Microsegregation
Variations in alloy concentration within individual grains on a microscopic scale. It happens because the first solidifying metal is purer than the remaining liquid pool.
Segregation means: During cooling of the weld, certain alloying elements like sulfur, phosphorus, or carbon travel with the solidification front and accumulate at the end between the crystals. These spots melt earlier and are weaker, so the weld tears quickly under stress – these are hot cracks. You can counteract by keeping the heat input low and not laying too wide beads. Use filler materials low in sulfur and phosphorus. For austenitic steels, a small ferrite content in the weld metal helps to better absorb the harmful elements. And don't let the interpass temperature get too high, otherwise the elements will accumulate at the grain boundaries even more.