Formation of Intergranular M 23 C 6 in Sensitized Type-347 Stainless Steel

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1 , pp Formation of Intergranular M 23 C 6 in Sensitized Type-347 Stainless Steel Tatsuya FUKUNAGA, 1) Kenji KANEKO, 1) * Rika KAWANO, 1) Kakeru UEDA, 1) Kazuhiro YAMADA, 1) Nobuo NAKADA, 1) Masao KIKUCHI, 2) Jonathan Simon BARNARD 3) and Paul Anthony MIDGLEY 3) 1) Department of Materials Science and Engineering, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi, Fukuoka, Japan. 2) Research Center for Steel, Kyushu University, 744 Motooka, Nishi, Fukuoka, Japan. 3) Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ UK. (Received on May 22, 2013; accepted on August 19, 2013) Formation of intergranular M 23C 6 carbides and thereafter Cr-depletion zones in commercially available type-347 stainless steels were observed by optical microscopy, conventional transmission electron microscopy and analytical transmission electron microscopy. At the early stage of sensitization, only NbC carbides were observed both intragranularly and intergranularly. At the later stage, formation of intergranular M 23C 6 carbides and intergranular corrosion were found. A model is proposed to describe the formation kinetics of intergranular precipitation of NbC and of M 23C 6, as well as Cr-depletion zones, thermodynamically. Evolution mechanisms of intergranular precipitates were found in the order of 1) the formation of intergranular NbC, 2) the coarsening of NbC, 3) the formation of M 23C 6, then 4) the coarsening of M 23C 6 and formation of Cr-depletion zones. KEY WORDS: carbides; precipitation; sensitization; OM; TEM. 1. Introduction Most standard method to suppress the formation of Crrich M 23C 6 carbide at grain boundaries and associated intergranular Cr-depletion zone is the intentional addition of strong carbide forming alloying elements, such as Ti, V, Zr, Nb, Hf and Ta with comparison to Cr. 1,2) Among these elements, Nb added austenitic stainless steel has been commercially available as type-347 stainless steel employed in applications up to K, in which Nb combines with C forming NbC, preferentially. 3,4) NbC carbides are found intragranular favorably, at dislocations, at stacking faults, or both, and the presences of those improve the strength and creep resistance of the steel. 5,6) The defects at grain boundaries can also act as the sites of intergranular NbC, such as steps, ledges, discontinuities and dislocations. 7,8) The preferential formation of NbC and consumption of free C in solid solution state would suppress the formation of Cr-rich M 23C 6 and that of Cr-depletion zone at grain boundaries, which eventually reduces the susceptibility to the corrosion at grain boundaries. It is obvious that the best corrosion resistance is achieved when the whole C atoms in solid solution state are combined to Nb atoms forming NbC completely. For this reason, the content (wt.%) of Nb should usually be large enough, more than ten times of that of C, 9) to deliver the maximum NbC precipitation in the stainless steel. In fact, there are two different purposes in adding Nb; one is * Corresponding author: kaneko@zaiko.kyushu-u.ac.jp DOI: to stabilize the material against intergranular corrosion, another one is to provide good creep resistance in the material. If the good creep resistance is the primary aim, a solution heat treatment should be performed to dissolve NbC. Nevertheless, the residual C in solid solution would still result in the formation of Cr-rich M 23C 6 carbide at the sensitization temperatures, K, and thus resulting in a poor corrosion resistance. It is therefore that the ductility and toughness may change after the long exposure to high temperature circumstances, so that the detail information between the microstructural evolution and the change of mechanical properties are always indispensable. The objective of the present investigation is to characterize the microstructural evolution of precipitates at different sensitization period, to gain a further understanding of intergranular precipitation phenomena in type-347 austenitic stainless steel during the realistic service conditions. In this study, optical microscopy (OM) and transmission electron microscopy (TEM) were performed to characterize precipitates at grain boundaries. Some additional analytical investigations were carried out further to investigate the distribution of Nb and Cr at grain boundaries by scanning-tem (STEM) with energy dispersive X-ray spectroscopy (EDS) and evolution kinetics of intergranular precipitates were discussed. 2. Experimental Procedure The chemical composition of the commercially available type-347 stainless steel (supplied by Nippon Steel & Sumikin Stainless Steel Corporation, Hikari, Japan) investi ISIJ 148

