  To assess long-term structural integrity under operational conditions in
nuclear reactors, SS 316LN with varying nitrogen content is subjected to
thermal aging for 20000 hours at 650{\deg}C. 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 for nitrogen concentration. Colour contrast in bright field
optical microscopy combined with transmission electron microscopy reveals the
presence of three major precipitate phases; M23(C, N)6, Fe2Mo intermetallics
and Cr2N. Contiguously formed hetero-structures of Fe2Mo and Cr2N are
quantified and extensive formation has been observed in steels with high
nitrogen content (0.14 wt. % and 0.22 wt. %). A unique occurrence of
morphological derivatives of intra-granular M23(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>{\gamma} and have crystallographic
variants. The conclusions are drawn from bright field colour contrast optical
micrographs in the present study demonstrate the potential of light optical
microscopy to be used as a significant tool to efficiently and accurately
process huge amount of data including the preferential growth planes/directions
({111}/<110>) of morphological derivatives of M23(C, N)6 precipitates, by
analyzing the symmetry and dense planes/directions of secondary phases. Overall
mechanical properties have been reformed by the precipitation strengthening
effect arising from the excess nitrogen content.
