Low Temperature Grain Boundary Diffusion of Chromium in SUS316 and 316L Stainless Steels

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1 Materials Transactions, Vol. 4, No. (4) pp. 94 to 9 #4 The Japan Institute of Metals Low Temperature Grain Boundary Diffusion of Chromium in SUS6 and 6L Stainless Steels Masaki Mizouchi, Yoshihiro Yamazaki *, Yoshiaki Iijima and Koji Arioka Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai , Japan Institute of Nuclear Safety System, Incorporated, Mihama-cho, Fukui 99-, Japan Grain boundary diffusivity of chromium in SUS6 and 6L stainless steels has been determined in the temperature range between 8 and 7 K. The magnitudes of the grain boundary diffusivities in four kinds of specimens are in the order of the cold-worked SUS6, the SUS6L, the SUS6 and the sensitized SUS6. The grain boundary diffusivities in these specimens are remarkably higher than those of previous works. The activation energies for the former are 8 9 kjmol, whereas those for the latter are 4 kjmol. (Received May 4, 4; Accepted August, 4) Keywords: grain boundary diffusion, SUS6 and SUS6L stainless steels, inter-granular stress corrosion cracking (IGSCC), chromium. Introduction Austenitic stainless steels have been widely used as structural materials in nuclear power plants because of their excellent strength at high temperature and good corrosion resistance. However, inter-granular stress corrosion cracking (IGSCC) of these stainless steels has been observed in service in Boiling Water Reactors (BWR) after a very long duration. ) Incidents of failure by IGSCC have led to various investigations of chemical, metallurgical and mechanical factors associated with the cracking of the austenitic stainless steels. ) Arioka and coworkers,) have studied the temperature dependence of IGSCC under hydrogenated water and observed a low activation energy of 7 kjmol. They have also noted that the grain boundary sliding (creep) might play a significant role on IGSCC.,) On the other hand, the activation energy for steady state creep in SUS6 above 798 K is 9 kjmol, while that below 798 K is kjmol. ) To understand the degradation and fracture processes of the stainless steels, information on grain boundary diffusion is important. In particular, experimental data on it at lower temperatures around 6 K, which is an ordinary temperature region in service of light water reactors (LWR), are essential. However, experiments on the grain boundary diffusion in austenitic stainless steels have been so far limited to the high temperature range above 8 K, 4) because experiments of grain boundary diffusion as well as volume diffusion at lower temperatures are not easy. Perkins et al. ) have measured the grain boundary diffusivity of Cr in Fe-7Cr-Ni alloy (concentration of solute is expressed in the unit of mass%) in the temperature range between 849 and K. They have obtained the preexponential factor : 4 m s and the activation energy kjmol. Smith 6) has measured the grain boundary diffusivity of Cr in Fe-6.8Cr-.6Ni-.Mo-.64Mn-.Si-.4C alloy in the temperature range from 9 to 8 K. The preexponential factor : m s and the activation energy 9 kjmol have been obtained. Čermák 7) has measured the diffusivity of Cr in Fe-8Cr- *Corresponding author, address: yamazaki@material.tohoku.ac.jp Ni and Fe-Cr-Ni alloys in the temperature range from 97 to K and obtained the diffusion parameters, : m s and 4 kjmol, for both of the alloys. Čermák et al. 8) have also studied the influence of carbon addition on the grain boundary diffusivity of Cr in Fe-Ni-Cr alloy. The grain boundary diffusivity decreases with increasing of the carbon content of. to.7 mass%. In all these experiments, grain boundary diffusivity has been deduced from the analysis of type B kinetics using the numerical value of the volume diffusion coefficient measured in a relatively high temperature region. Therefore, these data on grain boundary diffusion are also limited to the corresponding high temperature region. In view of these considerations, the present paper reports the volume and grain boundary diffusion coefficients of chromium using Cr tracer