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УДК 621.039.53
EFFECT OF GADOLINIUM ADDITION ON THE CORROSION, WEAR, AND NEUTRON ABSORBING BEHAVIORS OF DUPLEX STAINLESS STEEL SHEET
© 2015 г. Youl Baik*, Yong Choi*, Byung M. Moon**, Dong S. Sohn***, S. G. Bogdanov****, A. N. Pirogov****, *****
*Department of Materials Science and Engineering, Dankook University, 29-San Anseo-dong, Dongnam-gu, Cheonan, Chungnam 330-714, Korea **Department of Liquid Processing and Casting Technology R&D Group, KITECH, 7-47 Songdo-dong, Yeonsoo-gu, Incheon Metropolitan City, Korea ***Interdisciplinary School of Green Energy, UNIST, 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 689-798, Korea ****Institute of Metal Physics of RAS, 620990 Ekaterinburg, ul S. Kovalevskoi, 18, *****Institute of Natural Sciences, Ural Federal University, 620002 Ekaterinburg, ul. Mira, 19
e-mail: yochoi@dankook.ac.kr Поступила в редакцию 03.06.2014 г.; в окончательном варианте — 05.11.2014 г.
Abstract—In order to develop the neutron absorbing and shield materials, a hot-rolled 0.02%-Gd duplex stainless steel was prepared with 55 vol % of ferrite and 45 vol % of austenite. The a phase with an average grain size of 9—11 p.m in austenitic (y) grains tended to be elongated parallel to the rolling direction, with (100) poles concentrated towards the normal direction, and (110) poles located between the normal and radial directions (ND and RD, respectively). Most of the gadolinium existed as sub-micro-meter-sized Gd2O3 and GdCrO3 precipitates. The yield strength, ultimate tensile strength, elongation, and micro-hardness of the 0.02%-Gd duplex stainless steel were 522.8 MPa, 700.2 MPa, 38.1%, and 258.5-314.7 HV, respectively. The friction coefficient and wear resistance were 3.11 and 0.004 mg/kg/cycle, respectively. The corrosion potential and corrosion rate of the 0.02%-Gd duplex stainless steel were -0.448 VSHE and 1.263 x 10-3 A/cm2 for 1 M HCl, -0.544 VSHE and 2.619 x 10-3 A/cm2 for 1 M NaCl, -0.299 VSHE and 1.469 x 10-3 A/cm2 for 1 M H2SO4, and -0.607 VSHE and 2.295 x 10-3 A/cm2 for synthetic water, respectively. The coefficient of neutron transmission for the 0.02%-Gd duplex stainless steel sheet of 2 mm thickness at neutron beam wavelength of 0.48 nm was 0.6.
Keywords: Gd-duplex stainless steel, mechanical and corrosion properties, neutron adsorption.
DOI: 10.7868/S0015323015110133
1. INTRODUCTION
Excellent corrosion resistance and good mechanical properties are required for neutron absorbing structural materials for the storage and transportation of spent nuclear fuels and high level wastes, since they are under the conditions of long-term neutron and gamma radiation environment in dry or wet condition [1]. There are several neutron absorbing structural materials with boron and gadolinium, such as aluminum-boron carbide cermet, boron aluminum alloys, boron stainless steel, and Ni—Cr—Mo—Gd alloys [2—4]. Neutron absorbing structural materials, especially for storage racks and the transport of spent fuels, are generally of sheet shape. Boron carbide phase is arisen at boundaries of grains and defect structure is formed, when a plastic deformation of alumina matrix occurs at a fabrication of metal-matrix of boron alumina
sheets by cold roll [5]. It is difficult to get a homogeneous microstructure in boron-alumina carbide ceramic from mixture of ceramic and metal [6].
Stainless steels for neutron absorbing structural materials are more challenging than aluminum alloys, because of their higher melting point, higher mechanical properties, and excellent corrosion resistance. A boron-stainless steel has good durability and high corrosion resistance. However, the limited solubility of boron in stainless steels prevents the production of alloys with more than 2.25 wt % boron [7]. An Ni—Cr— Mo alloy with 2.1 wt % Gd is proven only for usage in the non-welded condition, because of the precipitation of the Ni5Gd gadolinide at grain boundaries. Accordingly, gadolinium addition to the matrix materials for a neutron absorbing material has various advantages such as free-generation of helium by the absorption
of neutrons during service and easy production of alloy and plastic deformation. In order to get boron-stainless steel and Gd steel with the same neutron absorption coefficients it is necessary to add much more boron than gadolinium because the B has a neutron absorption cross-section equaled to be aB = 4300 barn, whereas the natural Gd possesses aGd = 49000 barn (for neutron energy E = 0.025 eV) [8]. In case of matrix materials, stainless steelis plausible to use because of its high strength and high corrosion resistance [9]. Since duplex stainless steels of ferrite and austenite have an excellent corrosion resistance and high mechanical properties, the combination of gadolinium and duplex stainless steels are one of promising neutron absorbing materials.
