Interfacial Recrystallization
The nucleation of new, stress-free grains preferentially at existing phase or grain boundaries. This accelerates microstructural transformation under thermal influence.
Interfacial recrystallization basically means: during welding, new, often finer grains form at the grain boundaries. This happens when the material gets enough heat again after welding. If you achieve this in a controlled way, you get a more homogeneous structure and the part becomes tougher and has less residual stress. However, if it happens uncontrolled, the structure can become non-uniform. Then you have soft and hard spots next to each other, and that's exactly where it tends to crack. So you don't want extreme cooling or heat buildup right after welding. In practice, this means: control the heat input so that the part doesn't cool down suddenly. When welding thick parts, it's often better not to quench the weld immediately in ice water, but to let it cool evenly. A clean, calm weld pool and no overheated melt are the be-all and end-all.