Effects of molybdenum on precipitation behaviours in aged cast stainless steels

The effects of Mo on precipitation behaviours in aged cast stainless steels have been investigated. Mo-free CF3 steel and Mo-bearing CF3M steel, both of which consisted of a duplex structure of ferrite and austenite phases, were prepared. Microstructural evolution in the ferrite phase during ageing at 723 K has been observed by transmission electron microscopy (TEM). Precipitates in CF3 steel were identified to belong to the G-phase and possess lattice coherency with the ferrite phase at interphase boundaries. On the other hand, precipitates in CF3M steel were found to be nanodomains of not only to the G-phase but also to another phase enriched in Mo. Some of the nanodomains containing Mo exhibited diffraction patterns having pseudo five-fold symmetry, but others exhibited regular periodicity in high-resolution TEM images. The atomic structure of the Mo-related nanodomains is proposed to be a distorted χ-phase that retains coherency with the matrix.