Primary ledeburitic structure
A coarse microstructure originating from melt solidification consisting of primary austenite dendrites within the eutectic, typical for high-alloy tool steels and unalloyed cast irons. It requires subsequent forming or annealing steps for grain refinement.
The primary structure, as it exists directly after the melt solidifies, is coarse and irregular. You especially get this when the weld cools too quickly and the crystals can form unhindered. This coarse structure is bad for mechanical properties. It is brittle and prone to cracking. If you don't post-treat the part, it remains susceptible. To get the structure back under control, you must normalize or stress-relieve the component after welding. This makes the structure fine-grained again and increases toughness. The exact temperatures and times depend on the material, but slow cooling after heat treatment is always mandatory.