in the SUS6 and SUS6L, the sensitized SUS6 and the cold-worked SUS6 alloys over a temperature range of 8 7 K.. Experimental Procedure Plates mm in thickness of SUS6 and 6L austenitic stainless steels were supplied by Institute of Nuclear Safety System, Incorporated. The chemical compositions of the SUS6 and 6L stainless steels are shown in Table. Four kinds of specimens were prepared to examine the effect of carbon, carbide precipitation and cold work on grain boundary diffusion. At first, for the solution heat treatment the plates of SUS6 and 6L stainless steels were annealed at 4 K for 6 s and then quenched into water. Then, the sensitized SUS6 specimen was made by heating at 9 K for 7.8 ks in air. The cold worked SUS6 specimen was made by cold-rolling the plate mm down to 4 mm in Table Chemical compositions of SUS6 and 6L (N in mass ppm, others in mass%). N Specimen Cr Ni Mo Mn C Si P S Al (ppm) SUS SUS6L

2 946 M. Mizouchi, Y. Yamazaki, Y. Iijima and K. Arioka thickness reduction of %. After these treatments, four kinds of the plates were machined into the discs mm in diameter and mm in thickness. The surfaces of discs were electropolished at 7 K in an aqueous solution containing 4 vol% acetic acid and vol% perchloric acid. Some specimens of the SUS6 were used for measurements of volume diffusion of chromium, because the volume diffusion coefficient is required to deduce the grain boundary diffusivity in the analysis of type B kinetics. To cause grain growth, they were annealed at 6 K for 86.4 ks in a stream of hydrogen gas purified by permeation through a palladium alloy tube. The resultant grain size was 6 mm. On the other hand, the grain sizes of the specimens of the SUS6 and 6L and the cold-worked SUS6 were 4 8 mm, while that of the sensitized SUS6 was 6 mm. All the specimens were ground on abrasive papers and polished with fine alumina paste. The specimens for the experiment of volume diffusion were further annealed at 7 K for 7. ks in an evacuated quartz tube to obtain a stress-free surface. For the annealing, the specimen was wrapped in a tantalum foil to prevent volatilization of chromium from the surface of the specimen. Tantalum foil is effective in keeping away the specimen from the quartz tube to avoid the reaction between them. The distribution of grain boundary character was measured using an orientation imaging microscope. In particular, by compression of %, the ratio of the length of random grain boundary to the total length of grain boundary increases from 4 to 66%. The radioisotope Cr (-ray,. MeV; half-life, 7.7 d) was procured from ICN Biomedicals, Inc., U.S.A. in the form of CrCl in. kmolm HCl solution. It was electroplated onto the polished surface of the specimen with the current density 4 ma cm for 8 s. A quartz tube containing the specimen wrapped with a tantalum foil was evacuated under 4 Pa, and then it was put into an electric furnace controlled within K during diffusion annealing at a temperature in the range from 8 to 7 K for.4 to. ks. After diffusion anneal, the cylindrical surface of the specimen was reduced by a depth of..4 mm to remove the effect of surface diffusion. To measure the penetration profiles of the radioisotope into the specimen, two types of the serial sectioning methods were employed; the ion-beam sputter-microsectioning method for short penetration profiles of volume diffusion and the serial sectioning method by grinding on abrasive papers for long penetration profiles of grain boundary diffusion. The intensity of -ray emitted from each section was measured with the help of a well-type Tlactivated NaI detector attached to a 4 channel pulse height analyzer.. Results and Discussion. Volume diffusion of Cr in SUS6 The boundary conditions used in the present experiments are an instantaneous source and a semi-infinite medium. The appropriate solution of Fick s law is given by p CðX; tþ ¼ M= ffiffiffiffiffiffiffiffiffiffi D v t expð X =4D v tþ; ðþ X / -4 m for A and B C (888K,.8ks) A (7K, 7.ks) B (K,.6ks) where CðX; tþ is the concentration of the radioactive tracer at a distance X from the original surface, D v the volume or lattice diffusion coefficient, t the time of diffusion anneal and M the total amount of deposited tracer at X ¼ and t ¼. This equation represents the material transport within the lattice through volume diffusion. Figure shows the typical penetration profiles of ln CðX; tþ versus X for the diffusion of Cr in SUS6. The linearity observed in Fig. proves that eq. () will hold, and thus the volume diffusion has been observed. Figure shows the Arrhenius plot of the volume diffusion coefficient of Cr in SUS6 by the present work in comparison with those by Perkins et al. ) and Smith. 