Proc. 2nd Int. Symp. Science at J-PARC — Unlocking the Mysteries of Life, Matter and the Universe — JPS Conf. Proc. 8, 031015 (2015) http://dx.doi.org/10.7566/JPSCP.8.031015
Microstructure and Residual Strain Distribution in Cast Duplex Stainless Steel Studied by Neutron Imaging Yuhua SU*, Kenichi OIKAWA, Takuro KAWASAKI, Tetsuya KAI, Yoshinori SHIOTA1, Hirotaka SATO2, Takenao SHINOHARA, Yo TOMOTA3, Masahide HARADA, Yoshiaki KIYANAGI1, and Masatoshi ARAI J-PARC Center, Japan Atomic Energy Agency, 2-4 Shirane Shirakata, Tokai, Ibaraki 319-1195, Japan 1 Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan 2 Graduate School of Engineering, Hokkaido University, Kita-13 Nishi-8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan 3 Graduate School of Science and Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, Ibaraki 316-8511, Japan E-mail:
[email protected] (Received October 10, 2014) Pulsed neutron imaging experiments were conducted to study the spatial distribution of the microstructure in a cast duplex stainless steel. The microstructures of different specimens were successfully observed using two-dimensional imaging. Bragg-edge transmission spectra were analyzed using a Rietveld-type analysis code; microstructure information for two crystalline phases was obtained by total profile fitting. KEYWORDS: neutron imaging, Bragg-edge transmission, cast duplex stainless steel, microstructure, RITS, MLF, J-PARC
1. Introduction Neutron radiography under near Bragg-edge conditions provides qualitative contrast images of crystalline materials; however, the origin of the contrast is often unclear. In contrast, energy-resolved neutron imaging using a pulsed neutron source provides quantitative contrast images that contain microstructure information, which can be elucidated by Bragg-edge analysis. Sato et al. have developed a Rietveld-type analysis code for pulsed neutron Bragg-edge transmission imaging, RITS, and have quantitatively evaluated the texture and microstructure of a welded -iron plate [1,2]. In the present study, to evaluate the capability of the Bragg-edge imaging technique for the development of industrial materials, two-dimensional (2D) transmission spectra of cast duplex stainless steel (DSS) were measured at NOBORU of J-PARC. Three types of 2D detectors were used for the same specimens. Important factors that influence the properties of steel, including residual strain, phase distribution, volume fraction, crystallite size, and texture of the constituent phases in DSS, were expected to be
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characterized. 2. Experimental Procedures As listed in Table I, four cast ingots of a commercial DSS were used in the present study. These ingots were produced by different casting methods at different Table I. Ferrite–austenite duplex stainless steels studied Specimen
Casting method
Temperature (°C)
Thickness (mm)
No. 1 No. 2 No. 3 No. 4
Sand molding Metal molding Sand molding Metal molding
1490 1490 1545 1545
14 14 11 14
Grain size (μm) BCC FCC interval ~620 ~30 ~1220 ~12.5 ~2620 ~34.5 ~1490 ~14.1
temperatures, followed by isothermal holding at 1120 °C for 240 min and water quenching [3]. The chemical compositions of the steel were 0.071C–1.02Si–0.54Mn–6.88Ni–23.03Cr–1.88Mo (in mass%). Specimens were directly cut from the cast blocks into samples with dimensions of 50 mm × 30 mm and a thickness of 14 or 11 mm (dashed-line region in Fig. 1). The microstructure of the as-cast specimens consists of a two-phase structure: a mixture of ferrite (α-Fe, BCC structure) and austenite (γ-Fe, FCC structure). Optical microscopy observations [3] revealed ferrite grains with a columnar shape in three of the specimens, whereas only specimen no. 1 exhibited an equiaxed shape. Fine austenite grains were uniformly distributed in the coarse ferrite matrix. Fig. 1. Schematic diagram of a Bragg-edge imaging was performed on NOBORU cast block and examined position. at BL10 of MLF, J-PARC. We used three types of 2D detectors to investigate the feasibility of the microstructure analysis on the prepared DSS specimens. The detector used in each experiment and the corresponding beamline conditions, such as the collimator ratio (L/D), are summarized in Table II. A typical experimental setup is shown in Fig. 2. The distance from the moderator to the Fig. 2. Example of the experimental setup used in the present sample position was about 14 m, study.
