Effects of low-temperature neutron irradiation on the microstructure and tensile properties of duplex 2304 stainless steel and its electron-beam welds

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Abstract

A lean duplex stainless steel material (2304-grade) in its base metal and electron beam (e-beam) welded conditions were studied microstructurally and mechanically as a function of irradiation conditions to evaluate its use as a structural material at low temperatures (60–100 °C). Neutron irradiation up to a fluence of 1.40 × 1019 n/cm2 (E > 0.1 MeV) or ~0.011 dpa decreased the total elongation of both base metal and e-beam welded samples. Overall, radiation hardening was observed in all the samples. The transversely cut irradiated samples showed some nonuniform quasi-cleavage and shearing in their fracture surfaces, indicating the variance of ductile nature of the two-phased (deformable austenite and harder ferrite) duplex structure. The e-beam welded samples also showed quasi-cleavage fracture, which is a characteristic of radiation-induced embrittlement. These observations of the e-beam welded samples were attributed to the formation of coarse ferrites, grain boundary and intragranular phases such as γ2 and γ3, and minor impurity phases such as CrN and Cr2N in the weld pool and/or heat-affected zone of the samples. Radiation-induced elemental segregation was also identified in the post-irradiated base metal.

Introduction

Duplex stainless steels (DSSs) are made of two main phases: austenite and ferrite. Alloy constituent (elemental) concentrations of DSS vary depending on whether its grade is a low alloy, intermediate alloy, or highly-alloyed (superduplex) [1]. The chemical make-up of each phase (austenite and ferrite) is highly dependent on the processing conditions and can show differences for different grades of DSS Fe–Cr alloys, with ferrite being the matrix. This combination of austenite and ferrite phases is important compositionally and microstructurally since it gives attractive fracture toughness/higher mechanical strength and desirable corrosion or pitting corrosion resistance, especially in chloride-ion containing media, and better weldability compared to conventional austenitic stainless steels [2,3]. Therefore, the duplex structured alloys are of great importance in a number of industrial applications such as petroleum, chemical tanks, heat exchangers, offshore applications, and in other applications where corrosion resistance is essential [3,4]. Those are also important materials for nuclear power industry, especially in light water reactor applications, where duplex stainless steels can be used in primary pipping for example in pressurized water reactors [5]. Despite they are used in the nuclear industry, there is little data on the behavior of DSS after neutron irradiation. A significant amount of irradiation effect studies on austenitic stainless and martensitic/ferritic steels at elevated temperatures, some of which are compatible to commercial nuclear reactors, have been performed [[6], [7], [8]]. These studies have shown that radiation-induced formation of secondary phases such as carbides (η-M6C; coarse M23C6 can dissolve during irradiation), nitrides (CrxN), phosphide (M2P), silicides (γ′-Ni3Si and G (M6Ni16Si7) phase), intermetallic phase of Fe–Cr (χ), and Cr-rich (α′) and radiation-induced segregation (RIS) of elements can occur in these materials. Irradiation can also partially transform austenite into ferrite, especially in post-irradiated austenite materials; for example, 304 L SS irradiated at ~500 °C to ~3 × 1022 n/cm2 (E > 0.1 MeV) [9].

The DSS consists of a number of alloying constituents, which include Fe, Cr, Ni, Mo, Mn, and minor elements such as N and C. While some of these alloying elements (e.g., Cr and Mo) support resistance to corrosion, others such as Ni (and N) improves mechanical (e.g., tensile) properties [10]. Nitrogen also supports weldability and reformation of austenite via a diffusion mechanism, and it favorably affects the material's strength and corrosion resistance properties [11,12]. However, thermal aging embrittlement of DSS has been observed at ~250–450 °C, limiting their use to temperatures <300 °C [13]. The presence of secondary phases such as σ or nitrides such as Cr2N or CrN could act as detrimental phases lowering the favorable mechanical and chemical properties of DSS [14,15]. The formation of the σ phase is mainly controlled by Mo and is usually precipitated out along grain boundaries of ferrite-ferrite and ferrite-austenite phases, while Cr2N is formed in ferritic regions [[16], [17], [18]]. Literature also points out that the corrosion resistance of DSS is favorably affected by the presence of austenite formation, for example in TIG (tungsten inert gas) welded samples [19]. During DSS welding, the solidification in the fusion zone occurs via the ferritic phase formation, and furthermore, weld surface oxides that may form as impurity phases during welding can detrimentally affect the corrosion resistance of duplex stainless steel at varied temperatures [[20], [21], [22]]. Therefore, DSS welding has to be carried out in a controlled manner, so that the duplex base metal (BSM) properties are maintained in the weld pool and/or heat-affected zone (HAZ) by reforming austenitic phase to obtain reasonable ferrite to austenite phase ratio [23,24].

