Crystallographic details of precipitates in Fe-22Cr-21Ni-6Mo-(N) superaustenitic stainless steels aged at 900°C

Crystallographic features of second phases and the effect of nitrogen addition on the microstructural evolution in superaustenitic Fe-22Cr-21Ni-6Mo-(N) tall in wt pet) stainless steels during isothermal aging at 900 degrees C were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Both alloys with and without nitrogen contained sigma phase and M23C6 carbides in the solution treated condition. While four phases (sigma, M23C6, M6C, and chi) of intermetallics and carbides appeared sequentially as a function of aging time in the nitrogen-free alloy, two nitrides (Cr2N and AIN) were additionally observed after long time aging of the nitrogen-containing alloy. The addition of nitrogen into Fe-22Cr-21Ni-6Mo steel promoted a finer and more uniform distribution of precipitates during isothermal aging. The exact identification and crystallography of various second phases were confirmed from the analyses of selected area diffraction patterns from various orientations, stereographic projection, and energy dispersive spectroscopy. The orientation relationships between the precipitates and austenite matrix can be summarized as follows: (1) two carbides (M23C6 and MBC): cube-on-cube orientation relationship; (2) chi phase: Kurdjumov-Sachs (K-S) orientation relationship; (3) two nitrides (Cr2N and AlN): (11 (2) over bar 0)(nitrides) //(211)(gamma) and [0001](nitrides) //[(1) over bar 11](gamma), and (4) sigma phase: (1) ((1) over bar 11)(gamma)//(00 (1) over bar)sigma and [<(11)over bar>0](gamma)//[<(11)over bar>0](sigma) or (2) ((1) over bar 10)(gamma)//((1) over bar 10)(sigma) and [<(11)over bar>2](gamma)//[113](sigma). For the sigma phase, the former orientation relationship was predominant throughout aging, and the latter orientation relationship was occasionally observed under limited aging conditions.