Fe3Nb3N precipitates of the Fe3W3C type in Nb stabilized ferritic stainless steel

A Nb stabilized ferritic stainless steel with 0.45 wt.%Nb, 82 ppm C and 170 ppm N is investigated to reveal the nature of the precipitates present at 950 °C. In particular, Fe3Nb3X precipitates of the Fe3W3C type are analyzed with WDS and EELS to determine the light elements X stabilizing this phase in the steel. According to WDS on large precipitates after 500 h at 950 °C, the Fe3Nb3X phase contains 10.4 at.% N, 1.2 at.% O and 1.0 at.% C. Auger Electron Spectroscopy on the same precipitates confirms the presence of N. In addition, it is revealed that the C and O peaks observed with WDS result from surface contamination as they disappear after Ar sputtering. The presence of a N peak in the EELS spectra of small Fe3Nb3X precipitates which have formed after 6 min at 950 °C indicate that N stabilizes this phase already from the initial precipitation stage. With this analysis it is demonstrated that N is an effective stabilizer of Fe3Nb3X precipitates in ferritic stainless steels. The formation of this phase should therefore be considered when predicting the precipitation behavior of Nb in industrial Nb stabilized ferritic stainless steels containing residual N.

► The precipitation in Nb stabilized ferritic stainless steel at 950 °C is investigated.
► We characterized the Fe3Nb3X precipitates with SAED, EELS, WDS and AES.
► We found that Fe3Nb3X precipitates are stabilized by N and not by C or O.
► This insight is new and important for future development of this type of steel grade.