Alloy 2304 is a low-alloyed (or lean) duplex stainless steel grade and is usually known as a Cr–Mn duplex stainless steel [25]. This low alloy duplex 2304 is a high strength, corrosion resistant (closely similar to that of 316 L) stainless steel grade with low Ni and Mo concentrations: typically ~4 and ~0.1 wt % Ni and Mo, respectively [26]. These low amounts of Ni and Mo allow it to be a comparatively cheaper alloy and prevent formation of unwanted secondary intermetallic phases with detrimental effects on the materials mechanical and chemical properties [27]. In this study, the mechanical behavior of DSS 2304 was evaluated under different conditions, including neutron irradiation of base metal and electron-beam (or e-beam) weldments. It is also noteworthy to point out that most of the welding treatments on DSS or specifically the duplex 2304 are made using arc welding or friction stir welding, while few studies have also discussed the use of explosive welding of DSS [11], [[28], [29], [30]]. Neutron irradiation was performed in the 60–100 °C range as that was the targeted irradiation temperature range for this study, which was a part of the evaluation of novel methods to produce medical isotopes (e.g., Mo-99). A thorough characterization of the microstructure of DSS 2304 in its base metal and e-beam welded conditions was performed. Microstructural properties of the fracture surfaces of tensile tested materials in their pre- and post-irradiated conditions are also discussed. Since neutron irradiation data on DSSs are scarce in the literature, this paper should give some insight into the mechanical behavior of DSS 2304 in the post-irradiation conditions as well.

Section snippets

As-received material

DSS 2304 used in this research was purchased through Metal Suppliers. The material (heat number 6211695) was fabricated by Industeel under A923 specification. After being casted and rolled to a 0.75 in. (19.05 mm) thickness, the DSS 2304 underwent a final 980 °C solution annealing, which is an important step in processing DSS materials to retain their favorable microstructural and mechanical properties, followed by water cooling.[31] This procedure was followed to maintain the formation of

Properties of DSS 2304 in its base metal and e-beam welded conditions

XRD analysis of a control sample of DSS 2304 showed the presence of two phases: γ-austenite and α-(or δ)-ferrite (Fig. 2). The austenite phase is face-centered cubic (fcc) with a space group of Fm-3m (ICSD # 41,506), while ferrite phase is body-centered cubic (bcc) with Im-3m (ICSD 52258) unit cell space group. The determined phase contents of austenite and ferrite phases using full profile fit of the XRD pattern with Rietveld analysis were 65 and 35 wt %, respectively. Except for a slight

Conclusions

High tensile ductility was observed in the tensile specimens of nonirradiated lean DSS 2304 cut along the rolling direction and the transverse direction. The transversely cut samples showed a significant decrease in the tensile ductility after neutron irradiation up to 1.40 × 1019 n/cm2 (E > 0.1 MeV) as a result of different effects of the two phases present in the duplex structure. The fracture surface of both pre- and post-irradiated DSS 2304 specimens cut along the rolling direction showed

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

CRediT authorship contribution statement

Chinthaka M. Silva: Methodology, Formal analysis, Investigation, Writing – original draft, preparation. Keith J. Leonard: Conceptualization, Methodology, Investigation, Writing – review & editing, Funding acquisition, Supervision. Lauren M. Garrison: Investigation, Writing – review & editing. Chris D. Bryan: Writing – review & editing, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Funding for this research work was provided by the US Department of Energy's National Nuclear Security Administration, DOE/NNSA), Office of Material Management and Minimization's Molybdenum-99 Program. This work was also performed under the auspices of the U.S. Department of Energy (DOE) by Lawrence Livermore National Laboratory (LLNL) under Contract No. DE-AC52-07NA27344. The authors would also like to thank their coworkers at ORNL for their support in this research work: Tom Geer, Christopher

References (40)

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