Precipitation in Nb-Stabilized Ferritic Stainless Steel Investigated with in-situ and ex-situ Transmission Electron Microscopy

A Nb-stabilized Fe-15Cr-0.45Nb-0.010C-0.015N ferritic stainless steel is studied with transmission electron microscopy (TEM) to investigate the morphology and kinetics of precipitation. Nb_x(C,N)_yhbox{Nb}_{x}hbox{(C,N)}_y and MnS precipitates are present in the steel in the initial condition. Ex-situ TEM analysis is performed on samples heat treated at 973 K, 1073 K, 1173 K, and 1273 K (700 °C, 800 °C, 900 °C, and 1000 °C). Within this temperature range, both Fe₂Nbhbox{Fe}_2hbox{Nb} and Fe₃Nb₃X_xhbox{Fe}_{3}hbox{Nb}_{3}hbox{X}_{x} (with X = C or N) precipitates form. Fe₂hbox{Fe}_2Nb is observed at 1073 K (800 °C).   Fe₃Nb₃X_x;hbox{Fe}_{3}hbox{Nb}_{3}hbox{X}_{x} precipitates form at the grain boundaries between 973 K and 1273 K (700 °C and 1000 °C). Up to at least 1173 K (900 °C) their fraction increases with time and temperature, but at 1273 K (1000 °C) they lose stability with respect to Nb_x(C,N)_y.hbox{Nb}_{x}hbox{(C,N)}_{y}. With in-situ TEM, no phase transition is observed between room temperature and 1243 K (970 °C). At 1243 K (970 °C) the precipitation of Fe₃Nb₃X_xhbox{Fe}_{3}hbox{Nb}_{3}hbox{X}_{x} is observed in the neighborhood of a dissolving Nb₂hbox{Nb}_2(C,N) precipitate. For sections of grain boundaries where no Nb_x(C,N)_yhbox{Nb}_xhbox{(C,N)}_y precipitates are present, Fe₃Nb₃X_xhbox{Fe}_3hbox{Nb}_3hbox{X}_{x} does not form. It is concluded that the precipitation of Fe₃Nb₃X_xhbox{Fe}_{3}hbox{Nb}_{3}hbox{X}_x is directly related to the dissolution of Nb₂hbox{Nb}_2(C,N) through the redistribution of C or N.