Primary austenite dendrite
The tree-like branched crystal forming as the primary microstructural constituent during cooling below the liquidus temperature in hypoeutectoid steels.
When the melt cools, austenite dendrites first form in the steel, growing like small trees from the crystallization front into the liquid phase. How fine or coarse this structure turns out depends mainly on the cooling rate. The faster the melt solidifies, the finer the dendritic structure, which usually has a favorable effect on toughness. For practice, this means: if you want tough weld metal, you have to keep an eye on heat dissipation so that the melt solidifies quickly. Slow cooling in the critical range, on the other hand, promotes coarse dendrites, which can make the structure more brittle.