6) From the linear Arrhenius plot observed in the temperature range from 888 to 7 K the preexponential factor and the activation energy are obtained to be : 7 m s and 4 kjmol, respectively. The compositions of the alloys used by Perkins et al. ) and Simth 6) are similar to that used by the present work. Although the diffusion coefficients obtained by Perkins et al. at lower temperatures are nearly equal to those by the present work, at higher temperatures the values of the diffusion coefficients by Perkins et al. ) and Simth 6) are larger than those by the present work. However, strict comparison between them is unfruitful, because stainless steels contain usually many elements besides residual impurities, as shown in Table. The Arrhenius lines for grain boundary diffusivities in stainless steels obtained by many authors form a wide band. 4). Grain boundary diffusion of Cr in SUS6 and 6L To determine the diffusion coefficient within the lattice as well as the grain boundary simultaneously, we need an analytical procedure to separate the contributions of the 4 X / -4 m for C SUS6 Fig. Typical penetration profiles of ln CðX; tþ versus X for diffusion of Cr in SUS6. 4 6

3 Low Temperature Grain Boundary Diffusion of Chromium in SUS6 and 6L Stainless Steels 947 T / K present work Perkins et al. (97) Smith (97) SUS K,.6ks -8 K,.8ks -9-7K, 79.ks - 99K, 4ks T - / - K - X 6/ / -6 m 6/ Fig. Arrhenius plot of volume diffusion coefficient of Cr in SUS6 by present authors in comparison with those by Perkins et al. and Smith. Fig. Penetration profiles of ln C versus X 6= for diffusion of Cr in SUS6. lattice and the grain boundary to the overall depth profile of the diffusing species. When the value of mean diffusion distance ðd v tþ = is smaller than d= (d diameter of the grains), the grain boundary diffusion behavior can be described by the type B kinetics referred to Harrison. 9) For the case of diffusion from an instantaneous source in the type B kinetics, Suzuoka s solution is given by,) sd gb ¼ :8ðD v tþ = ln C=@X 6= Þ = ; when the parameter ð¼ sðd gb =D v Þ=ðD v tþ = Þ is larger than 4 as is the case of the present experiments. s is the segregation factor, is the width of grain boundary and C is the mean concentration of the tracer at a distance X from the original surface after a diffusion time t. D gb is the grain boundary diffusion coefficients of the tracer. The product sd gb represents the characteristic of grain boundary diffusion and is usually called the grain boundary diffusivity. On the other hand, when lattice diffusion is negligible and a significant transport of matter occurs only within the grain boundary, the average concentration of indiffused material, as determined by a sectioning experiment, will vary with the penetration distance in the same way as lattice diffusion. This situation refers to the Type C kinetics by Harrison. 9) Under the condition that ðd v tþ = is smaller than =, the tail region in a logarithmic plot of C will vary linearly with the penetration distance squared and the slope will yield D gb itself. Figures and 4 show the penetration profiles of ln C versus X 6= for the diffusion of Cr in the SUS6 and 6L, respectively. Each penetration profile exhibits two regions: one is the near-surface region and the other the deeper region of the profile. The former mainly corresponds to the volume diffusion process, while the latter is due to a high-diffusivity path: the diffusion through this path is faster than that in the grain. The slopes of the highdiffusivity contribution in Figs. and 4 are used for the ðþ SUS6L 7K, 79.ks 99K, 4ks 4 X 6/ / -6 m 6/ Fig. 4 Penetration profiles of ln C versus X 6= for diffusion of Cr in SUS6L. calculation of the grain boundary diffusivity sd gb using eq. (). The calculated values of sd gb are listed in Table as well as the grain boundary diffusion parameters, where the conditions of type B kinetics are satisfied. The Arrhenius plots for the grain boundary diffusivity of Cr in the SUS6 and 6L are shown as solid circles and squares, respectively, in Fig. in comparison with those by the previous authors. 