Identification of sigma and chi phases in duplex stainless steels

Abstract

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 Chiphase (χ-phase) in duplex stainless steels (UNS S32205 and UNS S32750). The microstructure was characterized after a solution annealing at 1080oC followed by an isothermal heating at 830oC for different time ranges, ranging from 1 minute to 9 hours, 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 χphase 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 σphase and χ-phase.

Experimental and Thermodynamic Study of Selected in-Situ Composites from the Fe-Cr-Ni-Mo-C System

“…Cieslak et al [26] found that χ phase can form in the solid state during high cooling rates, which occur in welds from 316 austenitic steel. Additionally, similar morphology of χ phase as in alloy 2, was reported by Llorca-Isern et al [27]. During precipitation, the χ phase consumed Cr and Mo, which causes the surrounding areas to be depleted in these elements.…”

Double Kinetics of Intermetallic Phase Precipitation in UNS S32205 Duplex Stainless Steels Submitted to Isothermal Heat Treatment

“…In this work, however, the presence of the chi phase at temperatures between 850 °C and 950 °C was not identified. This behavior could be attributed to the solubilization treatment before isothermal aging, which would cause the development of different microstructural conditions (grain size, and different morphology of the austenite and ferrite clusters), leading to changes in the kinetics 7 and favoring rapid consumption of the chi phase. Wong et al 15 showed that dissolution of the sigma phase could occur at 860 to 910 °C and at 910 to 930 °C under continuous heating.…”

“…Differences in the solution-treated state of the non-aged metal alter the duration of each transformation mechanism, due to changes in the initial grain size and the dislocations density of the base metal. The initial solutiontreated condition of the base metal plays a very important role in the kinetic behavior of the precipitation 7,16 .…”

“…Chi phase can precipitate at the α/α interface, and both sigma and chi phases also precipitate at triple α/α/γ points. Studies have proposed that the precipitation of hexagonal chromium nitride (Cr 2 N), which precedes the precipitation of chi and sigma phases, favors the precipitation of these deleterious intermetallic phases [6][7][8] . The precipitation of the sigma phase occurs according to a eutectoid reaction in which a secondary austenite with low chromium content precipitates.…”

“…As the secondary austenite has lower chromium content, its presence close to the sigma phase controls the formation of sigma. However, this secondary austenite can also be considered a deleterious phase because it presents low corrosion resistance 7 . The kinetics of phase transformations in heterogeneous materials such as duplex stainless steel can be determined using the Johnson-Mehl-Avrami (JMA) model 8,9 , described by Equation 1:…”