Longitudinal shrinkage
Contraction of the component along the weld seam axis due to cooling of the heated zone. It generates high tensile residual stresses along the seam.
When welding, you heat the material locally and strongly. The hot zone wants to expand but is hindered by the cold rest and plastically compressed. During cooling, the compressed material wants to shrink again, but the cold rest holds against it. This creates tensile stresses in the weld and the component shortens measurably. Tensile stresses can cause buckling in thin sheets, and cracks in hard, quenched and tempered steels. The more heat you put in, the greater the distortion. Keep weld cross-sections small, weld quickly and without unnecessary weaving. Weld from the center outwards or in steps so that stresses can distribute. Do not clamp the part too rigidly, otherwise distortion will only get worse. For critical steels, preheating helps, and if necessary, stress-relieve the part after welding.