Fragmented martensite
A fine-structural transformation state of martensite where internal dislocations and substructures are disrupted by mechanical stresses or thermal effects during quenching. This locally enhances ductility within the otherwise brittle microstructure.
Fragmented martensite is basically a microstructural state that arises from internal stresses and dislocations in the crystal lattice. When welding, it's rarely the direct goal – usually you don't want to promote it, but rather avoid hard, crack-sensitive martensite forming from too rapid cooling. If the structure breaks up finely through heat management, it can improve toughness a bit, but you shouldn't overdo it, or the structure becomes coarse and brittle again. In practice, this means you have to control the cooling of the weld. If you cool too fast, you get hard martensite that cracks. If you manage heat input and interpass temperature sensibly, you can achieve a finer, tougher structure. But don't rely on fixed values – it depends heavily on the base material. If unsure, stick to the welding procedure or ask the foreman.