Grain boundary strengthening
Grain boundary strengthening increases mechanical strength by reducing grain size in metals. Grain boundaries act as obstacles to dislocation movement of atoms, which increases strength.
Physical-Chemical Mechanism of Action
Grain boundary strengthening is based on the fact that smaller grains in metals have more grain boundaries. These boundaries act as insurmountable obstacles to the movement of dislocations (atoms) within the crystal. The more grain boundaries present, the harder it is for atoms to shift, which increases mechanical strength.
Metallurgical Relevance & Welding Risks
Excessively large grain size can lead to lower strength and increases the risk of cracking and distortion. In the weld, it is important to keep the grain size in the structure as small as possible to ensure high toughness. However, too rapid cooling can lead to uneven grain size and coarsen the structure.
Workshop Practice & Action Instructions
To maximize grain boundary strengthening, controlled cooling should be ensured during welding. This can be achieved by using cooling water or special cooling media. The cooling rate should be set so that the structure cools uniformly and finely. Too fast cooling (e.g., cooling time t8/5 < 5 s) can lead to uneven grain size and a dangerous martensitic structure, while too slow cooling (t8/5 > 25 s) coarsens the structure and reduces toughness.