Grain-Refined Ferrite

Fine-grained ferrite forms during steel production by rolling in a specific temperature range. This makes the structure fine-grained, which makes the steel tougher and stronger.

When welding, you have to be careful not to destroy this fine structure again. If you introduce too much heat or cool too slowly, the grain coarsens again and the steel loses its good properties.

So keep the heat input low and don't let the weld cool too slowly. Then the structure in the heat-affected zone remains fine and the steel retains its toughness.