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. \(\hbox{Nb}_{x}\hbox{(C,N)}_y\) 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 \(\hbox{Fe}_2\hbox{Nb}\) and \(\hbox{Fe}_{3}\hbox{Nb}_{3}\hbox{X}_{x}\) (with X = C or N) precipitates form. \(\hbox{Fe}_2\)Nb is observed at 1073 K (800 °C). \(\;\hbox{Fe}_{3}\hbox{Nb}_{3}\hbox{X}_{x}\) 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 \(\hbox{Nb}_{x}\hbox{(C,N)}_{y}.\) 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 \(\hbox{Fe}_{3}\hbox{Nb}_{3}\hbox{X}_{x}\) is observed in the neighborhood of a dissolving \(\hbox{Nb}_2\)(C,N) precipitate. For sections of grain boundaries where no \(\hbox{Nb}_x\hbox{(C,N)}_y\) precipitates are present, \(\hbox{Fe}_3\hbox{Nb}_3\hbox{X}_{x}\) does not form. It is concluded that the precipitation of \(\hbox{Fe}_{3}\hbox{Nb}_{3}\hbox{X}_x\) is directly related to the dissolution of \(\hbox{Nb}_2\)(C,N) through the redistribution of C or N.