Magnetism induced by electrochemical nitriding on an austenitic stainless steel Takashi Watanabe, Akio Sagara, Yoshimitsu Hishinuma, Sadatsugu Takayama, Teruya Tanaka, and Saburo Sano Citation: AIP Advances 5, 047138 (2015); doi: 10.1063/1.4919112 View online: http://dx.doi.org/10.1063/1.4919112 View Table of Contents: http://aip.scitation.org/toc/adv/5/4 Published by the American Institute of Physics AIP ADVANCES 5, 047138 (2015) Magnetism induced by electrochemical nitriding on an austenitic stainless steel Takashi Watanabe,1 Akio Sagara,2 Yoshimitsu Hishinuma,2 Sadatsugu Takayama,2 Teruya Tanaka,2 and Saburo Sano3 Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka, 432-8561, Japan National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu, 509-5292, Japan National Institute of Advanced Industrial Science and Technology, 2266-98 Shimo-Shidami, Moriyama-ku, Nagoya, Aichi, 463-8560, Japan (Received March 2015; accepted 15 April 2015; published online 22 April 2015) The surface of a Fe-Ni-Cr Alloy (SUS316L) plate was electrochemically nitrided in molten LiF-KF salt including Li3N at 873K The crystal structure changed from fcc structure to bct structure with nitrogen introduction The Nitrogen diffusion layers were predominately formed at nitrogen concentration of 23 at% The nitriding process drastically also changed its magnetic property from non-magnetic to ferromagnetic The magnetic field of 20 kOe saturated the magnetic moment with its magnetization of 81 emu/g at 10K The anisotropic magnetization is ascertained Based on CrN formation and Cr extraction from the original Fe-Ni-Cr system, the induced ferromagnetism was discussed C 2015 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4919112] We are developing a molten salt plant for a fusion reactor.1 It works for cooling and tritium breeding using large quantity of extremely hot fluoric molten salt over 450 ◦C When steel is employed as a structural material, its corrosion resistance should be enhanced against the hot molten fluoric salt One of pathbreaking approaches is nitriding treatment Then, we reported an electrochemical nitriding for a stainless steel surface.2 Although we focused on formation of a dense CrN and/or ε-Fe2-3N surface layer, a thick dilute nitrogen diffusion layer was also formed under the CrN layer The original face centered cubic crystal (fcc) structure simultaneously transformed to body centered tetragonal (bct) structure Then, it was found that the nitriding treatment has accompanied with drastic magnetic change Since SUS316 is often employed due to a non-magnetic material, its magnetization is ill-received The drastic magnetic change is a significant factor for machine and plant designing In this article, we present the magnetic change of the nitrided stainless steel A stainless steel SUS316L, the appellation by JIS G4303-2012 and also ASTM A240-15, was employed as a base material The plate of 4.0 × 6.0 × 0.25 mm3 was electrochemically nitrided in molten fluoric salt prepared at a molar ratio of LiF : KF : Li3N = 49 mol% : 49 mol% : mol% The specimen was bind at an end of the nickel working electrode with a nickel wire As counter and the reference electrodes, aluminum rods were employed To avoid influence of humidity to nitriding, the electrochemical equipment was assembled in a glove box filled with dry argon gas These electrodes were dipped into the molten fluoric salt including Li3N at 873 K The voltage of 1.0V vs Li/Li+ was applied to the working electrode for 460 minutes After the nitriding process, the specimen was cooled to room temperature with taking for 10 hours To remove the fluoric salt, it was washed in water and rinsed in acetone Finally, it was dried in dry air Structural observation and chemical analysis were carried out by a field emission type scanning electron microscope (SEM, JEOL JSM-7100F) with energy dispersive X-ray analysis (EDX, Oxford X-act) The cross-section of the specimen was analyzed about iron, chromium, nickel, and nitrogen by EDX The crystal structural characterization was carried out by X-ray diffraction (XRD, Rigaku