Heterogeneous nucleation
Nucleation occurring preferentially on existing surfaces such as foreign particles, grain boundaries, or mold walls. It requires significantly less activation energy than spontaneous nucleation in a pure melt.
Heterogeneous nucleation happens at impurities, grain boundaries, or the surface. The crystals need less energy to start there, so the melt solidifies faster and more unevenly. The risk is that you get a coarse or non-uniform structure and pores form because the crystals don't grow together cleanly. In practice, this means: keep the weld pool clean, remove scale and dirt before welding. If you don't control the cooling, the structure quickly becomes brittle and crack-prone. Clean material and calm, controlled welding prevent most problems.