The sheet production of duplex stainless steels is required to precisely control the precipitation of the a phase, and to avoid M23C6 forming edge cracking during hot rolling. The addition of a strong ferrite stabilizer (Cr, Si, Mo) into the stainless steels completely transforms the a phase to the a + y2 cellular structures at the aging at 1100°C for 0.5 h, and at 800°C for 10 h [10]. Although Gd-duplex stainless steel sheet has been successfully produced by hot rolling [11], little information is available about its corrosion, mechanical properties, and neutron absorbing behavior, especially the precipitate controlled Gd-duplex stainless steels.
Hence the objectives of this research are to fabricate precipitate-controlled Gd-duplex stainless steel sheet, and to study the effect of gadolinium addition on the corrosion, wear, and neutron absorbing behaviors of Gd-duplex stainless steel sheet. In our study, the content of gadolinium in a duplex stainless steel was selected as 0.02% in order to achieve the same value of neutron adsorption as in the commercial boron-stainless steels with 1.2% boron.
2. EXPERIMENTAL METHOD 2.1. Specimen preparation
Specimen has been prepared by melting, casting and rolling processes. The mother alloys of Fe—Mo, Fe—Gd, and Fe—Cr—Ni were prepared by vacuum arc melting (PAM-Plasma, Japan). A remelting process to produce the duplex stainless steel with gadolinium was carried out, using a high-frequency induction melting furnace (Inductotherm, USA) with the parent alloys and engineering grade of pure iron (>99.99, Sam-chang, Korea), chromium (>99.99, Samchang, Korea), nickel (>99.99, Samchang, Korea), manganese (>99.99, Samchang, Korea), and silicon (>99.99, Samchang, Korea).The total melting time during induction heating, the maximum melting temperature and the pouring temperature were 0.5 h, 1700, and 1640°C, respectively. The casting poured into a mold
was mechanically sectioned from gating and risering-parts before hot-rolling. The hot-rolling (M-tech, Korea) was performed three times under forward feeding condition above 950°C, after keeping the sheet at 1200°C for 1.5 h, followed by rapid-heating at 13°C/min for a solution treatment at 1070°C for 50 min, to remove intermetallic compounds, like the a and x phases.
2.2. Microstructure observation
The microstructure was observed by scanning electron microscopy (JEOL, JSM 6400, Japan).The crys-tallographic texture was determined by an electron backscattered diffraction method (JEOL, JSM-7100F, Japan). Chemical analysis and the morphology of precipitates was observed by energy dispersive spectroscopy (Oxford, X-MaxN 80 T, UK) and transmission electron microscopy (JEOL, JTM-2100F, Japan), respectively. The final thinning of the TEM work was performed by twin jet polisher (Struers, TenuPol-5, Denmark),under an operating voltage of 25 V at —40°C, using 10% HClO4 and 90% CH3OH solution, until a small hole was formed on the sample.
2.3. Characterization
The micro-hardness was determined by micro -Vickers hardness tester (HUATEC, DHV-1000, China) at a 9.8 N load. Tensile testing was performed by universal test machine (Shimadzu, AG-300kNX, Japan) .A pin-on-disk type wear test was carried out by the pin-on-disk type wear tester (R&B, Triboss PD-102, Korea),to determine the wear-loss and coefficient of friction. The load and RPM of the test were 300 g and 150 rpm, respectively. A potentio-dynamic corrosion test was carried out to determine corrosion resistance with a potentiostat (Gamry 100, USA) in various aqueous solutions, like 0.1 M HC1, 0.l M NaCl, 0.1 M H2SO4 solution, and synthetic seawater.
A coefficient of neutron absorption has been measured by means of D6 diffractometer of small angle polarized neutron scattering, mounted on a horizontal channel of IVV-2M reactor (Zarechny, Russia). The
average neutron wavelength was X = 0.48 nm and a resolution in neutron lengths was 30%. The polarization of incident neutron beam is P0 = 0.93. The cross-section dimensions of the neutron beam were 5 x 1 mm. We applied an external magnetic field = 0.4 T to avoid a neutron beam depolarization. We prepared two samples: one sample was steel without gadolinium and the other was steel with 0.02 % Gd. Both samples were in shape of plate with dimensions 40 x 40 x 2 mm.
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Fig. 1. Typical scanning electron micrographs of the 0.02%-Gd duplex stainless steels with hot-rolling directions: (a) surfaces perpendicular to the (a) rolling direction (ND), (b) radial direction (RD), and (c) tangential direction (TD): (+) designates (a) Gd2O3,
(b) cd phase, and (c) □ phase.
3. RESULTS AND DISCUSSION 3.1. Microstructure observation
Fig. 1 is a typical scanning electron micrograph of the cast 0.02% Gd duplex stainless steels. As shown in Fig. l, the duplex phases contained 55 vol % ferrite (8) and 45 vol % austenite (y) phases. Ferritic grains tended to be parallel to the rolling direction, which structure was related to the lower interface energy between the 8 and y phases, than between the 8/8 and y/y grain bo
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