Available online at Procedia Engineering 55 (2013 ) on Creep Exposure

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1 Available online at Procedia Engineering 55 (2013 ) th International Conference on Creep, Fatigue and Creep-Fatigue Interaction [CF-6] Microstructural Modifications due to Tungsten and Tantalum in 9Cr Reduced Activation Ferritic Martensitic Steels on Creep Exposure R. Mythili a, Ravikirana b, J. Vanaja a, K. Laha a, S. Saroja a, T. Jayakumar a, M.D. Mathew a, E. Rajendrakumar c a Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam , India b Senior Research Fellow, Homi Bhabha National Institute, Indira Gandhi Centre for Atomic Research, Kalpakkam , India c Institute for Plasma Research, Gandhinagar , India Abstract Replacement of Mo and Nb by W and Ta in modified 9Cr-1Mo class of steels has been significantly useful for achieving reduced activity and improved mechanical properties. The addition of W and Ta strongly influences microstructure which comprises of the substructural changes and precipitation kinetics under both thermal and stress exposure. This study focusses on the effect of W and Ta on microstructural changes on thermal and creep exposure at 823K in 9Cr-W-Ta-0.2V- 1 to 0.1C steels, with W and Ta concentrations varying from 1 to 2% and 0.06 to 0.14% respectively. Increase in W from 1.4 or 2%, was beneficial to retain lath structure and retard the coarsening of M 23 C 6 during long term thermal aging. Detailed microstructural investigation of the steesl after creep deformation showed extensive subgrain formation and coarsening of M 23 C 6 carbides than just thermal exposure. Influence of W on subgrain size was more pronounced than Ta. Increase in W was found to be useful to control the coarsening rate of M 23 C 6 carbides, even under stress, while MX precipitate size showed no significant change The Published Authors. Published by Elsevier by Elsevier Ltd. Selection Ltd. Open access and/or under peer-review CC BY-NC-ND under license. responsibility of the Indira Gandhi Centre for Atomic Selection Research and peer-review under responsibility of the Indira Gandhi Centre for Atomic Research. Keywords: Reduced activation ferritic/martensitic steel; thermal aging; creep; microstructure 1. Introduction 9 Cr Reduced Activation Ferritic/Martensiticc (RAFM) steels with addition W and Ta are reported [0,0] to be candidate structural material for the first-wall and blanket modules of ITER and future fusion power systems, due to their excellent resistance to radiation-induced swelling, He embrittlement and improved high temperature mechanical properties [0,0]. Study of microstructural evolution during long term creep exposure at elevated temperatures is necessary, since it induces significant microstructural changes like coarsening of Corresponding author. address: rm@igcar.gov.in The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of the Indira Gandhi Centre for Atomic Research. doi: /j.proeng

