Subgrain Boundary Migration
The directed displacement of low-angle grain boundaries to reduce the overall residual energy of the crystal lattice. It is an essential sub-process of recovery.
Subgrain boundary migration is a recovery process in the microstructure that can occur after welding. Low-angle grain boundaries move in the crystal lattice to lower the internal energy of the material. This happens on its own if the component stays at an elevated temperature long enough after welding, i.e., doesn't cool too quickly. If you cool the material too fast, the microstructure has no time for it, and you risk internal stresses and, in the worst case, cracks. So: after welding, don't force-cool the part, but let it cool slowly or directly apply a heat treatment, then the microstructure can settle and become more stable.