Fragmented martensite substructure

Martensite forms during rapid cooling and is naturally hard and brittle. If you further deform the material or introduce high stresses, the structure can become finer, but the risk of cracks increases significantly. When welding, this means: fast, controlled cooling is important, but don't overdo forced cooling or excessive heat input. If you want to post-treat the weld, rather do it through tempering or stress relief annealing, not by deliberately breaking down the structure. Keep cooling uniform and avoid abrupt temperature changes to maintain a stable structure.