Quantitative Description of the Microstructure of Duplex Stainless Steels Using Selective Etching

: The properties of duplex stainless steels (DSSs) depend on the ferrite–austenite ratio and on the contents of secondary phases. Therefore, it is necessary to control the volume fractions, morphologies, and distribution patterns of all phases. The phases in the samples were identified using thermodynamic modeling and scanning electron microscopy. Investigated specimens were obtained after different heat treatments, such as solution annealing and quenching from 1050 to 1250 °C to obtain different amounts of ferrite and annealing at 850 °C to precipitate the σ -phase. Therefore, a metallographic technique for assessing the phases in DSSs based on selective etching and subsequent analysis according to ASTM E 1245 was developed. It was shown that the developed method of quantitative analysis based on selective etching and metallographic assessment according to ASTM E 1245 allows obtaining much more accurate results compared to the proposed ASTM E 562 method, which correlates well with the XRD quantitative phase analysis.


Introduction
The properties of duplex stainless steels (DSSs) depend on the ferrite-austenite ratio and on the contents of secondary phases [1].The actual phase content depends on the chemical composition of the steel, and heat treatment of the final product [2].Therefore, it is necessary to control the volume fractions, morphologies, and distribution patterns of all phases.The existing physical methods of assessing phase volume fractions, such as magnetic response or X-ray diffraction (XRD) quantitative phase analysis, work either with reference to certain databases to interpret the results or in very narrow determination ranges [3,4].These methods also require the preparation of additional specimens, while metallographic assessment can be carried out using only specimens made after mechanical tests.There is a wide range of metallographic standards and methods for assessing grain size [5], non-metallic inclusions content [6], and microstructural banding [7] for carbon steels.However, there are few such standards for DSSs.Therefore, the aim of this work was the development of a metallographic technique for assessing the phases in DSSs, based on selective etching and subsequent analysis according to ASTM E 1245.

Materials and Methods
In this study specimens of as-cast duplex steels of different grades were investigated (Table 1, Steels 1, 2, 3, 4).The specimens were examined after different heat treatments.The experimental steel was produced in an open induction furnace [8].The raw materials were high-purity electrical steel, metallic chromium, metallic nickel, ferromanganese-95, and ferromolybdenum-60.After heating and melting at 1485 °C, liquid steel was poured into a cylindric copper mold with a diameter of 40 mm and a height of 100 mm.The chemical composition of the experimental steels was determined by spark optical emission spectroscopy.The content of sulfur and phosphorus in all steels did not exceed 0.01%.The samples of steel were heat-treated in a programmable muffle furnace: heated to different temperatures of 1050-1250 °C with a step of 50 °C, held at each temperature for 60 min, and quenched in water.In Steel 4, the formation of a σ-phase is possible [9].To form various contents of a σ-phase, samples of Steel 4, were subjected to annealing at a temperature of 850 °C for 15-120 min.
Heat-treated steels were used to produce metallographic specimens by molding them in a phenolic compound, grounding, and polishing.The structures of these specimens were revealed using various etchants.
Metallographic studies were performed using the light optical microscopy method.The microstructure was investigated using the inverted Reichert-Jung MeF3A microscope equipped with a Thixomet Pro image analyzer.
The chemical composition of each phase was studied using a Zeiss Supra scanning electron microscope equipped with an energy dispersive spectrometer.To confirm the results of quantitative metallography, the phase contents in the same specimens were determined by Bruker's X-ray diffractometer.Thermodynamic modeling was carried out by the ThermoCalc software package with the TCFE database [10].

DSS's Etchants Overview
Numerous works are known in which one or another reagent was used [11][12][13][14][15] to reveal the structure of DSSs (Table 2).However, there is no unified approach that allows a sufficiently accurate quantitative assessment of the DSS's structure.The well-known method for determining the volume fraction by systematic manual point count allows the measurement of the amount of ferrite or austenite even with low etching quality.Modern image analyzers can significantly increase the accuracy and speed of measurements, provided that an image of the structure is suitable for recognition.Table 2 lists the most common reagents used to identify the structure of DSSs.The articles describe different applications of these reagents.For example, etching with the Murakami etchant was used in [11]; however, in [12], it was shown that it is rather difficult to identify ferrite if its fraction is small using such etching.Another equally important issue is the selective etching of the σ-phase.The most common method for its recognition is XRD, and there is no definite etchant for its revealing [11,13,14], as well as for other harmful phases in DSSs.

