High Temperature Precipitation Behavior of High-Nitrogen Duplex Stainless Steel

Precipitation behavior of high-nitrogen duplex Fe-24Cr-7Mn-4Ni-4Mo-0.43N stainless steel aged at 850 degrees C was investigated using scanning transmission electron microscopy. Based on the analyses of selected area diffraction patterns, four kinds of precipitates (intermetallic sigma (sigma) and chi (chi), Cr2N and secondary austenite) were identified. At the ferrite/austenite phase boundary, the a phase and secondary austenite were formed via alpha -> gamma + sigma eutectoid reaction. The precipitation of Cr2N occurred at the austenite grain boundary as well as the interior of the ferrite. The intermetallic chi phase also formed within the ferrite and showed a cube-cube orientation relationship with the ferrite. Further aging produced a lamellar structure composed of Cr2N and austenite along the ferrite/austenite boundary and enhanced the precipitation of the chi phase. The crystallographic features of the precipitates were also examined in terms of the orientation relationship with the austenite or ferrite matrix.