7) The grain boundary diffusivity in SUS6L is higher than that in SUS6. Furthermore, it should be noted that the diffusivities in SUS6 and 6L are about two-three orders of magnitude higher than those observed in austenitic

4 948 M. Mizouchi, Y. Yamazaki, Y. Iijima and K. Arioka Table Grain boundary diffusivities and diffusion parameters for Cr in SUS6 and 6L. Specimen T/K t/s sd gb /m s SUS6 7 6 :8 8 : 6 SUS6 8 4: 9 4:6 SUS :8 9 :77 6 SUS :84 9:9 6 SUS6L 7 79 :6 9 :4 6 SUS6L 99 4 : 9 :78 7 SUS6 SUS6L 7K,.6ks K,.8ks T / K 8 present work, SUS6 present work, SUS6L Perkins et al. (97) Smith (97) Cermák ˇ (99) 7K, 79.ks 99K, 4ks 7K, 79.ks 99K, 4ks X 6/ / -6 m 6/ Fig. 6 Penetration profiles of ln C versus X 6=, where the short depth region in Figs. and 4 is enlarged stainless steels by the previous authors. It is well known that the magnitude of grain boundary diffusivity depends greatly on the grain boundary character, ) even if the diffusivity is calculated from a linear tail in the plot of ln C versus X 6= under the conditions of type B kinetics. As mentioned earlier, the mean diffusion distance ðd v tþ = is an important parameter in the analysis of penetration profiles. In the paper of Perkins et al., ) the penetration depth was not shown. On the other hand, Smith 6) and Čermák 7) have determined sd gb from the tail slope where the ratio of the penetration depth to ðd v tþ = is in the range from to 6 6) and from 8 to, 7) respectively, whereas in the present experiments the ratio is in the range from to 7. Figure 6 shows the same penetration profiles as Figs. and 4. However, the short depth regions are enlarged, where the linear slopes are also observed. Then, the value of sd gb is recalculated and plotted in Fig. as open circles and squares. The recalculated values of sd gb are in a similar range that observed by the previous authors. 7) This means that the present experiment has observed the grain boundary diffusion via faster paths than those observed by Smith and Čermák. Figures 7, 8, 9 and show the penetration profiles of ln C versus X for the diffusion of Cr in the SUS6 and 6L, the sensitized SUS6 and the coldworked SUS6, respectively. In all profiles, two regions are. T - / - K - Fig. Arrhenius plots for grain boundary diffusivities of Cr in SUS6 and 6L.... observed as in Figs. and 4. From the linear tail in these profiles, the grain boundary diffusion coefficients D gb of Cr in these alloys are calculated and listed in Table. The condition of Type C kinetics: ðd v tþ = is smaller than =,is fully satisfied if m is put into, as in the usual case of metals. ) The Arrhenius plots for the grain boundary diffusion coefficients D gb of Cr in the SUS6 and 6L, the sensitized SUS6 and the coldworked SUS6 are shown in Fig. in comparison with those converted from the grain boundary diffusivity sd gb by putting s ¼ m. The grain boundary diffusion coefficients obtained by the present work are remarkably higher than those of the previous authors. The linear Arrhenius plots of the grain boundary diffusion coefficients obtained by the type C kinetics can be expressed as follows: Solution-treated SUS6 D gb ¼ : : þ:8 expð 87: :6 kjmol =RTÞ m s Solution-treated SUS6L D gb ¼ :84 : þ: expð 9:6 : kjmol =RTÞ m s Sensitized SUS6 D gb ¼ 4:87 þ9:9 :89 6 expð 8: 7:8 kjmol =RTÞ m s Cold-worked SUS6 D gb ¼ :6 :4 þ:7 expð 9: : kjmol =RTÞ m s As seen in Fig., the values of D gb deduced from the values of sd gb are on the lines extrapolated from the ðþ ð4þ ðþ ð6þ

