Effects of Mo on the Precipitation Behaviors in High-Nitrogen Austenitic Stainless Steels

Precipitation behaviors of Fe-18Cr-18Mn-0.63N and Fe-18Cr-18Mn-2Mo-0.69N high-nitrogen austenitic stainless steels during isothermally aging at 850 degrees C have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The experimental results show that precipitation displays a discontinuous cellular way and the precipitates are identified as Cr2N in Fe-18Cr-18Mn-0.63N steel. The addition of Mo makes precipitation occur not only at the grain boundary but also inside the grain and precipitation also displays discontinuous cellular way. The precipitates at the grain boundary and in the cell are both identified as Cr2N phase and chi phase and the precipitates inside the grain are identified as chi phase in Fe-18Cr-18Mn-2Mo-0.69N steel. The nucleations of chi phase and Cr2N phase at the grain boundary are both governed by the diffusion of Cr atoms. The formation and growth of chi phase inside the grain are induced by the impoverishment of N atoms with increasing aging time.