Microstructural evolution of heat-resistant steel Super304H during 650 ℃ long term aging

The long-term structure stability of heat-resistant steel Super304H, which is used as superheater/reheater tubes for ultra-supercritical power plant boilers, was studied by means of SEM, TEM and 3DAP (three-dimensional atom probe). The results show that Cu-rich phase, MX and M₂₃C₆ are the main precipitates in Super304H steel. With increasing aging time, M₂₃C₆ carbide coarsens quickly and its morphology changes from separate particles to continuous precipitating at grain boundaries. The number of MX increases with increasing aging time and its average size is about 150 nm. Cu-rich phase dispersive particles precipitate in grains and their size keeps in the range of 3 to 35 nm. It is indicated that most of the strengthening effect in Super304H attributes to Cu-rich phase, with the complement of MX and partially M₂₃C₆ carbide.