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whereas that from the sample to the detector was approximately 30 mm. Filters, pinhole size, and field of view were carefully determined from the measurement results of each blank to eliminate the counting loss of each detector. Table II. Measurement conditions and types of detectors [4-6]. Beam-time w/wo sample 3/0.3 MWh 1.65/1 MWh 2.5/1.5 MWh
Filter @7.5 m Pb 50 mm Borosilicate glass 1 mm Bi 50 mm
Pinhole @8 m and L/D Sq. 3.2 mm and 1600 Sq. 3.2 mm and 1800 Sq. 10 mm and 600
Collimator @12.5 m Sq. 47 mm Sq. 47 mm Open
Detector type WLS scintillator detector μPIC-based detector GEM type detector
Best spatial resolution
Nominal count rate
Sq. 0.52 mm
Sub-M cps
Sq. 0.1 mm
Few M cps
Sq. 0.8 mm
10 M cps
3. Results and Discussion 3.1 2D images and Bragg-edge spectra by different detectors Figure 3(a) shows the transmission image for wavelengths between 3.32 and 3.60 Å, with a pixel size of 0.1 × 0.1 mm2, as collected using the micro-pixel chamber (μPIC) detector [5]. A particular contrast for each sample is clearly observed, where brighter areas indicate higher neutron transmission. All the grains in specimen no. 1 appear equiaxed and uniform; however, the other three specimens exhibit coarse grains with elongated shapes along the casting direction, which is in good agreement with the microstructure observation results reported by Takahashi et al. [3]. Bragg-edge transmission spectra of specimen nos. 1 and 3 (boxed in Fig. 3(a) with a red dashed line) obtained by the Gas Electron Multiplier (GEM) detector [6] are shown in Fig. 3(b). The transmission spectrum of specimen no. 1 exhibits a sharp saw-tooth shape, consistent with the peak positions of hkl = 90° for the BCC and FCC phases. In contrast, the spectrum for specimen no. 3 exhibits numerous additional dips caused by the very large grains (up to millimeter scale) of the ferrite phase. Similar features were observed in the spectra of specimen nos. 2 and 4.
Fig. 3. Neutron imaging results: (a) 2D images near wavelengths of 3.32–3.6 Å collected using the μPIC-based detector; (b) Bragg-edge spectra of specimen nos. 1 and 3 collected using the GEM detector.
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3.2 Analysis results of the Bragg-edge transmission spectra using RITS code The neutron wavelength calibration was performed via a prior measurement of a pure Fe standard sample under the same experimental conditions. Bragg-edge spectra analyses for specimen nos. 2, 3, and 4 failed because of the effect of these specimens’ coarse grains, as exemplified in Fig. 3(b). The 2D transmission spectra of specimen no. 1 (position dependent spectrum collected over the wavelength region between 1.5 and 5.5 Å using the GEM detector) were analyzed using RITS code. To clarify the microstructure differences between positions near the surface and those in the interior of the cast specimen, positions A and B in Fig. 3(a), each with an area of approximately 5 × 5 mm2, was used for analysis. Figures 4(a) and 4(b) show the fitting results for each transmission spectrum. Here two-phase analysis was performed using the chemical compositions of the Fig. 4. RITS-code total profile fitting of the spectrum for measured specimens. Parameters specimen no. 1. for lattice constants, crystallite sizes, and degree of crystallographic anisotropy (evaluated by the March–Dollase (MD) coefficient) were refined. Table III summarizes the refined parameters for the two positions. The results show that the lattice constants of both the BCC and FCC phases at position B are larger than those at position A. In addition to residual stress, another possible reason for the Table III. Refined parameters of positions A and B in specimen