US EPA

4839735 

Journal Article 

Identification of sigma and chi phases in duplex stainless steels 

Llorca-Isern, N; Lopez-Luque, H; Lopez-Jimenez, I; Victoria Biezma, M 

2016 

1 

Materials Characterization
ISSN: 1044-5803
EISSN: 1873-4189 

112 

20-29 

10.1016/j.matchar.2015.12.004 

WOS:000370109200003 

The aim of this work is to find out the most suitable method for detecting and analyzing accurately the formation conditions of secondary phases, particularly Sigma-phase (sigma-phase) and Chi-phase (chi-phase) in duplex stainless steels (UNS 532205 and UNS S32750). The microstructure was characterized after a solution annealing at 1080 degrees C followed by an isothermal heating at 830 degrees C for different time ranges, ranging from 1 min to 9 h, in order to enlighten the controversial point concerning the mechanism of chi-phase nucleation in relation with the sigma-phase. Etched samples were observed using optical microscopy (MO), and scanning electron microscopy (FESEM) with a backscattered electron detector (BSE) was used on unetched samples. Compositional microanalysis (EDS) was carried out for identifying the different phases present in the steels. Sigma phase was easily observed using different etching procedures, whereas chi-phase was only clearly detected with FESEM-BSE on unetched samples. The compositional analyses showed that the molybdenum content in chi-phase almost doubles the content of this element in sigma-phase, and as a result the kinetics of nucleation and growth were also found to be remarkably faster when the alloy content in the steel is higher. In addition, chromium nitrides and carbides were also observed to precipitate as a result of the heat treatments and, in the case of the chromium nitrides, they act as a favorable site for the nucleation of sigma-phase and chi-phase. (C) 2015 Elsevier Inc. All rights reserved. 

Duplex stainless steel; Superduplex stainless steel; Sigma-phase; Chi-phase; Aging; Nucleation and growth mechanism