Creep damage is the slow plastic flow of metal under stress and at high temperature, typically about 50% of the absolute melting temperature. The result is a very slow viscous flow of the metal which ends in sudden failure. This problem occurs in alloy steels that have been used in steam generators, turbines, and pipelines in power plants operating at high temperatures, typically in the range 500°–600°C, and under stress for an extended period of time. Creep failure occurs by a process of cumulative damage which involves plastic deformation, nucleation and growth of cavities at the grain boundaries, subsequent linkage of these cavities to form microcracks, and the propagation of these microcracks until failure.