Twin Martensite Microstructure
A tetragonal microstructure containing internal transformation twins, primarily formed in high-carbon steels during martensitic transformation. This structure results in extreme hardness while simultaneously increasing brittleness.
Twin martensite is a hard, brittle structure that can form during rapid cooling of certain steels. If you get this in the weld or heat-affected zone, you'll quickly get cracks and can hardly machine the part anymore. The problem is the cooling. If you cool the steel too quickly after welding, this structure forms. Especially with higher-alloyed steels, you have to be careful. Let the component cool slowly and evenly, for example with a cover shell or by holding the heat. If you don't control the cooling, the material becomes rock-hard and you can forget it. Then only expensive annealing or grinding away and rewelding helps.