2 Table 1. Chemical composition of the austenitic stainless steel investigated (in wt.%). C Si Mn P S Ni Cr Mo Cu Ti Nb Al N gated in this paper is given in Table 1. Samples were solution treated at K for 3.6 ks in prior to the isothermal ageing for four different sensitization periods, 3.6 ks, 36 ks and 360 ks at 973 K. Samples were then mechanically polished and electrolytically etched with the aqueous solution of 10% oxalic acid for OM observation. Standard procedure of TEM characterization, namely bright-field TEM and selected area electron diffraction pattern (SAEDP) of aged samples at 973 K, were conducted by computer-controlled fully digitized TEMs (Tecnai-F20, FEI, Eindhoven, the Netherlands; ARM-200F, JEOL, Japan) with a scanning-tem high-angle annular dark-field (STEM- HAADF) detector. EDS mappings were also performed by STEM to achieve two-dimensional elemental distributions using drift-correcting programs (TIA of FEI and Analysis Station of JEOL). 3. Results 3.1. OM Observation The microstructures of the sample without ageing, and the samples aged for different periods, 3.6 ks, 36 ks and 360 ks, were observed by optical microscopy. The optical micrographs let us visualize the clear view of the formation of Crrich M 23C 6 at longer exposure to the sensitization. Figure 1 shows a typical set of the microstructural appearance of these samples after electrolytical etching, which showed the absences of intergranular precipitates from the sample without ageing and with 3.6 ks of sensitization, Figs. 1(a) and 1(b), respectively. Several intergranular precipitates started appearing from the sample with 36 ks of sensitization, as shown in Fig. 1(c). After 360 ks of sensitization, the traces of intergranular corrosion were found as the thick black lines along the grain boundaries, as appeared in Fig. 1(d). These results strongly suggested that the nature of intergranular precipitates present in 36 ks sensitized sample is different from 360 ks one. Occasional appearances of undissolved intragranular precipitates were seen from whole samples TEM Observation Homogeneous dispersion and distribution of fine intragranular NbC were found in the matrix from the sample after 3.6 ks of sensitization, as shown in Fig. 2, which were probably formed during the manufacturing and the solution annealing for the duration of material process. Several intergranular precipitates started appearing when the sample was sensitized for 36 ks, as shown in Fig. 3(a). Analysis of these precipitates by SAEDP obtained by TEM revealed that these precipitates were fcc structure with a lattice parameter about 0.45 nm, which also maintained cubecube orientation relationship to one of the neighboring matrix, as seen from SAED patterns obtained from the intergranular precipitates and the neighboring grain in Fig. 3(b). Furthermore, those intergranular precipitates were clearly found rich in Nb from elemental maps, as shown in Fig. Fig. 1. Fig. 2. The microstructures of sensitized samples for different periods, (a) untreated, (b) for 3.6 ks, (c) for 36 ks and (d) for 360 ks. A typical BF-TEM image of sensitized sample for 3.6 ks. 3(c), so that the newly formed intergranular precipitates were confirmed as NbC. The elemental distribution map of carbon is very faint due to the insensitiveness of EDS for light elements. Furthermore, coarsened intergranular precipitates with their sizes about a few hundred nm were found from the sample after 360 ks of sensitization treatment, indexed as A and B, as shown in Fig. 4(a). According to SAEDP analyses of these precipitates, they were both fcc structured with their lattice parameters about 0.46 nm and about 1.07 nm, respectively. Both of them maintained cube-cube orientation relationship to one of the neighboring austenite grain, as can be seen from SAEDPs in Fig. 4(b). In addition, elemental distribution maps obtained by STEM-EDS from similar region showed that the type A precipitates were found rich in Nb but poor in Cr, and the type B precipitates were found the other way around. The type A precipitates were therefore confirmed as developed NbC, as in the case of intergranular precipitates found in 36 ks sensitized sample, and the newly formed type B precipitates at the vicinity of NbC were confirmed as Cr-rich M 23 C 6. According to Sasmal, M 23 C 6 can be formed around undis ISIJ

3 Fig. 3. (a) and (b) is showing a TEM image and an SAED pattern, respectively, obtained from the region of running precipitates. Figure 3(c) is showing a typical set of STEM-DF image and the elemental distribution maps obtained from one of the running precipitate by STEM-EDS. Fig. 4. (a) is showing a typical TEM image obtained the sensitized sample for 360 ks showing two precipitates indexed as A and B. Figures 2(b) and 2(c) is showing SAED patterns obtained from these precipitates. A typical set of STEM-DF image as well as the elemental distribution maps obtained by STEM-EDS are also shown in Fig. 2(d) ISIJ 150