RINT-2200/VTK) Magnetic moment was measured by a superconducting quantum interference device (SQUID, Quantum Design MPMS-7) and a vibrating sample magnetometer (VSM, 2158-3226/2015/5(4)/047138/6 5, 047138-1 © Author(s) 2015 047138-2 Watanabe et al AIP Advances 5, 047138 (2015) FIG FE-SEM and EDX element mapping images of the cross-section of the nitrided specimen (a)FE-SEM image (b)Nitrogen, (c)iron, (d)chromium and (e)nickel mapping images Toei Industry, VSM-5) While SQUID was used for measuring it at low temperatures from 10 to 300 K, VSM was for measuring its anisotropy Figure demonstrates the SEM and element mapping images The FE-SEM image of Fig 1(a) illustrates that the cross-section with 255 µm width is embedded in resin The tone difference in the cross-section demonstrates that it consists of three layers The element mapping images of Figs 1(b)1(e) demonstrate element distributions in the cross-section; (b)nitrogen, (c)iron, (d)chromium and (e)nickel, respectively The nitrogen mapping image indicates that nitrogen was introduced from the surface to the depth of 74 µm and that the width corresponds to the tone difference described about the SEM image of Fig 1(a) As shown in figs 1(c), 1(d) and 1(e), metal distributions in the nitrided layer also fluctuated along with the nitrogen introduction Thus, these images indicate that an intact SUS316L layer is sandwiched between nitrogen introduced layers with 74 µm width The numbered squares in the FE-SEM image of fig 1(a) indicate the compositional analyzed areas in the cross section Sites and are located in the nitrided layers; site is located in the remained immaculate SUS316L layer Table I summarizes the analytical results for these analyzed areas At sites and 3, the electrochemical nitriding introduced nitrogen about 8.40 weight % into the layers, that corresponds to 23.25 atomic % Then, the mapping images indicate that metal concentration was attenuated in the nitrogen introduced layers And it was also confirmed that, at the site 2, the compositional ratio of the inside layer corresponds to that of SUS316L defined by JIS and ASTM While nickel does not form any nitrides, chromium easily form chromium nitrides, CrN and Cr2N Nitrogen atoms can also form some iron nitrides, dissolve into iron interlayer and form an interstitial solid solution.3 047138-3 Watanabe et al AIP Advances 5, 047138 (2015) TABLE I Chemical composition analyzed by EDX measurement Weight% Analyzed spot N O Si Cr Mn Fe Ni Mo Total Site Site Site 8.45 0.00 8.35 0.81 1.05 0.90 0.63 0.62 0.61 16.44 17.82 16.39 1.60 1.69 1.60 58.61 64.33 58.68 11.23 12.10 11.20 2.23 2.39 2.27 100 100 100 Figure shows XRD patterns of the specimens Miller indices were assigned to the peaks in the XRD patterns by PDF JCPDS database and Cohen method Table II summarizes crystal characterization of the specimens Figure 2(a) shows the pattern of bare SUS316L which was obtained before the electrochemical nitriding process The peaks, which are labeled by Miller index with γ, attribute to austenitic pattern, i.e., fcc pattern The unit cell is defined with lattice parameter a = 0.359 nm which corresponds to typical value of austenitic steel On the other hand, as shown in Fig 2(b), the electrochemical nitriding process drastically changed the XRD pattern When considering those XRD patterns and the EDX mapping images, it would be natural that its attenuation of metal concentration in the nitrided layers attributes to the crystal structural change According to PDF verification and Cohen’s method, the pattern shown in Fig 2(b) consists of CrN (PDF JCPDS #110065) and ferritic patterns Miller indices labeled with α and CrN attribute to ferritic pattern and CrN pattern, respectively Crystal analysis began with assigning an fcc structure with its lattice parameter a = 0.419nm as CrN pattern The other peaks were subsequently assigned as a bct pattern stemming from Mex(x>4)N, where Me indicates metals such as Fe, Cr and/or Ni This pattern change indicates that, with forming CrN, the nitrogen introduction rearranged the crystal structure from fcc structure to bct