Evaluation of the low corrosion resistant phase formed during the sigma phase precipitation in duplex stainless steels

Abstract

The duplex stainless steels, having a volumetric fraction of 50% ferrite and 50% austenite, conciliate high corrosion resistance with good mechanical properties. But, in many circumstances different phase transformations may occur, such as that responsible for sigma phase precipitation, which make the steel susceptible to localized corrosion. During the sigma phase precipitation a new austenitic phase is formed with a very low corrosion resistance. In the present research the composition of this new austenitic phase was evaluated in four duplex stainless steels, with different Mo, N and Cu contents. After the solution anneal at 1050 °C, samples of these steels were aged at 850 °C during 1 h and 5 h for sigma phase precipitation. Using the ferritoscope and an image analyzer it was possible to determine the volumetric fractions of ferrite and sigma phase, respectively, while those of austenite and the new austenitic phase were determined by difference to 100% volume. Finally, by using mass balance it was possible to determine theoretically the composition of the new austenitic phase. This phase is poor in Cr and Mo free, which explains its poor corrosion resistance

Sigma Phase Formation and Embrittlement of Cast Iron-Chromium-Nickel (Fe-Cr-Ni) Alloys

“…These fine, dispersed carbides contribute to the creep strength of the alloy. The most susceptible site for the formation of sigma phase is the grain boundaries for its high-energy [42]. In other words, the smaller the size of the area the greater tendency of the formation.…”

Sigma Phase in Superduplex Stainless Steel: Formation, Kinetics and Microstructural Path

“…This assumption was adopted in this work. Several works about this subject are found in literature [1][2][3][4][5][6][7][8][9][10] . These works based on sigma phase kinetics generally considered a theory by Johnson and Mehl 11 , Avrami [12][13][14] and Kolmogorov 15 , the JMAK theory or JMAK modified which may be written as:…”

“…Therefore, in this work it will be studied the formation of sigma phase in SDSS in isothermal aging at 700ºC, 750ºC and 800ºC. Several authors 2,6,8,22 found around of 850ºC as the temperature of maximum sigma phase kinetics. We selected temperatures below of 850ºC.…”

Characterization of thermal oxide films formed on a duplex stainless steel by means of confocal-Raman microscopy and electrochemical techniques

“…After that (Figures 2h to 2l), the amount of ferrite decreases and the colour of most of the sample becomes bright blue. This decrease in ferrite oxides can be related with the drop in the ferrite phase as a consequence of the formation of new intemetallic phases [17][18][19].…”