Screw dislocation
A line defect around which the atomic planes form a spiral ramp through the crystal lattice.
Dislocations are lattice defects in the metal that arise during plastic deformation. During welding, the heat input creates stresses that can form such dislocations and pile them up in the heat-affected zone. When they pile up at grain boundaries or inclusions, stress peaks occur that can lead to cracks—especially when hydrogen is involved. Additionally, a high dislocation pile-up makes the material brittle. In practice, this means: keep the heat input low so that not too many dislocations form. For higher-strength steels, preheating is useful to slow down the cooling and relieve stresses. After welding, stress-relief annealing helps to reduce dislocations and restore toughness.