Microstructural characterization of δ/γ/σ/γ2/χ phases in silver-doped 2205 duplex stainless steel under 800 °C aging
Introduction
The microstructure of duplex stainless steel (DSS) mainly consists of two phases, which are the Cr, Mo-rich δ-ferrite and the Ni, N-rich γ-austenite. The volume fractions of each phase must be not less than 30% [1]. DSS has excellent performance with high strength, high corrosion resistance and excellent weldability, so that it is widely used in the more severe environments, such as the nuclear power plant, offshore platforms and chemical factory [2], [3].
Since duplex stainless steel is also adopted as antibacterial material which can apply to kitchens and food processing factories by doping specific alloy (Ag, Cu, Bi, Al, etc.) [4]. Antibacterial stainless steel is divided into Ag-bearing stainless steel and Cu-bearing stainless steel in recent years. Silver-/copper-contained antibacterial materials possess good properties of long lasting, stability, safe and broad-spectrum antibiosis, overcome the drawbacks of or-ganic antibacterial materials, and have high antibacte-rial activity against both bacteria and funguses [5]. Some studies [6], [7], [8] have reported silver is doped on the material surfaces could exhibit excellent antibacterial characteristics; however, the silver had lower toxicity to human cells as compared with copper. Addition of copper is difficult to maintain both corrosion resistance and antibacterial property; moreover, it is high production cost to obtain Cu precipitations for antibacterial effect by aging treatment [9], [10], [11]. Therefore, the selected test material focuses on the silver which gets added to DSS to have well antibacterial.
Due to adding amount of multiple alloy elements (Cr, Ni, Mo, W, Si, etc.) in DSS, the precipitation of secondary phases easily occurred in the process of the annealing treatment. The intermetallic phases which contain sigma(σ) phase, chi(χ) phase, γ2, M23C6, Cr2N, and so on [12], [13] were generally precipitated between 600 °C and 1000 °C. Topolska and Łabanowski [14] found that the toughness of DSS decreased when the σ phase was precipitated. Pohl et al. [15] reported that the χ phase often appeared with the σ phase, and the formation of the χ phase was prior to the σ phase. The precipitation of σ and χ phases would decrease the ductility, toughness and corrosion resistance, and the operating temperature; hence, DSS did not usually use over 500 °C as a result.
It is well known that 2205 DSS was hard to avoid generating the intermetallic compounds in the process of annealing treatment. However, 0.2 wt.% Ag alloyed 2205 DSS is chosen as test material to investigate the relationship between doping Ag and microstructural evolution in this study. It also focus on phase transformation of δ/γ/σ/γ2/χ in 2205 DSS with 0.2 wt.% silver under 800 °C heating treating for 1, 10, and 100 h. Therefore, the purpose of long-term heating treating for silver-doped 2205 DSS is intended to systematically establish the relationship between intermetallic compounds and aging treatment.
Section snippets
Material preparation
2205 DSS samples with 0.2 wt.% silver-containing (called 2205-0.2Ag) was chosen for this work, and the chemical composition of the steel is given in Table 1. The material was prepared in a high-frequency induction furnace under a nitrogen atmosphere and homogenized at 1120 °C for 2 h to eliminate micro-segregation after casting. This sample was subsequently cut into specimens with the dimension of 10 mm × 10 mm × 5 mm. To investigate the effects of aging heat treatment in the microstructural
Phase identification and microstructure analysis
Fig. 3 shows the microstructures of 2205-0.2Ag annealed for various aging times at 800 °C were obtained by Murakami’s reagent solution. In the case of as-prepared sample (0 h), the microstructure results only show the γ phase as pale-islanded and brown δ phase. The blue massive precipitates appear at the δ/γ grain boundaries while the aging treatment is at 800 °C for 1 h, and then dark brown ones are generated for 100 h. EPMA elemental mapping of 2205-0.2Ag DSS are shown in Fig. 4. The mapping
Conclusion
In this study, 0.2 wt.% Ag alloyed 2205 duplex stainless steels is adopted for aging treatment at 800°Cto investigate the characterization of intermetallic compounds and precipitation behavior. The results reveal that microstructure results only show the γ phase and δ phase in the case of as-prepared sample, and Ag particle distribute over matrix uniformly with circular morphology. During aging treatment, Mo-enriched χ phase initiated at the δ/γ grain boundaries. and χ phase is also the
Acknowledgement
This study was funded in part by the National Science Council of Taiwan under Grant NSC 101-2811-E-390-001-.
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2019, Materials LettersCitation Excerpt :Combination of good mechanical properties together with high corrosion resistance in duplex stainless steel (DSS) makes it an attractive choice for using in marine environments, petrochemical and chemical industries [1–3]. These unique characteristics of DSSs arise from the two-phase microstructure consisting of austenite (γ) and delta ferrite (δ), which gives the opportunity to have combined properties of austenitic and ferritic stainless steels [4–6]. However, some serious problems may appear due to the microstructural changes occurring during exposure to high temperatures, associated with heat-treatment or welding [7,8], resulting in the precipitation of different compounds such as chromium nitrides, χ-phase, σ-phase, and carbides, to list a few [9–13].