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Articles • Matéria (Rio J.) 30 • 2025 • https://doi.org/10.1590/1517-7076-RMAT-2025-0241 linkcopy

Open-access Evaluation of sigma and laves phases in a niobium modified austenitic-ferritic stainless steel using FactSage software

AuthorshipSCIMAGO INSTITUTIONS RANKINGS

    ABSTRACT

    In austenitic-ferritic or duplex stainless steels, the microstructure is basically determined by the chromium and nickel contents, which are ferritic or austenitic phases stabilizers. Currently, duplex stainless steels are used to replace austenitic stainless steels in industrial applications, where requirements for corrosion resistance and mechanical strength are greater. In these steels, solidification starts at about 1450 oC with the formation of ferrite (α), which gives origin to austenite (γ) near 1300 oC. The σ phase nucleates preferentially in austenite/ferrite interface incoherent with the matrix in the range of 600 to 950 oC and compromises the cast steels toughness. Laves phase (Fe2Nb) is favored in stainless steels containing niobium. In this context, the objective of this research was to evaluate the effect of niobium on the formation of sigma and Laves phase in conventional austenitic-ferritic stainless steel and modified with 0.2, 0.5 and 1.5% niobium. The steels were heated at 1050 ºC for one hour and subsequently at 650 °C too, with cooling in water. The chemical compositions of sigma and Laves were determined semi-quantitative by energy dispersive X-rays (EDS). The amounts of Laves and sigma were obtained by computational thermodynamic simulation with the FactSage software. The results show that the thermodynamic simulation predicts Laves in the four steels, although in conventional stainless steel the microstructure is free from this phase. Regarding microstructural analyses by SEM, it is possible to observe the sigma phase in all steels, only after aging, while Laves appears in those modified with niobium in both heat treatment conditions. Whit regard to niobium, it favors the formation of sigma and Laves phase, however, the thermodynamic simulation results are different from the experimental ones.

    Keywords:
    Stainless steel containing niobium; FactSage; Laves phase; Sigma phase

    1. INTRODUCTION

    Austenitic-ferritic stainless steels, or duplex, are used in off and on shore platforms and in the manufacture of pumps that operate in an aqueous environment with chlorides, due to the excellent compromise between mechanical properties and corrosion resistance [1]. In these steels, the microstructure is mainly influenced for the chromium and nickel content, which are stabilizers of the ferritic and austenitic phases.

    According to the literature, solidification of these steels begins around 1450 oC with the formation of ferrite (α) which gives rise to austenite (γ) close to 1300 oC. In the range of 950 to 1050 oC, M7C3 carbides precipitate at grain boundaries. Below 950 oC, M23C6 carbides are formed. The sigma phase (σ), with a tetragonal crystalline structure, nucleates preferentially at the austenite/ferrite interface at 600 to 950 oC. This phase is not magnetic at room temperature and impairs the toughness of the steel [2,3,4]. In this same temperature range, the chi phase precipitates with a much smaller fraction and reduces toughness too [5].

    In austenitic-ferritic stainless steels, other different phases are formed during solidification. When nickel alloy scrap, containing niobium, is used in steels melting, the Laves (Fe2Nb) formation is favored. In this phase, iron can be replaced by chromium [6]. Laves has a compact hexagonal structure, with different stoichiometries such as Cr2Nb, Fe2Mo, Fe2Ti, among others [7]. The formation mechanisms of Laves phase were attributed to a divorced eutectic reaction [8]. Precipitation occurs in the form of laths in the matrix and is associated with alloy embrittlement. Laves phase formation led to increase in hardness and decrease fracture toughness [9, 10]. Nevertheless, Fe2Nb can act in grain refining and contribute to creep resistance at high temperatures [11].

    Literature results show that niobium favors the grain refinement, increases corrosion resistance and produce a great effect on solid solution reinforcement, due to the larger diameter and the coordination number equal to twelve [12, 13]. In this context, the objective of this research is to evaluate the effect of niobium on the formation of sigma and Laves phase, in the austenitic-ferritic stainless steel SEW 410 Nr.14517 with different niobium contents. The microstructural analyses were compared with FactSage simulations using the chemical compositions of four different steels according to Table 1. This software is an integrated computational system with access to thermodynamic information on liquid and solid metallic solutions, and it allows predicting the quantities, stoichiometries and temperatures of formation of intermetallic phases [14]. The program was developed by the CRCT (Center for Research in Computational Thermochemistry – Montreal) and GT – Technologies (Aachen) groups and consists of a series of information modules, databases, calculations, and simulations. It is used in hundreds of universities and companies around the world, as a research and educational tool [15].

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    Table 1
    Chemical compositions of austenitic-ferritic stainless steels (wt. %).

    2. MATERIALS AND METHODS

    2.1. Steel production

    The austenitic-ferritic stainless steels SEW 410 W.Nr. 1.4517, conventional and modified with 0.2, 0.5 and 1.5% niobium, were prepared in an industrial induction furnace with a capacity of 300 kg, at Fundição Grupo Metal in Tietê/SP. The liquid metal was poured into molds made of sand bonded with urethane phenolic resin and the chemical compositions are presented in Table 1.

    2.2. Phase identification with FactSage software

    The amounts of sigma and Laves phases were determined with FactSage software version 8.0, from the FSteel database, and in the range of 0 to 1500 ºC using the chemical compositions from Table 1.

    2.3. Heat treatments and microstructural characterization

    Samples of the four steels were heated at 1050 °C followed by water quenching. Subsequently, a new heating was done at 650 °C followed by water quenching. The warm ups were carried out for an hour. After conventional metallographic preparation and etching with Behara reagent (25 ml HCl, 3 g ammonium bifluoride, 125 ml H2O, 0.4 g of potassium metabisulfite), the microstructures were observed in a scanning electron microscope (SEM).

