Contiguous Hetero-structures & Co-existing Morphological Derivatives of Preferentially Grown Carbo-nitrides in Long Term Aged SS 316LN with Varying Nitrogen Concentration

TL;DR

This study investigates the long-term microstructural evolution of nitrogen-alloyed SS 316LN steel under thermal aging, revealing nitrogen-dependent precipitate phases, hetero-structures, and morphological derivatives that influence mechanical properties.

Contribution

It provides detailed characterization of nitrogen-induced secondary phases and morphological derivatives in aged SS 316LN, highlighting the role of nitrogen in microstructural evolution and strengthening.

Findings

Presence of three major precipitate phases: M23(C, N)6, Fe2Mo intermetallics, Cr2N.

Extensive formation of hetero-structures in high nitrogen steels.

Identification of morphological derivatives of M23(C, N)6 with specific crystallographic orientations.

Abstract

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.