Etching Method Development
To develop a quantitative assessment technique, it is necessary to select an etchant that allows obtaining a contrast image of phases for automatic analysis.The structure of the specimen of Steel 1 with 21% Cr (Table 1), quenched from a temperature of 1200 °C was investigated (Figure 1).Three Vickers indentations were made on this specimen with a load of 180 g to indicate the investigated field of view and to make an image of the revealed microstructure after each subsequent etching.In order to always observe the same structure, a polishing after each iteration was carried out with minimal metal removal.Some etchants (reagents 1-3, Table 2), recommended by other authors, did not work on the investigated DSSs.Thus, chemical etching with inhibited ferric chloride and sodium hydroxide did not reveal the structure.The modified Beraha's reagent showed a very undistinguishable structure.
Electrolytic etching is one of the most common techniques for revealing the structure of DSSs [11].Figure 1a shows the microstructure revealed by electrolytic etching of NaOH, which consists of a dark ferrite matrix and light austenite islands [16].Using 60% nitric acid as an electrolyte gives the same result.This etching method provides stable highquality images of the DSS's structure, but the contrast between phases is insufficient for automatic classification by the image analyzer.Therefore, this etching technique is only suitable for measuring by the manual point count method [17].
Other etchants according to Table 2 (Glyceregia in Figure 1b, Carpenter in Figure 1c, Murakami's etchant in Figure 1d) showed similar results.These etchants, similar to electrolytic etching, do not provide sufficient contrast in images for the automatic quantitative description of the microstructure of DSSs.Moreover, the Murakami etchant solution requires heating to a temperature of 80-100 °C, which is unsafe and requires additional equipment.
Etching with Beraha's etchant (reagent 4, Table 2) makes it possible to obtain a sufficient contrast of ferrite (Figure 1e), which is darkened, opposed to unetched austenite.The contrast level in grayscale is sufficient to carry out binarization at a given brightness threshold and determine the volume fraction of phases according to the ASTM E1245 standard in an automated mode (Figure 1f).
The σ-phase leads to the strengthening of DSSs and to a simultaneous decrease in corrosion properties.The reagent selection for detecting it is also an important problem.Figure 2 shows the microstructure of a specimen of Steel 4 after annealing at 850 °C.It was revealed using Beraha's etchant (reagent 4, Table 2), which showed the best result earlier in identifying ferrite and austenite.This etching reveals dark areas of ferrite that have not transformed into a σ-phase (Figure 2a), light gray areas of the σ-phase, and brown austenite.The image obtained after such etching can be binarized over specified ranges and the volume fraction of each phase can be determined (Figure 2b).The volume fraction of austenite (magenta in Figure 2b) and ferrite (blue in Figure 2b) was found.The content of the σ-phase was calculated as balanced.
The content of the remaining elements (manganese and silicon), taking into account the error in their determination, was the same.In the austenite of Steel 4, which was subjected to annealing to precipitate a σ-phase at 850 °C for 15 min (Figure 3b, Table 3, points 5, 6), the lowest concentration of chromium and molybdenum (25% and 4.2-4.6%,respectively) was determined, but the concentration of nickel was found to be the highest.In the ferrite (Figure 3b, Table 3, points 7, 8) the content of chromium and molybdenum was higher (28.2-28.5% and 5.1% respectively).In the σ-phase (Figure 3b, Table 3, points 9, 10) the chromium concentration was slightly lower than in the ferrite, and the molybdenum content was the highest, reaching 12.3-12.4%.Thus, the proposed etching method (Beraha's etchant and automatic image analysis) makes it possible to detect austenite, ferrite, and the σ-phase in DSSs and to obtain high contrast of the phases in images for automatic quantitative assessment according to ASTM E1245.

Analysis of DSS Specimens Obtained Using Different Heat Treatments
Depending on the chemical composition and the heat treatment of the DSS, it is possible to obtain different ratios of austenite and ferrite, and also to precipitate the σ-phase.The austenite content in specimens of Steels 1, 2, and 3 (Table 1), quenched from different temperatures, was determined using a technique based on etching with Beraha's reagent and image analysis (Table 4).The measurement results obtained by automatic image analysis correlate with the results obtained by the manual point count method.Both methods correlate with the pre-dicted phase contents according to thermodynamic calculations too (both weight and volume percent, providing that difference in densities of the phases is negligible).However, the evaluation area and the statistical significance of the measurements in the analysis after etching with Beraha's reagent were higher than in the analysis by the manual point count method.
Measurement accuracy was confirmed by the results of XRD of the same specimens that were quantified by the metallographic method.Figure 4 shows examples of diffraction patterns for samples of Steel 2, quenched from 1050 °C and 1250 °C, respectively.According to XRD, 51% and 22% of austenite was found in the specimens quenched from 1050 °C and 1250 °C, respectively.To test the method for quantifying the σ-phase, specimens of Steel 4 quenched from 1050 °C and annealed at 850 °C were investigated.The results shown in Table 5 are consistent with the data on the kinetics of the σ-phase [18].Upon annealing for 60 min, the proportion of the σ-phase increased slightly to a content of about 17.4-20.4%,and then its proportion abruptly increased to 57.2%.

Conclusions
A technique for the quantitative assessment of DSS microstructure based on etching with Beraha's etchant solution and subsequent analysis of the content of austenite, ferrite, and σ-phase using the ASTM E 1245 method was developed.Analysis of a large number of other etchants showed that they are not suitable for the investigated steels.
Using a scanning electron microscope, the composition of the phases in the investigated specimens was determined.The phases were identified and compared with the structure observed with an optical light microscope.
It is shown that the results of measurements after etching with Beraha's etchant correlate with thermodynamic calculations and with measurements obtained using XRD.It is possible to use the developed technique as a basis for the development of new compositions and technology for the production of DSSs.

Figure 2 .
Figure 2. Dark ferrite, light gray σ-phase and brown austenite (a) in Steel 4 after annealing at 850 °C for 15 min, and a phase identification using an image analyzer (b).

Figure 3 .
Figure 3. SEM images of ferrite and austenite in Steel 1 after quenching from 1200 °C (a) and in Steel 4 after annealing at 850 °C for 15 min (b).

Table 1 .
Chemical compositions of experimental steels.

Table 2 .
Used etchants and features of their application.

Table 3 .
Chemical composition of phases in DSS specimens.

Table 4 .
Comparison of methods for assessing the volume fraction of phase components.

Table 5 .
The results of assessing the amount of phase components in specimens of Steel 4 after annealing at 850 °C.