Diffusion-Controlled Growth
The enlargement of new phase nuclei whose velocity is limited by the mass transport of solute atoms through the surrounding lattice. This governs the kinetics of precipitation and transformation processes.
Diffusion-controlled growth means in plain terms: the atoms have to migrate first before the new phase can grow properly. If you cool too fast during welding, the atoms don't get time, and the structure remains incomplete – the material becomes hard and brittle, and in the worst case, cracks appear. To prevent this, you have to slow down and control the cooling. For higher carbon steels, that means: don't forget preheating and keep the cooling slow enough in the critical range so the transformation runs cleanly. If you control the cooling, you control the structure.