Fine Scale Microstructure in Cast and Aged Duplex Stainless Steels Investigated by Small Angle Neutron Scattering

摘要

Small angle neutron scattering (SANS) allows clustering phenomena to be studied in systems for which the constituent atoms do not differ greatly in atomic number. This investigation used SANS to characterize the fine scale microstructure in two cast and aged duplex stainless steels; aging times extended up to eight years. The steels differed in ferrite content by about a factor of two. The scattering at lowest q was dominated by magnetic scattering effects associated with the ferrite phase. In the range 0.025 less than or equal to q less than or equal to 0.2A sup -1 , additional scattering due to a precipitating phase rich in Ni and Si was observed. This scattering was rather intense and revealed a volume fraction of precipitate, in the ferrite, estimated to be 12 to 18% after long time aging. After about 70,000 hours at 400 sup 0 C, there were about 10 sup 18 precipitate particles per cm sup 3 some 50A in mean diameter, and they were distributed in a nonrandom manner, i.e., spatially, short-range-ordered. This investigation suggests that after aging some 70,000 hours at 400 sup 0 C, the precipitate in the ferrite phase is undergoing Ostwald ripening. The present data are insufficient to indicate at what time this ripening process began. (ERA citation 11:047921)