5 Low Temperature Grain Boundary Diffusion of Chromium in SUS6 and 6L Stainless Steels 949 X / -8 m for D X / -8 m for A SUS6 sensitized SUS6 A (698K,.4ks) B (668K,.9ks) A (6K, 7.ks) C (6K, 7.ks) B (98K, 84.6ks) D (48K, 6.8ks) X / -8 m for A, B and C Fig. 7 Penetration profiles of ln C versus X for diffusion of Cr in SUS6. 4 C (48K, 6.8ks) X / -8 m for B and C Fig. 9 Penetration profiles of ln C versus X for diffusion of Cr in sensitized SUS6. SUS6L. X / -8 m for B. cold-worked SUS6 68K, 48.ks 48K, 6.8ks A (6K, 7.ks) B (68K, 48.ks) K,.ks C (48K, 6.8ks) X / -8 m Fig. 8 Penetration profiles of ln C versus X for diffusion of Cr in SUS6L. Arrhenius relations of eqs. () and (4), suggesting that the assumption of s ¼ m for these alloys is reasonable. The magnitudes of D gb among the SUS6 and 6L and the sensitized SUS6 can be easily understood, because the value of D gb decreases with increasing the carbon content 8) and carbide ) in the grain boundary. The value of D gb in the cold-worked SUS6 is largest among the four kinds of specimens as shown in D (K,.ks) X / -8 m for A, C and D Fig. Penetration profiles of ln C versus X for diffusion of Cr in coldworked SUS6. Fig.. According to the measurements of grain boundary character, the ratio of the length of random grain boundary to the total length of grain boundary increases from 4 to 66% by thickness reduction of %. Substantially, the cold-work increases faster diffusion paths, because the diffusion through random grain boundaries is faster than the diffusion through

6 9 M. Mizouchi, Y. Yamazaki, Y. Iijima and K. Arioka Table Grain boundary diffusion coefficients for Cr in SUS6 and 6L, sensitized SUS6 and cold-worked SUS6. Specimen T/K t/s D gb /m s SUS :7 SUS :6 SUS6 6 7 : SUS :8 SUS :7 SUS :8 SUS :9 4 SUS :8 4 SUS6L : SUS6L 6 7 :76 SUS6L : SUS6L :8 SUS6L :7 4 SUS6L 4:46 4 SUS6L 8 66 :7 4 sensitized SUS : sensitized SUS :4 sensitized SUS :78 sensitized SUS :64 sensitized SUS :68 4 sensitized SUS :9 4 sensitized SUS :9 cold-worked SUS :6 cold-worked SUS :4 cold-worked SUS6 6 7 :96 cold-worked SUS :79 cold-worked SUS :6 cold-worked SUS : 4 cold-worked SUS6 4:7 4 cold-worked SUS :4 4 sub-grains and small angle grain boundaries. It should be emphasized that the grain boundary diffusion of chromium in SUS6 is much faster than that recognized by the previous works and the activation energy for it is much smaller than that known so far. 4. Conclusion Grain boundary diffusion coefficients of Cr in SUS6 and 6L have been measured in the temperature range between 8 and 698 K using the type C kinetics and between 99 and 7 K using the type B kinetics. The present results show remarkably high diffusivity and low activation energy in comparison with those obtained by previous authors, suggesting that the previous authors have observed grain boundary diffusion through relatively slow paths such as subgrains and small angle grain boundaries. REFERENCES -7 present work cold-worked SUS6 SUS6L SUS6 sensitized SUS6 Perkins et al. (97) Smith (97) Cermák ˇ (99). T / K ) S. H. Bush and R. L. Dillon: Stress Corrosion Cracking and Hydrogen Embrittlement of Iron Base Alloys, (National Association of Corrosion Engineers, Texas, 977) p. 6. ) K. Arioka, Y. Kaneshima and T. Yamada: J. Inst. Nuclear Safety System (). ) K. Arioka, Y. Kaneshima, T. Yamada and T. Terachi: Proceedings of the th International Symposium on Environmental Degradation of Materials in Nuclear Power Systems, (Stevenson, USA, ). 4) I. Kaur, W. Gust and L. Kozma: Handbook of Grain and Interphase Boundary Diffusion Data, (Ziegler Press, Stuttgart, 989) p. 68. ) R. A. Perkins, R. A. Padgett, Jr. and N. K. Tunali: Metall. Trans. 4 (97) 4. 6) A. F. Smith: Metall. Sci. 9 (97) ) J. Čermák: Z. Metallk. 8 (99) ) J. Čermák, J. Ru žičková, A. Pokorná and B. Million: Mater. Sci. Eng. A8 (99) ) L. G. Harrison: Trans. Faraday Soc. 7 (96) ) T. Suzuoka: Trans. JIM (96). ) T. Suzuoka: J. Phys. Soc. Japan 9 (964) ) A. P. Sutton and R. W. Balluffi: Interfaces in Crystalline Materials, (Clarendon Press, Oxford, 99) p ) T.-F. Chen, G. P. Tiwari, Y. Iijima and K. Yamauchi: Mater. Trans. 44 () T - / - K - Fig. Arrhenius plot for grain boundary diffusion coefficients of Cr in SUS6 and 6L, sensitized SUS6 and cold-worked SUS6..