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. Nbx(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 Fe2Nb and Fe3Nb3X x(with X = C or N) precipitates form. Fe2 Nb is observed at 1073 K (800 °C). Fe3Nb3Xx 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 Nbx(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 Fe3Nb3Xx is observed in the neighborhood of a dissolving Nb2 (C,N) precipitate. For sections of grain boundaries where no Nbx(C,N)y precipitates are present, Fe3Nb3Xx does not form. It is concluded that the precipitation of Fe3Nb 3Xx is directly related to the dissolution of Nb 2 (C,N) through the redistribution of C or N. © 2011 The Minerals, Metals & Materials Society and ASM International.status: publishe