Quantitative Description of Duplex Stainless Steels Microstructure Using Selective Etching

Abstract

The properties of duplex stainless steels (DSSs) depend on the ferrite–austenite ratio, on the content of secondary phases and on the contamination with non-metallic inclusions. To assess the quality of DSSs, it is necessary to use an integrated approach which includes controlling for the volume fraction, the morphology and the distribution of all phases and non-metallic inclusions. Samples of several grades of DSSs were obtained using various heat treatments, such as solution annealing and quenching from 1050 to 1250 °C to obtain different amounts of ferrite and to provoke annealing at 850 °C to precipitate σ-phase. As a result, a metallographic technique of phase analysis in DSSs based on selective etching and subsequent structure parameters estimation according to ASTM E1245 was developed. We demonstrated that the developed method of quantitative analysis based on selective etching and metallographic analysis according to ASTM E1245 allows us to obtaining much more accurate results, compared to the point count method described in ASTM E562 and to the XRD method.

Development of a Methodology for the Quality Management of Duplex Stainless Steels

“…The simplest and most effective method is to use an automatic image analyzer, but this requires obtaining a phase-contrast image. Various authors suggest a variety of etchants [ 31 ], but a later analysis [ 30 ] carried out by our team showed that even the use of electrolytic etching ( Figure 2 a) does not allow sufficient phase contrast to be obtained for automatic analysis.…”

“…Today, traditional tools for the development of alloys, which are also used to improve the quality of DSSs, include thermodynamic modeling [ 30 ], which has been successfully used for many applications, along with the use of various quantitative methods for structural evaluations to compare simulated and experimental results [ 31 ]. We considered the applicability of these methods to the experimental steels.…”

“…Beraha (20 mL of HCl, 80 mL of water, and 1 g of K 2 S 2 O 5 ) is the most effective etchant for DSSs [ 30 ], which makes it possible to reveal the precipitation of both the main phases—austenite and ferrite ( Figure 2 b)—and the secondary phases, i.e., the sigma phase ( Figure 2 c), chromium nitrides, and Laves phases ( Figure 2 d). The images obtained after etching with Beraha are quite contrasting, and allow us to evaluate all phases in DSSs [ 30 ], while also making it possible to estimate the volume content, average and maximum sizes, distribution density, and other metric parameters. The images obtained using Beraha are suitable for the application of automatic analysis methods provided, for example, in ASTM E 1245, which can subsequently be integrated into the quality management system during the production of DSSs.…”

“…However, in these steels, there is a clear maximum of the pitting potential E pit at 65% ferrite ( Figure 8 b). Super duplex Steel 8 with PREN 38 has no microalloying and small content of nitrogen, so there are no secondary phases [ 30 ]. The CR values increase with distance from the optimal phase composition.…”

“…Their contents decrease as the quenching temperature increases; therefore, despite the departure from the optimal phase composition, the properties increase. The behavior of E pit is also determined by the presence or absence of secondary phases [ 30 ] in the super DSSs.…”