Formation of oxides particles in ferritic steel by using gas-atomized powder

Oxides dispersion strengthened (ODS) ferritic steel was prepared by using gas-atomized pre-alloyed powder, without the conventional mechanical alloying process. By adjusting the volume content of O2 in the gas atmosphere Ar, the O level in the ferritic powder can be well controlled. The O dissolves uniformly in the ferritic powder, and a very thin layer of oxides forms on the powder surface. After hot deformation, the primary particle boundaries, which retain after sintering, can be disintegrated and near fully dense materials can be obtained. The oxide layer on the powder surface has a significant effect on the microstructural evolution. It may prevent the diffusion in between the primary particles during sintering, and may dissolve and/or induce the nucleation of new oxides in the ferritic matrix during recrystallization. Two kinds of oxide particles are found in the ferritic steel: large (∼100 nm) Ti-rich and fine (10–20 nm) Y–Ti-rich oxides. The hardness of the ferritic steel increases with increasing annealing temperatures, however, decreases at 1400 °C, due to the coarsening of precipitates and the recrystallization microstructure.