2 296 R. Mythili et al. / Procedia Engineering 55 ( 2013 ) precipitates and decrease in dislocation density, decreasing the precipitation strengthening effect and the work hardening effect respectively, finally leading to reduced long-term creep strength. Also extending the upper operating temperature limit to beyond 773K requires the optimisation of the chemical composition of the steel. Hence, a detailed study on the effect of alloying elements mainly W and Ta on microstructural stability in RAFM steels has been initiated. Attempt has been made to understand the role of W and Ta on microstructural evolution in these steels under long term creep exposure. 2. Experimental methods 9Cr RAFM steels with different W and Ta concentrations in the range of 1-2wt% and wt% respectively, are being studied, mainly w.r.t microstructural evolution during exposure to high temperature and stress for different durations. The composition of the steels is given in Table 1. The steels were subjected to a normalising treatment at 1253 K for 30 minutes followed by tempering at 1033 K for 60 minutes and aircooled. The microstructures of the steels from both grip and guage portions of the specimens subjected to a constant load creep test at 823 K in air at a stress level of 220 MPa creep tested were analysed to study the effect of thermal aging and creep deformation respectively. Preliminary microstructural analysis of the normalised as well as normalised and tempered (N&T) steels were carried out using Optical and Scanning Electron Microscopy. Detailed TEM investigations were carried out on thin foils of the steel in the N&T, thermal and stress exposed conditions in a Philips CM200 ATEM fitted with X-Max SDD detector for EDS analysis. Thin foils were prepared by mechanical thinning followed by ion milling/electrolytic jet thinning, while carbonextraction replica was prepared by conventional etching. The identification of phases was carried out by a combination of selected area electron diffraction (SAED) and EDS (Energy Dispersive Spectroscopy) analysis. Table 1. Chemical composition (wt.%) of the melted RAFM steels. Element/Steel Cr C Mn V W Ta N O P S Fe Steel Rest Steel <0.002 <0.001 Rest Steel Rest Steel < Rest 3. Results and discussion 3.1. Microstructural characterisation of normalised and tempered steels The microstructure of the normalised steels is presented in Figs. 1(a d), showing the presence of a fully martensitic structure. Presence of -ferrite was not observed even in Steel 3 with the highest W concentration.the backscattered electron (BSE) microphs of the normalised steels in Figs. 1(e and f) clearly shows the presence of undissolved primary carbides in Steels 3 and 4. It was also observed that the size and number density of the primary carbides increases with increase in W and Ta. Also, the prior austenite grain size decreased, from 25 to 13±2 μm with increase in W from 1 to 1.4 % and Ta from 0.06 to 0.14 %, while further increase in W from 1.4 to 2 % did not show significant change. This decrease in prior austenite grain size suggests the beneficial effect of pinning of austenite grain boundaries by the undissolved carbides during normalising [0]. The decrease in prior austenite grain size also manifested in the increase of hardness of the steels with increase in W and Ta concentration.

3 R. Mythili et al. / Procedia Engineering 55 ( 2013 ) Fig. 1. Martensitic structure of the steels normalized at 1253K (a) Steel 1 (b) Steel 2 (c) Steel 3 (d) Steel 4; BSE images showing the primary carbides in normalised (e) Steel 3 and (f) Steel 4. A typical microstructure of the normalised and tempered steels is shown in Fig. 2(a) showing a tempered martensite structure. Laths with high dislocation density induced by martensitic transformation due to the normalization heat-treatment was observed predominantly along with the presence of subgrains in some regions due to the recovery of laths during tempering. The lath size was between 0.3 and 0.4 μm. It is observed that both inter and intralath precipitate occur with different morphologies like globular and lenticular. Most of the interlath precipitates were coarse with an average the size of 70 nm, while fine intralath precipitates were of the size nm. Analysis of electron diffraction pattern of coarse interlath precipitates showed that they are of M 23 C 6 type, while fine intralath precipitates are of MX type (inset of Fig. 2(a). EDS spectrum from both the types of precipitates is shown in Fig. 2(b), which shows that M 23 C 6 carbides are enriched in Cr with a high solubility for W also, while MX type precipitates have an enrichment of Ta and V. a b Fig. 2. (a) Tempered martensite structure of normalized and tempered Steel 1 along with diffraction patterns from M 23C 6 and MX precipitates along [12 2 ]and [121] zone axis respectively and (b) EDS spectra showing enrichment of Cr, W in M 23C 6 and Ta, V in MX precipitates respectively. Increase in W and Ta concentration of the steel brought about the following changes. Hardness of the tempered steels increased with increase in W and Ta, suggesting the retardation of the recovery of the martensitic substructure. The number density of carbides increased with increase in both W and Ta. The average size of the precipitates showed an increasing trend with W though the actual coarsening was very less, which could be mainly attributed to the coarsening of M 23 C 6 carbides. The slow increase in size of M 23 C 6 carbides could be understood due to the solubility for W, which increased with W concentration of the steels. The concentration of Fe in M 23 C 6 carbides decreased with increase in W, indicating the replacement of Fe by W in these carbides.

