Coherent to incoherent transition of precipitates during rupture test in TP347H austenitic stainless steels

Abstract

Precipitation of various particles and their growth during rupture test have been investigated in TP347H austenitic stainless steels using a transmission electron microscopy. Various precipitates of MnS, Nb-rich MC, and MnS+MC and MnS+M2P complexes are observed in the gamma matrix after rupture test at 750 degrees C. The MnS particles formed independently in the gamma matrix show a coherency or semi-coherency with the gamma matrix. The Nb-rich MC carbides show also a coherency with the gamma matrix. The Nb-rich MC carbides showing a semi-coherency with the MnS also form on the surface of the coherent or semi-coherent MnS particles, and they show a cube-cube orientation relationship with the MnS particles. The MnS + MC complex loses the initial coherency with the gamma matrix, as the MC in the complex grows. The Nb-rich M2P precipitates formed on the surface of the MnS particles do not show an orientation relationship with the MnS particles or the gamma matrix. The MnS particles in the MnS+M2P complex hold the initial coherency with the gamma matrix. Effects of MnS precipitation followed by the formation of the complexes on rupture life of the TP347H austenitic stainless steels are discussed from the viewpoint of MnS precipitates acting as sinks of free sulfur segregating to the grain boundaries. (C) 2016 Elsevier Inc. All rights reserved.