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.
Highlights
IntroductionThe duplex stainless steels are Fe-Cr-Ni alloys presenting an approximate volumetric fraction of 50% ferrite (α) and 50% austenite (γ) in their microstructure
The duplex stainless steels are Fe-Cr-Ni alloys presenting an approximate volumetric fraction of 50% ferrite (α) and 50% austenite (γ) in their microstructure. Their main feature is that they compromise the favorable corrosion resistance of the austenitic stainless steels with good mechanical properties
Notwithstanding these advantages, duplex stainless steels are susceptible to the precipitation of additional phases that affect both the corrosion and the mechanical properties
Summary
The duplex stainless steels are Fe-Cr-Ni alloys presenting an approximate volumetric fraction of 50% ferrite (α) and 50% austenite (γ) in their microstructure Their main feature is that they compromise the favorable corrosion resistance of the austenitic stainless steels with good mechanical properties. Notwithstanding these advantages, duplex stainless steels are susceptible to the precipitation of additional phases that affect both the corrosion and the mechanical properties. These phases may be formed during the solidification of the alloy or in the subsequent heat treatments or plastic deformation processes, or even due to the aging processes during its use, causing a marked effect upon the workability and the useful life of the material. The sigma phase in stainless steels is an intermetallic compound, non-magnetic[1,2], with a complex tetragonal structure, and when present has deleterious effects upon both the mechanical and the corrosion properties[3]
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