no. 1. larger lattice parameters at position B is the Position A B biased separation of Phase BCC FCC BCC FCC chemical elements Lattice constant (Å) 2.8703 3.5949 2.8725 3.5972 during the casting and Crystallite size (μm) 23.908 6.534 21.798 9.045 rapid quenching process. Preferred orientation The refined lattice vector and MD coefficient 0.6455 0.7610 0.6509 0.8735 constants of the BCC phase are slightly larger than the reference one of pure Fe because of the formation of a solid solution with a high concentration of alloy elements in the specimens. Furthermore, the fitted crystallite size of the BCC phase is approximately 3–4 times greater than that of
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the FCC phase. The obtained crystallite sizes for the BCC and FCC phases were smaller than the grain size determined by optical microscopy observations (Table I). This result is reasonable because the subgrains were also included in the RITS fitting. The MD formula is commonly used in preferred orientation corrections in structure analysis programs such as GSAS [7]. Furthermore, the MD function is applicable to crystallite orientation corrections in Bragg-edge spectra analyses because the textured profile along a Debye–Scherrer ring is averaged in the Bragg-edge profile. Note that the MD coefficient, R, obtained by the RITS provides the degree of crystallographic anisotropy; R = 1 for an isotropic sample and R < 1 ( // beam) or R > 1 ( ⊥ beam) for a textured sample, where represents preferred orientation vector. In the present study, texture evaluation by RITS was conducted under the assumption of a orientation of the BCC phase and a orientation of the FCC phase along the neutron-beam transmission direction. According to the fitted results, similar textures are present at positions A and B. Strong BCC preferred orientations and relatively weak FCC orientations exist in the beam direction. 4. Conclusion The microstructure comprising coarse ferrite and austenite grains in cast DSS blocks were successfully imaged by high-resolution 2D neutron transmission data using a μPIC detector. Quantitative microstructure information was obtained by applying RITS code analysis to the Bragg-edge transmission spectra of specimen no. 1 collected using the GEM detector. However, analyses of the spectra of other samples with large grains were unsuccessful because of the highly complicated transmission patterns. Acknowledgment This work was supported by the Photon and Quantum Basic Research Coordinated Development Program of the Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors would like to thank NIDAK Co. for preparing the specimens. References [1] H. Sato, T. Kamiyama and Y. Kiyanagi: J. Materials Trans. 52 (2011) 1294. [2] H. Sato, T. Shinohara, R. Kiyanagi, K. Aizawa, M. Ooi, M. Harada, K. Oikawa, F. Maekawa, K. Iwase, T. Kamiyama and Y. Kiyanagi: J. Phys. Procedia 43 (2013) 186. [3] O. Takahashi, M. Yabe, Y.Shibui, Y. Tomota: J. Tetsu-to-Hagané 100 (2014) 102 (in Japanese). [4] T. Hosoya, T. Nakamura, M. Katagiri, A. Birumachi, M. Ebine, K. Soyama: J. Nucl. Instr. Meth. A. 600 (2009) 217. [5] J.D. Parker, M. Harada, K. Hattori, S. Iwaki, S. Kabuki, Y. Kishimoto, H. Kubo, S. Kurosawa, Y. Matsuoka, K. Miuchi, T. Mizumoto, H. Nishimura, T. Oku, T. Sawano, T. Shinohara, J. Suzuki, A. Takada, T. Tanimori, K. Ueno: J. Nucl. Instr. Meth. A. 726 (2013) 155. [6] S. Uno, T. Uchida, M. Sekimoto, T. Murakami, K. Miyama, M. Shoji, E. Nakano, T. Koike, K. Morita, H. Sato, T. Kamiyama, Y. Kiyanagi: J. Phys. Procedia 26 (2012) 142. [7] A. C. Larson and R. B. Von Dreele, Los Alamos National Laboratory Report LAUR 86-748, Los Alamos National Laboratory (2004).
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