Phase transformations during aging of a nitrogen-strengthened austenitic stainless steel

An analytical electron microscopy study was undertaken in order to characterize intergranular and matrix precipitation accompanying intermediate temperature aging in NITRONIC 50, a nitrogen-strengthened austenitic stainless steel. Extensive precipitation on most grain boundaries had occurred after aging for 24 hours at 675 °C. The primary intergranular phase at that time was Cr-rich M₂₃C₆, and energy dispersive spectra taken on grain boundary segments between these carbides indicated Cr-depletion and Fe- and Ni-enhancement relative to the matrix. After aging for 336 and 1008 hours at 675 °C, M₆C (eta-carbide) precipitates were also present on grain boundaries. These precipitates were distinguished from M₂₃C₆ on the basis of their lattice parameters and chemistries, with M₆C containing less Cr and Fe, and more Ni, Mo, and Si than M₂₃C₆. The differences in chemistry were clarified by a statistical treatment of the spectra. The statistical analysis also showed that precipitates with a range of chemistries between M₂₃C₆ and M₆C coexisted with these phases on the grain boundaries. Associated with this shift in precipitate stoichiometry was an increase in the average concentration of Cr and a decrease in the average concentration of Ni at the grain boundaries. Intergranular sigma phase was also observed after times 24 hours at 675 °C, with sigma precipitating on grain boundaries containing carbides. Intragranular precipitates observed to be stable up to 1008 hours at 675 °C included Z-phase, a complex nitride which had formed during solution annealing; M₇C₃ carbides, which nucleated at Z-phase/austenite interfaces; M₂₃C₆ carbides, which precipitated on incoherent twin boundaries; and Cr-rich MN precipitates, which nucleated on dislocations.