Phase equilibrium
A thermodynamic state in which the chemical potentials of the participating phases no longer change over time and no driving forces for net mass transfer exist. It forms the theoretical basis for evaluating constitutional diagrams.
Phase equilibrium simply means that no more transformation is taking place in the material because the temperature is uniform everywhere. In welding, this is important because the structure in the weld and heat-affected zone depends on how fast the material cools and whether it cleanly passes through the critical transformation points in between. If you don't watch out, the structure will flip – you'll get hard, brittle martensite or coarse grain, and the part will tear under load. Keep the interpass temperature in range and don't let the component cool too fast, then the structure stays calm and you won't have any nasty surprises.