Dendritic Crystallization
Tree-like branched crystal growth form occurring during the solidification of metals under highly undercooled conditions. It significantly determines the primary microstructure and the distribution of alloying elements.
Dendrites are the typical tree-like crystal forms that form when a weld seam solidifies. That's perfectly normal and happens in every weld pool. The shape and size of these dendrites depend heavily on the cooling rate. If you cool too quickly, the structure becomes finer and harder. That can make the seam brittle and increase the risk of cold or hot cracks. You need to be careful especially with higher-alloy steels. In practice, this means: don't let the weld pool cool too quickly. A calm travel speed and clean, well-tempered heat input are the be-all and end-all. If you let the seam cool too quickly after welding, that can be just as problematic as a too-hot pool.