or clamping it to a matched dummy, keeps the face registered while the weld cools. Tack welds placed opposite each other hold the relationship until the final passes are run.
Backing the weld with a chill bar or a copper plate pulls heat out of the joint. The faster the area cools, the less time there is for the surrounding material to be dragged out of shape. Purging the inside also protects the root from oxidation, which indirectly helps because a clean root needs less heat on the second pass.
A thicker flange resists bowing better than a thin one, and a flange with a raised face keeps the sealing surface away from the weld. Where weight is critical, recessing the weld into a pocket so that heat starts further from the face is a common compromise between mass and flatness.
Spacing the bolt holes well out from the tube also helps, because the plate then has material between the weld and the load path. A flange whose bolts sit close to the weld has nowhere to spread the strain, and it will move no matter how carefully it is welded.
Lay a straight edge across the face and look for gaps, then check the same face against its partner before bolting. A flange that rocks on a flat surface has already bowed. Doing this check at the bench saves pulling the part off the car later to chase a leak.
After bolting up, check that the tube has not been pulled out of line. If the flange needs force to close a gap, the load goes into the weld and the tube, and a crack at the flange root is the usual long-term result. A joint that bolts up by hand is one that was made correctly.
