Precipitation Mechanisms and Crystallographic Study of Co-precipitation Carbides in Super Austenitic Stainless Steel

Abstract

This study focuses on the microstructural evolution mechanisms of co-precipitated M23C6 and M6C carbides in S32654 super austenitic stainless steel. It is found that the crystallographic characteristics of carbides largely determine their morphology, which has an important influence on the properties of stainless steel. Therefore, an accurate description of the crystallographic characteristics of carbides is essential for an in-depth understanding of their effects on material properties. The orientation relationships between the matrix γ, M23C6, and M6C phases were determined as [-1 1 0]γ//[-1 1 0]M23C6//[-1 1 0]M6C and (1 1 1)γ//(1 1 1)M23C6//(1 1 1)M6C. The M23C6/γ interface provided favorable sites for the nucleation of M6C, and M6C/γ exhibited identical orientation relationships and interface orientation as M23C6/γ. Due to the structural similarity between M23C6 and M6C, the lattices matched well at all orientations, and no repeated habit plane was observed. Based on STEM-EDS analysis, the chemical formula of M23C6 was confirmed as (Cr17Mo3Fe3)C6. An appropriate increase in Mo content enhanced the stability of M23C6, but when exceeding a specific threshold, M23C6 will gradually transform to M6C. First-principles calculation verified the feasibility of this transformation process in terms of the system energy. This work promotes the understanding of the precipitation mechanism of multiple carbides in super austenitic stainless steel.