Identification of sigma and chi phases in duplex stainless steels

• Published in: Materials characterization 2016-02, Vol.112, p.20-29
• Main Authors: Llorca-Isern, Núria, López-Luque, Héctor, López-Jiménez, Isabel, Biezma, Maria Victoria
• Format: Article
• Language: English
• Subjects: Aging; ANNEALING; CARBIDES; Chi-phase; CHROMIUM; Chromium nitride; CHROMIUM NITRIDES; Duplex stainless steel; Duplex stainless steels; ELECTRON SCANNING; ETCHING; Heat treatment; MATERIALS SCIENCE; MICROANALYSIS; MICROSTRUCTURE; MOLYBDENUM; NANOSCIENCE AND NANOTECHNOLOGY; NUCLEATION; Nucleation and growth mechanism; OPTICAL MICROSCOPY; Phases; PRECIPITATION; SCANNING ELECTRON MICROSCOPY; Sigma-phase; STAINLESS STEELS; Steels; Superduplex stainless steel
• ISSN: 1044-5803; 1873-4189
• DOI: 10.1016/j.matchar.2015.12.004

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 (σ-phase) and Chi-phase (χ-phase) in duplex stainless steels (UNS S32205 and UNS S32750). The microstructure was characterized after a solution annealing at 1080°C followed by an isothermal heating at 830°C for different time ranges, ranging from 1min to 9h, in order to enlighten the controversial point concerning the mechanism of χ-phase nucleation in relation with the σ-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 χ-phase was only clearly detected with FESEM–BSE on unetched samples. The compositional analyses showed that the molybdenum content in χ-phase almost doubles the content of this element in σ-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 σ-phase and χ-phase. [Display omitted] •Microscopy was used on heat treated duplex steels for microstructure identification.•FESEM–BSE observation on unetched samples provided the best contrast between phases.•Analyses of carbides, nitrides, chi and sigma phases were possible by EDS and WDS.•Chromium nitrides act as favorable site for the nucleation of chi and sigma phases.•Secondary phases nucleation kinetics are faster in superduplex than in duplex steels.