SS 316LN with varying nitrogen content is subjected to thermal aging for 20,000 h at 650 °C to assess long-term structural integrity under operational conditions in nuclear reactors. While the annealing twins and heterogeneous grain evolution by bimodal division are independent of chemical composition, the type, size, and morphology of evolving secondary phases are characteristic of nitrogen concentration. Color contrast in bright field optical microscopy combined with transmission electron microscopy reveals the presence of three major precipitate phases; M 23 (C, N) 6 , Fe 2 Mo intermetallics, and Cr 2 N. Contiguously formed hetero-structures of Fe 2 Mo/Cr 2 N are quantified, and extensive formation has been observed in steels with high nitrogen content (0.14 wt. % and 0.22 wt. %). In addition to the grain boundary carbo-nitrides, the morphological derivatives of intra-granular M 23 (C, N) 6 precipitates with cube-on-cube orientation relationship such as (1) isolated cubes, (2) primary stringers/strings of cubes, (3) secondary stringers/clusters of branched primary strings, and (4) clusters of laths are identified. The stringers and laths are preferentially grown in {111}/ < 110 > γ. The growth plane and crystallographic variants of primary and secondary stringers are exclusively identified from optical micrographs. All varieties of M 23 (C, N) 6 are prominent in the sample with 0.22 wt. % nitrogen whereas, clustering of stringers and laths are absent in steels with lower nitrogen concentration. Overall mechanical properties reform by the precipitation strengthening effect due to the excess nitrogen content. Graphical abstract