Welding distorts everything it touches
Differential heating and cooling, and how fabricators fight it.
Welding distorts everything it touches because heating a small region of metal to its melting point while the surrounding material stays comparatively cool forces the hot region to expand against metal that has not expanded to match. When the assembly cools, the shrinkage is just as uneven, leaving a permanent, built-in warp that no amount of care during the weld can fully prevent.
A tug of war the weld always loses
Metal expands when heated and contracts when it cools, and a weld pushes that ordinary behaviour to its extreme, taking a narrow band to melting point in seconds while metal a short distance away stays near room temperature. The hot band tries to grow into the cooler metal around it, which resists. Once the weld solidifies, it tries to shrink back while anchored to metal that never moved, and in that tug of war the shrinking weld pulls the surrounding structure slightly out of shape.
None of this means the weld was done badly. The distortion follows directly from how localised the heating had to be for the weld to form, so skill at the torch can only manage and redirect it. Part of the fabricator's job is planning where the inevitable shrinkage will go.
The warped record
A vinyl record left too close to a heater warps into a shallow, permanent dish because one side heats and softens before the other, and by the time it has cooled the uneven heating and cooling has baked a curve into its shape. A welded assembly distorts for the same reason, and once it cools, the unevenness is locked into its final shape. Pressing the record flat under weight rarely restores it, since the vinyl has taken a new set, and a distorted welded structure resists correction in the same stubborn way: straightening it usually takes real force and can introduce fresh stress of its own.
Small pulls that add up
A single weld pass may move a joint by only a fraction of a millimetre or a fraction of a degree. A structure built from many welds sums those movements, and if ten joints along a frame each pull half a degree the same way, the frame ends up five degrees out, enough to throw an assembly well outside its tolerance. Nobody signing off any single weld would have flagged it, which is why accumulated distortion so often goes unnoticed until a nearly finished assembly is checked as a whole and no longer fits.
Fabricators therefore fight distortion deliberately. They sequence the welds so that an earlier weld's pull is partly cancelled by a later one, clamp and fixture the structure rigidly enough to restrain it while it cools, or pre-bend a part in the opposite direction so that the weld pulls it back to the shape wanted. Process choice matters too, since a lower-heat process, or brazing that never melts the parent metal, leaves less distortion behind than a high-heat fusion weld on the same joint.
A thick, heavily braced structure has far more resistance to being pulled out of shape than a thin, lightly supported one, so distortion control gets the most attention on thin-walled precision assemblies. It makes a fitting end to this set. Each earlier article dealt with a gap between how a weld looks and what it is doing, and distortion is that gap made visible in the finished part's shape, a permanent record of every uneven heating and cooling cycle it went through.