Dislocation annihilation
Dislocation annihilation is the mutual cancellation of dislocations of opposite sign at elevated temperature. This lowers the dislocation density in the crystal lattice. Result: hardness and strength decrease, toughness increases.
Physical-Chemical Mechanism of Action
Dislocations are line-shaped lattice defects. They form during plastic deformation. At high temperature, atoms get enough energy to rearrange. Dislocations can glide and climb. When two dislocations of opposite sign meet, they annihilate – they disappear. It's like plus and minus: they cancel each other out. The stored strain energy in the lattice is released. The lattice becomes more regular.
Metallurgical Relevance & Risks in Welding
When welding, a heat-affected zone forms. There, the base material is briefly heated. In cold-worked steels (e.g., cold-rolled sheet), the heating causes dislocation recovery. Exactly in the transition zone between the weld and unchanged base material, a soft zone forms. This soft zone has lower strength than the rest. Under load, deformation or fracture can occur there. This is a typical problem when welding cold-worked materials. Also in multi-pass welding, the heat from the next pass can trigger recovery in the previous pass. Hardness drops.
Shop Practice & Action Instructions
• Before welding, check: Is the base material cold-worked? (E.g., by cold rolling or drawing.)
• If yes: Plan for a soft zone. Do the strength verification with reduced values in the HAZ.
• Limit weld heat: Choose low heat input (e.g., short arc times, low heat input, avoid preheating if not needed). Less heat = less recovery.
• If possible: After welding, do a heat treatment to homogenize the entire structure – but then keep in mind that the overall strength drops, not just locally.
• Alternative: Use a material with a higher recrystallization temperature or replace the cold-worked condition with precipitation hardening.