Dendritic segregation
Chemical non-uniformity between the branches of tree-like crystals and the spaces between them. It can be partially homogenized by high-temperature diffusion annealing.
When the weld cools, the edge area solidifies first, the middle stays liquid longer. Alloying elements and impurities like sulfur or phosphorus are pushed into the remaining melt and accumulate there. If this happens too quickly, these elements can no longer distribute evenly and remain concentrated in spots. These zones melt more easily and can crack under stress – that's called hot cracking. With stainless steel, local chromium depletion can also ruin the corrosion protection. You can counteract with clean filler material that has low sulfur and phosphorus. Don't run too hot or too cold, stick to the specified heat input, otherwise the structure becomes unstable. If it's critical, post-weld heat treatment can even out the differences again.