structure Since, even when pure iron surface is nitrided, the nitrided layer consists of several phases such as ε-Fe2-3N, γ-Fe4N, and α-Fex(x>8)N, it would be natural that multiphase formation is unavoidable Then, the majority is α-Fex(x>8)N.4 By analogy with this, the nitrided SUS316L would accompany with generation of a dilute nitrogen diffused and dispersed layer The lattice parameters, a and c, were estimated at 0.510 and 0.542 nm, respectively Figure shows magnetic properties measured by SQUID, i.e., temperature dependence of saturation magnetization σs The inset demonstrates hysteresis loops: (a) the nitrided SUS316L and (b) the bare SUS316L These loops were measured by normal magnetic field to the specimen between −20 and 20 kOe While SUS316L was non-magnetic, the nitrided SUS316L was ferromagnetic FIG XRD patterns (a) Bare SUS316L and (b) Nitrided SUS316L 047138-4 Watanabe et al AIP Advances 5, 047138 (2015) TABLE II Crystal structure of the specimens Specimen SUS316L Nitrided SUS316L α′′-Fe16N212,13 Crystal structure lattice parameter [nm] fcc CrN fcc Mex(x>4)N bct bct a = 0.395 a = 0.419 a = 0.510, c = 0.541 a = 0.572, c = 0.629 Due to negative spin interaction of chromium incorporated in Fe-Ni system, SUS316L is stable antiferromagnetic steel Even though fcc structure is formed, it is not-ferromagnetism.5,6 On the other hand, the nitrided layer of SUS316L was ferromagnetic The electrochemical nitriding drastically switched from non-magnetic to ferromagnetic This suggests that the nitriding attenuated the negative spin interaction The temperature dependence of σs was measured from measurements at 10, 77, 100, 200 and 300 K Although σs decreased with raising temperate, it was reversibly regained with lowering temperature When normalized by the weight of the nitrided layers in the specimen, we obtained σs of 81.0 emu/g at 10 K and 63.7 emu/g at 300 K And, when extrapolated to K, it reaches to 89.0 emu/g We could explain the enhanced magnetism under chromium extraction from the Fe-Ni-Cr system, SUS316L When, due to CrN formation, chromium concentration decreases in the Fe-Ni-Cr system, fcc structure would be converted to bcc or bct structure, which corresponds to ferritic or martensitic stainless steel Since CrN is antiferromagnetic, the remained Fe-Ni-Cr system would consequently contribute to the magnetism Then, assuming iron, nickel, chromium and molybdenum as remained metals, we can correct it using weight factor using f as follows: Cr,Fe,Ni,Mo m=element Rm f = N,Si,Mn,Cr,Fe,Ni,Mo , (1) Rm m=element where Rm is weight % shown in Table I Then, using the corrected moment m = σs/f , we can obtain magnetic moment per atom as follows à[àB] = 1.078 ì 1020 Mm , NA (2) FIG Magnetic property measured by SQUID Hysteresis loops and temperature dependence of saturation magnetization σs of the nitrided SUS316L 047138-5 Watanabe et al AIP Advances 5, 047138 (2015) where M is mean atomic mass defined by atomic % and atomic mass about iron, manganese, iron, nickel, molybdenum, and NA is Avogadro number Even if chromium remains more than 25% from the original SUS316L composition, µ is estimated at 1.7 µB On the other hand, due to containing chromium, its magnetism would depart from SlaterPauling rule7 and be plotted on a branch inside its triangle curve.8 Even though so, we can explain its magnetic property up to certain points by a rigid band model.9 Then, effective magnetic moments per atom of −0.5, 2.2 and 0.6 µB are allocated for chromium, iron and nickel respectively.10 Since manganese and molybdenum would paramagenetically act, those seldom contribute the magnetic moment of nitrided SUS316L layer Then using those effective magnetic moment and the atomic ratio, Cr,Fe,Ni,Mo m=element µm Rm/Mm µ= , (3) Cr,Fe,Ni,Mo m=element Rm/Mm where Mm is atomic mass Rm/Mm corresponds to atomic ratio Then, µ is estimated at 1.7 µB This is consistent with the experimental result described above Although the magnetization of nitrided SUS316L is far inferior to that over 3.0 µB of α′′Fe16N2, its enhanced ferromagnetism and bct transition evoke those of α′′-Fe16N2.11–14 