    3. RESULTS AND DISCUSSION

    3.1. Steel production

    Table 1 shows the chemical compositions of austenitic-ferritic stainless steels with different niobium contents.

    3.2. Phase identification with FactSage software

    The quantities of austenite, ferrite, sigma and Laves phases, according to the simulation, are presented in Figures 1 to 4, considering 100 g of each steel. Other steel components were not considered.

    thumbnail of Figure 1
    Figure 1
    Duplex conventional steel without Nb.
    thumbnail of Figure 2
    Figure 2
    Duplex steel with 0.2% Nb.
    thumbnail of Figure 3
    Figure 3
    Duplex steel with 0.5% Nb.
    thumbnail of Figure 4
    Figure 4
    Duplex steel with 1.5% Nb.

    Considering the simulation results, it is possible to observe that sigma and Laves phases increase with the niobium content. However, microstructural analysis reveal that Laves is not observed in steels without niobium, as well as results from literature [16, 17]. Table 2 shows the maximum values of sigma and Laves phases obtained in the Figures 1, 2, 3 and 4.

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    Table 2
    Maximum quantities of sigma and Laves with FactSage simulation (g).

    The Laves amounts are similar in the range of 6.5 to 6.8 g. Nevertheless, the quantities of Laves increase with niobium content, as can see in Figures 6 to 8, after heating at 1050 °C. This precipitation during cooling is favored by the niobium content above the solubility limit in the matrix. This phase, in the form of platelets, is mainly composed of iron, niobium and chromium and, unlike sigma, does not solubilize at the usual heat treatment temperatures.

    thumbnail of Figure 5
    Figure 5
    Conventional duplex steel (heated 1050oC).
    thumbnail of Figure 6
    Figure 6
    Duplex steel with 0.2% Nb (heated 1050oC).
    thumbnail of Figure 7
    Figure 7
    Duplex steel with 0.5% Nb (heated 1050oC).
    thumbnail of Figure 8
    Figure 8
    Duplex steel with 1.5% Nb (heated 1050oC).

    The results corroborate those in the literature and show the effect of niobium in the formation of sigma and Laves. Niobium is an alphagenic element and destabilizes austenite, increases the volumetric fraction of ferrite, as well as the amount of molybdenum and chromium in this phase [17, 18].

    3.3. Heat treatments and microstructural characterization

    Figure 5 shows the ferritic matrix with elongated austenitic grains in conventional duplex stainless steel, after heating at 1050 °C. In this condition, the sigma and Laves phases do not appear. Unlike simulation that contemplates these phases, as it considers thermodynamic equilibrium, the different thermomechanical process can alter these results.

    Figures 6, 7 and 8 shows the Laves Fe2Nb as needles in different quantities in the steels containing niobium. Laves phase is insoluble even after heat treatment at 1050 °C. Laves can dissolve carbon and is favored over sigma phase, as niobium is poorly soluble in austenite [19].

    After heating at 1050 °C, followed by aging at 650 °C, it is possible to observe in Figures 9 to 12, sigma in all steels, distributed along the grain boundaries and inside the matrix. The austenitic grains and Laves as needles appear dispersed in matrix.

    thumbnail of Figure 9
    Figure 9
    Conventional duplex steel (aged).
    thumbnail of Figure 10
    Figure 10
    Duplex steel with 0.2% Nb (aged).
    thumbnail of Figure 11
    Figure 11
    Duplex steel with 0.5% Nb (aged).
    thumbnail of Figure 12
    Figure 12
    Duplex steel with 1.5% Nb (aged).

    According to the literature, the transformation occurs with reduction of chromium and molybdenum in regions near sigma phase. Sigma is primarily responsible for increased corrosion in austenitic-ferritic stainless steels.

    Sigma phase, with a predominance of iron, chromium and molybdenum, nucleates preferentially at the austenite/ferrite interface and grows towards the ferritic phase [20, 21]. Laves phase, has a predominance of iron, chromium and niobium. Table 3 shows chemical compositions of sigma and Laves phase obtained by X-ray energy dispersive (EDS).

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    Table 3
    Chemical composition of sigma and Laves phases (wt. %).

    Figures 13 and 14 shows the diffraction spectra of the sigma and Laves phases by EDS.

    thumbnail of Figure 13
    Figure 13
    Diffraction spectra of sigma (EDS).
    thumbnail of Figure 14
    Figure 14
    Diffraction spectra of Laves (EDS).

    Sigma and Laves phases are enriched with chromium and reduce the content of this element in the matrix, compromising corrosion resistance [22, 23]. These phases provide an increase in the hardness and reduce the toughness of the matrix. In this case, the addition of niobium in austenitic-ferritic stainless steels is interesting when wear resistance is an important factor to consider [17, 18].

    4. CONCLUSIONS

    • Laves phase is insoluble at usual heating temperatures;

    • Niobium favors the formation of the Laves and sigma phases in austenitic-ferritic stainless steels;

    • Niobium is soluble in the Laves phase and insoluble in sigma;

    • Sigma phase is present in steels after aging;

    • The thermodynamic simulation results may be different from the experimental one, and that is due to different manufacturing processes.

    5. ACKNOWLEDGMENTS

    The authors thank FAPES for the scientific initiation and researcher scholarship, Grupo Metal for the steels and Structural Characterization Laboratory of UFSCar for the microstructures images.

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    Publication Dates

    • Publication in this collection
      01 Dec 2025
    • Date of issue
      2025

    History

    • Received
      16 Mar 2025
    • Accepted
      08 Sept 2025
    Creative Common - by 4.0
    This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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