Influence of the addition of gadolinium on the microstructure and mechanical properties of duplex stainless steel

Abstract

The aim of this study is to investigate the effects of gadolinium addition on the microstructure and mechanical properties of duplex stainless steel (DSS) fabricated using a normal casting method. The oxygen content in the cast DSS alloy with gadolinium decreased because of the high reactivity of gadolinium with oxygen. The area fraction and size of non-intermetallic inclusions in the alloy decreased from 0.80±0.12% to 0.58±0.04% and from 6.9±0.7 to 5.8±0.4 μm upon gadolinium addition, respectively. Notably, the ultimate tensile strength and strain at break of the cast alloy significantly increased with the addition of gadolinium from 919±25 to 969±8 MPa and from 24.8±1.9% to 28.4±1.1%, respectively. The hardness of the cast alloy with gadolinium increased from 23.6±1.3 to 25.0±1.2 HRC. A significant increase in the impact energy of the cast alloy was observed and the brittle-to-ductile transition temperature slightly decreased by approximately 10 °C with the addition of gadolinium.

Introduction

Duplex stainless steels (DSSs), which comprise ferrite (α) and austenite (γ), have been widely investigated because of their favorable mechanical properties and corrosion resistance [1], [2], [3]. DSS has higher strength and better stress corrosion and pitting and crevice corrosion resistance than pure austenitic grades [4], [5]. With its combination of excellent corrosion resistance and tensile strength, DSS can be used in a variety of applications including oil, pulp and paper, petrochemical, and marine industries [1], [6], [7].

While chromium (Cr), molybdenum (Mn), and nickel (Ni), the main components of DSS, make DSS strong and corrosion-resistant, they also form detrimental tertiary phases such as sigma (σ) phases that only appear in ternary Fe–Cr–Mo systems and chi (χ) phases that are present in ternary Fe–Cr–Mo and quaternary Fe–Cr–Ni–Mo systems [8], [9], [10], [11]. These intermetallic σ and χ phases have negative effects on the corrosion and mechanical properties of DSS because of embrittlement [12], [13]. Previous studies by other groups reported that the precipitation of the σ phase occurs when steels are annealed below 1000 °C, and it can be removed using a solution treatment process that provides control of the ferrite and austenite proportions [14], [15], [16], [17], [18], [19]. Moreover, non-metallic inclusions in steel are detrimental to the corrosion and mechanical properties [20], [21], [22]. In particular, owing to their brittle character, they cause crack formation and fatigue failure in steel during deformation processes such as forging and flattening [23]. For instance, nitrogen, which is used to strengthen DSS, reacts with other elements with high affinity to nitrogen and forms nitrides such as Cr₂N. Further, oxides that are detrimental to corrosion properties can form during the manufacturing process. Therefore, various attempts have been made to produce high-quality DSS by inhibiting the formation of intermetallic phases and non-metallic inclusions.

Recently, the addition of rare earth metals to stainless steel has shown to reduce the formation of intermetallic phases and non-metallic inclusions [24], [25]. Some researchers have also demonstrated that the addition of rare earth metals to steel promotes solid solution hardening and suppresses the formation of precipitates, such as σ phases, by occupying voids and vacancies in the ferrite matrix and reducing the diffusion rates of the elements involved in precipitate formation [26], [27], [28], [29], [30]. In addition, the standard free energies of formation for rare earth oxide and sulfide formation are lower than those for other non-metallic inclusions, such as manganese sulfide (MnS) and chromium oxide (Cr₂O₃); therefore, the addition of rare earth metals can retard the deleterious actions of non-metallic inclusions [24]. The addition of cerium (Ce), a representative rare earth metal, to steels has shown to improve their mechanical and corrosion properties [24], [25], [26], [31]. Kim et al. and Jeon et al. reported that pitting corrosion resistance was enhanced following the addition of Ce to DSS [25]. Liu et al. revealed that the sizes and morphologies of non-metallic inclusions and the mechanical properties of 2205 DSS were positively changed after adding Ce [23], [32].

Gadolinium (Gd) is one of the more abundant rare earth metals and has special characteristics such as a high neutron cross-section [33]. It has been used in neutron therapy applications for targeting tumors, and recently, it has been introduced as a neutron absorber material for the shielding of nuclear fuels in nuclear reactors [34]. DSS, when successfully fabricated with Gd addition, may benefit from these characteristics through enhancement of its corrosion resistance and mechanical properties and be ideally suited for a variety of applications in the nuclear industry.

The present study thus focuses on investigating the influence of Gd on the microstructure and mechanical properties of DSS. DSS with a pitting resistance equivalent number (PREN=wt% Cr+3.3 wt% Mo+30 wt% N) of 50 was used [25] and 0.1 wt% of Gd was added to DSS and alloyed through casting, and the microstructure and inclusions in the alloy were investigated via field emission scanning electron microscopy (FE-SEM) and x-ray crystallography (XRD). The mechanical properties were examined using Charpy impact, hardness, and tensile strength tests.