Its lattice parameters a and c are 0.572 and 0.629 nm To discuss magnetism more, we should define spin configuration of 3d and 4f electrons by identification of nitrogen atom position in its bct structure and definition of its DOS function by CPA calculation.15 The coersive force Hc was 0.4 kOe at 10 K This feeble Hc is scarcely able to retain magnetic charge after applying magnetic field to the nitrided layer In general, including SUS316L, stainless steels contain some other impurities along with iron, nickel and chromium And its metallographic structure also consists of grain and lath structure, which is specific to stainless steel Although magnetic defects stemming from these were expected to enhance the coersive force, those seldom affected on it Figure indicates the anisotropy of hysteresis loop measured by VSM We also examined the direction dependence of the magnetic field on the magnetic properties The inset illustrates the direction of the magnetic field applied to the specimen Although the orthogonal magnetic fields were applied to the specimen, normal (a) and horizontal (b), the obtained σs was 66.2 and 67.5 emu/g, respectively Although there is approximately no difference between those σs, the magnetic susceptibility χ depended on the direction When measured by normal and horizontal magnetic fields, those χ were 12.9×10−3 and 81.8×10−3 emu.g−1.Oe−1., respectively The nitrided SUS316L FIG Anisotropy of hysteresis loops of the nitrided SUS316L These loops measured by VSM (a) The hysteresis loop of the nitrided SUS316L measured by normal magnetic field, (b) that of the nitrided SUS316L by horizontal magnetic field The inset illustrates direction of magnetic field applied to the specimens 047138-6 Watanabe et al AIP Advances 5, 047138 (2015) demonstrated anisotropic ferromagnetism During electrochemical nitriding process in the molten salt, nitrogen was introduced into the specimen by the normal electric field to the specimen surface and deeply diffused into the specimen Although it is unknown why χ depends on the direction of the magnetic field, the direction of nitrogen introduction would affect on the orientation of crystal structure and magnetic domain structure It might consequently make its magnetic response anisotropic This electrochemical nitriding can form bulky ferromagnetic layer on the steel surface The magnetic property of iron nitrides strongly depends on its crystal structure and nitrogen amount Non-stoichiometric nitrogen is easy to diffuse in metal crystals Since the nitriding drastically changes magnetic property of austenitic steel, plant designing about employing the surface nitrided austenitic steel would require a consideration for its magnetization In conclusion, the surface of austenitic stainless steel, SUS316L plate was electrochemically nitrided in molten LiF-KF salt including Li3N at 873K XRD measurement revealed that the nitriding process changed its crystal structure from austenitic fcc structure to bct structure Then, the XRD peak patterns were precisely assigned to those derived from CrN and Mex(x>4)N According to EDX analysis on the cross-section, the electrochemical nitriding introduced nitrogen atoms from the surface into the depth of 74 µm and that a nitrogen diffusion layer was predominately formed at nitrogen concentration of 23 atomic% VSM and SQUID measurements revealed that the electrochemical nitriding process drastically changed its magnetic 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ADVANCES 5, 047138 (2015) Magnetism induced by electrochemical nitriding on an austenitic stainless steel Takashi Watanabe,1 Akio Sagara,2 Yoshimitsu Hishinuma,2 Sadatsugu Takayama,2 Teruya Tanaka,2... property of austenitic steel, plant designing about employing the surface nitrided austenitic steel would require a consideration for its magnetization In conclusion, the surface of austenitic stainless. .. transition evoke those of α′′-Fe16N2.11–14 Its lattice parameters a and c are 0.572 and 0.629 nm To discuss magnetism more, we should define spin configuration of 3d and 4f electrons by identification