4 Fig. 5. Schematic diagrams showing the sequences of the formation of Cr-depletion zone in connection with intergranular precipitation of NbC. solved NbC particles in a stabilized austenitic stainless steel, which probably took place at grain boundaries for the case of 360 ks sensitized sample. 6) 3.3. The Evolution Kinetics at Sensitization Temperatures After these microscopic investigations, four stages are proposed as the evolution kinetics during the sensitization treatment. 1 st stage: Fig. 5(a) Although some undissolved intragranular NbC were also found, Nb, Cr, C and other minor elements were assumed to be almost fully dissolved in the matrix since there was no Cr-rich M 23C 6 found at the initial stage. 2 nd stage: Fig. 5(b) Dissolved Nb and C atoms within the matrix and at grain boundaries become intergranular NbC. 3 rd stage: Fig. 5(c) The content of Nb and C become less during the formation and the growth of NbC at the vicinity of NbC as well as along grain boundaries. 4 th stage: Fig. 5(d) Free C atoms in solid solution state react with Cr atoms at the region of less Nb content and forms Cr-rich M 23C 6 as well as Cr-depletion zones. Further coarsening of M 23C 6 may take place at longer sensitization period. Fig. 6. (a) presents the schematic diagram of NbC and the matrix, as 10 nm and 1 μm, respectively. (b) and (c) show the simulated result of the diffusivities of Nb and C at 973 K from the interface Thermodynamics and Kinetics Simulation of Nb Depletion Formation The concurrent formation of intergranular M 23C 6 after NbC suggests the presences of compositional region where M 23C 6 could be stable carbide, strongly. As schematically drawn in Fig. 5(b), the formation and growth of intergranular NbC probably caused the decrease of Nb content at the vicinity of NbC and at grain boundaries, which resulted the formation of intergranular M 23C 6. The diffusivities of Nb and C at 973 K were therefore simulated via DICTRA (ver.26 database: SSOL2&Mob2) with the sizes of NbC and the matrix as 10 nm and 1 μm, respectively, as shown in Fig. 6(a). The starting composition of NbC and the matrix were set to be pure NbC and Fe 17.41Cr 0.45Nb 0.03C, respectively. The variations of Nb and C contents at the vicinity of NbC/matrix interfaces with respect to the distance and the time were calculated as Figs. 6(b) and 6(c), respectively. The content of Nb in the matrix reached almost zero mass% at the interface, then gradually increased. Furthermore, longer the period of sensitization treatment, the wider the distance of low Nb content. C also diffuses as in the case of Nb, Fig. 7. Phase equilibrium diagram at 973 K, there the C content is the content of C in solid solution obtained by ThermoCalc. but the content of C could not reach less than 0.2 mass%, so that C-depletion region is not present due to its long-range diffusivity as shown in Fig. 6(c). These phenomena can also be explained from the isothermal model simulated by ThermoCalc (database: SSOL2) at 973 K, as shown in Fig. 7, where NbC is only the stable carbide in the case of the matrix with the compositional region A, and both NbC and M 23C 6 become the stable carbides in that with the compositional region B. It is therefore that when there is a region with small amount of Nb at the interface close to NbC, as region B, M 23C ISIJ

5 becomes stable carbides. Namely, the kinetics of M 23C 6 formation becomes somewhat favorable due to the decrease of Nb content. It has been reported that the M 23C 6 would nucleate at the interface between the undissolved NbC particles 6) and the austenite matrix without cube-cube orientation relationship. M 23C 6 particles may grow into the austenite matrix around these particles with supplied free C from the matrix. The region with less Nb content would probably become the nucleation sites of Cr-rich M 23C 6, in particular, at the vicinity of NbC and along the grain boundary. The interface between NbC particles and the austenite matrix with cubecube orientation relationship is less favorable to act as the nucleation site of M 23C 6 than the grain boundaries, energetically. 4. Conclusion In conclusion, from microstructural evolutions of type- 347 SS at three different sensitization periods, 3.6 ks, 36 ks and 360 ks, were examined by OM and TEM. Not only the standard TEM procedure, such as bright-field imaging and selected area diffraction pattern, but also elemental distribution mapping were carried out. According to OM, the various precipitates at different periods were easily observed, such as intragranular precipitates without sensitization treatment and apparent intergranular corrosion after 360 ks of sensitization treatment. Evolution mechanisms of intergranular precipitates were then assumed from results obtained by SAEDPs and elemental distribution maps. They were probably in the order of 1) the formation of intergranular NbC, 2) the coarsening of NbC with the decrease of Nb content locally, 3) the formation of M 23C 6, then 4) Cr-depletion zone as well as the coarsening of M 23C 6. Acknowledgements We gratefully acknowledge the supply of type 347 austenite stainless steel from Nippon Steel & Sumikin Stainless Steel Corporation and financial support in part by a Grantin-Aid for Scientific Research (No ) from the Iron and Steel Institute of Japan. P.A.M. thanks the Isaac Newton Trust for funding and the Royal Academy of Engineering and the Leverhulme Trust for the award of Senior Research Fellowships. REFERENCES 1) H. K. D. H. Bhadeshia: ISIJ Int., 41 (2001), ) A. H. Cottrell: Chemical Bonding in Transition Metal Carbides, The Institute of Materials, London, (1995). 3) J. S. T. van Aswegen, R. W. K. Honeycombe and D. H. Warrington: Acta Metall., 12 (1964), 1. 4) N. Terao and B. Sasmal: Trans. Jpn Inst. Met., 22 (1981), ) A. F. Padilha, R. L. Plaut and P. R. Rios: ISIJ Int., 43 (2003), ) B. Sasmal: J. Mater. Sci., 32 (1997), ) A. R. Jones, P. R. Howell and B. Ralph: J. Mater. Sci., 11 (1976), ) H. Uno, A. Kimura and T. Misawa: Corrosion, 48 (1992), ) A. Y. Kina, V. M. Souza, S. S. M. Tavares, J. A. Souza and H. F. G. de Abreu: J. Mater. Process. Technol., 199 (2008), ISIJ 152