A weld heats a small region of a much larger structure. That region expands while hot, but surrounding colder material restrains it. During cooling it contracts again under restraint. This nonuniform thermal strain can leave both distortion and residual stress after the weld returns to ambient temperature.

A simple free thermal strain would be

$$\varepsilon_{th}=\alpha\Delta T.$$

If that strain is constrained, internal stress develops instead of free expansion or contraction.

Common distortion modes include angular change, longitudinal shrinkage, transverse shrinkage and buckling of thin panels.

Residual stress can remain even when external force is zero. Tensile residual stress near a weld can affect fatigue, brittle fracture and stress-corrosion behavior.

Control strategies include:

• reducing unnecessary heat input;
• balancing weld placement and sequence;
• using fixtures or preset geometry;
• minimizing excess weld volume;
• applying suitable preheat or post-weld heat treatment when material and design permit.

For example, welding only one side of a thin plate joint produces an asymmetric thermal cycle and can rotate the plates toward the weld. A balanced sequence on opposite sides can reduce net angular distortion.

Distortion and residual stress are therefore consequences of compatibility between thermal expansion, plastic deformation and structural restraint, not merely workmanship defects.