4 298 R. Mythili et al. / Procedia Engineering 55 ( 2013 ) Ta addition did not result in a significant increase of the average size of the precipitates, suggesting the increased nucleation of fine MX precipitates Microstructural changes on creep exposuree The creep rupture time of these indigeneous steels is reported to increase with increase in W. Figure 3 shows the TEM micrographs of the steels after long term thermal exposure at 823 K. Fig. 3. TEM micrographs after long term thermal exposure at 823K showing (a) partial recrystallization in Steel 1 after 5000h and (b) retention of lath structure with increase in W in Steel 3 after 5978 h. Increase in W from 1 to 2 % showed the following features: Partial recrystallization in Steel 1 after 5000 h, while retention of lath structure in Steel 3 even after 5978 h. No significant coarsening of M 23 C 6 carbides, due to the slow diffusion of W, while MX precipitates did not show any significant change in size with change in composition of the steel. The retention of lath structure is thus attributed to the effective pinning of lath boundaries by M 23 C 6 carbides and the slow recovery of martensitic substructure by the fine MX precipitates. However, creep deformation produced significant changes both in the substructure and precipitation kinetics, which was also dependent upon the W and Ta content of the steels. Extensive subgrain formation (Fig. 4) was observed in all the creep tested steels, due to the combined action of temperature and stress [0]. Also, the subgrain size of the steels and average carbide size was found to be higher under creep deformation than on just thermal exposure, indicating the higher rate of recovery under stress. However, the size of MX carbides did not show a significant difference between thermal and creep exposure, showing the stability of these precipitates and improves the creep resistance to the steel by pinning the moving dislocations. Fig. 4. TEM micrographs of steels with high W and Ta content showing extensive recrystallization on creep exposure at 823 K at 220 MPa in (a) Steel 3 and (b) Steel 4.

5 R. Mythili et al. / Procedia Engineering 55 ( 2013 ) Increase in W and Ta concentration resulted in the following changes: An increase in the subgrain size from 0.65 to 0.95 μm with increase in W concentration from 1 to 1.4 %, and a decrease to about 0.86 μm in Steel 3 with 2 %W. Slight decrease in the average size of M 23 C 6 carbides due to the enhanced nucleation of fresh carbides under stress, pinning the subgrain boundares. Thus, the higher creep strength of Steel 3 is attributed to the synergistic effect of fine subgrains, fresh nucleation of precipitates and increase in solid solution strengthening effect of W [0]. Increase in Ta from 0.06 % to 0.14 % resulted in an increase of subgrain size to 1μm. In fact, increase in Ta resulted in a shorter rupture time, which shows that the presence of coarse incoherent primary carbides in the initial microstructure are not very effective to control the recovery of the martensitic substructure. 4. Conclusions Systematic studies on the effect of W and Ta on microstructural evolution in 9Cr RAFM steels in normalised and tempered, thermal and creep exposed conditions have been carried out. Salient features of the study are as below: Increase in W and Ta content of refines the prior austenite grain size Higher influence of W in retarding the recovery of lath structure during creep deformation than Ta. Reduced coarsening kinetics of M 23 C 6 carbides by the presence of W, which is accelerated by stress. Insigficant coarsening of fine MX carbides even under creep deformatiostress, though the presence of coarse undissolved primary MX carbides decreases the creep resistance. References [1] R.J.Kurtz, A.Alamo, E.Lucon, Q.Huang, S.Jitsukawa, A.Kimura, R.L.Klueh, G.R.Odette, C.Petersen, M.A.Sokolov, P.Spätig, J.-W. Rensman, J. Nucl. Mater., 411(2009) [2] H.Sakasegawa, T.Hirose, A.Kohyama, Y.Katoh, T.Harada, K.Asakura, T.Kumagai, J. Nucl. Mater., 490(2002) [3] N.Baluc, R.Schaublin, P.Spatig, M.Victoria, Nucl. Fission, 44(2004)56. [4] G.L.Butterworth, J. Nucl. Mater., 135(1991) [5] R.L.Klueh, Intl. Metall. Reviews, 50(2005)287. [6] H.Sakasegawa, T.Hirose, A.Kohyama, Y.Katoh, T.Harada, K.Asakura, Fusion Engg. and Design, 671(2002) [7] K.Sawada, M.Takeda, K.Maruyama, R.Ishii, M.Yamada, Y.Nagae, R.Komine, Mater. Sci.Engg., A267(1999)19.