6.8 EVALUATION OF LONG-TERM CREEP PROPERTIES OF HASTELLOY XR IN SIMULATED HIGH-TEMPERATURE GAS-COOLED REACTOR HELIUM

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1 6.8 EVALUATION OF LONG-TERM CREEP PROPERTIES OF HASTELLOY XR IN SIMULATED HIGH-TEMPERATURE GAS-OLED REACTOR HELIUM Yuji KURATA, Yutaka OGAWA*. Tmi SUZUKI, Masami SHINDO, Hajime NAKAJIMAand Tatsu KONDO** Japan Atmic Energy Research Institute.JAPAN * nw Research Institute fr Metals, Thku University.JAPAN ** nw Faculty f Engineering, Thku University.JAPAN ABSTRACT Creep prperties are amng the imprtant basic items f material perfrmance fr design f high temperature cmpnents f high-temperature gas-cled reactrs(htgrs). In rder t evaluate creep prperties f Hastelly XR (a mdified versin f the cnventinal Hastelly X) develped fr the High-Temperature Engineering Test Reactr(HTTR), lng-term creep tests were carried ut in simulated HTGR helium at 800, 900 and looo'c. The test results up t abut 50,000h shwed n significant degradatin in creep prperties. The creep-rupture strength btained thrugh the lng-term tests was abve the level crrespnding t the design allwable creeprupture stress f the HTTR. Rupture lives culd be estimated with sufficient accuracy using Larsn-Miller parameter. The values f the stress expnent were 4.5 t 5.7 when the stress dependence f the steady-state creep rate was expressed in terms f the Nrtn equatin. It was judged that dminant creep prcess was dislcatin creep. The relatinship between the steady-state creep rate and the rupture life was expressed in terms f the Mnkman-Grant equatin. Carburizatin during creep in simulated HTGR helium did nt degrade creep prperties f this ally. Internally frmed cavities and cracks were initiated at sites f precipitates at grain bundaries, grwing nearly perpendicular t the stress axis. Creep fracture was caused by the nucleatin, grwth and link-up f grain bundary cavities in lng-term tests. Tw phases, Cr-rich carbide and M-rich carbide, c-exisred in specimens after lng-term creep tests

2 1. Intrductin The HTTR, which uses helium gas as the primary clant, is currently under cnstructin as the first HTGR in Japan( 1 ). Creep prperties are amng the imprtant basic items f material perfrmance t be cnsidered in high-temperature structural design f the HTTR. Althugh pure helium gas is inert, the primary clant helium, which passes thrugh the large mass f ht graphite cre structure, cannt avid cntaining slight amunt f impurity gases, i. e., H2, H2O,, 2 and CH4. It is cnsidered that these impurities can. cause xidatin and carburizatin r decarburizatin f heat-resistant allys and pssible effects upn creep prperties are suspected. Fr this reasn many research wrks n creep tests in flwing helium cntaining a small amunt f impurities have been carried ut. We have als cnducted creep tests in simulated HTGR helium with special care n impurity cntrl in helium. An intermediate heat exchanger(ihx) f the HTTR is being manufactured using Hastelly XR. High temperature cmpnents such as the IHX were designed using High- Temperature Structural Design Cde^2) and Design Allwable Limits^. At the time f the design f the HTTR a lt f creep data were used t generate the Design Allwable Limits fr which the allwable stress fr lng-time service was determined thrugh sme reasnable extent f extraplatin frm the test data btained up t that time. The allwable stress up t 10 5 h was determined using data up t abut 25,000 Ir 3 '. Therefre, it is necessary t cnfirm whether the allwable stress is sufficiently marginal r nt when creep data in lnger tests are btained. We reprted that there was n significant difference between helium and air in the creep rupture lives at 800 t looo'c up t abut 10,000h and that the test results shwed n significant degradatin in creep prperties^4' 5 ). Since a series f creep tests fr Hastelly XR in simulated HTGR helium have been accmplished, all results including creep data up t abut 50,000 h are analyzed in this study in cnjunctin with carburizatin behaviur, micrstructure bservatin and mechanism estimatin f creep defrmatin and fracture. 339

3 2. Experimental prcedure 2.1 Specimens The material tested in this study is Hastelly XR develped fr high temperature cmpnents f the HTTR. Hastelly XR is an imprved versin f Hastelly X. Table 1 shws the chemical cmpsitin f Hastelly XR tgether with the specificatin f Hastelly X(ASTM B435). Hastelly XR has a basal cmpsitin fr the majr cnstituents cmmn with that f Hastelly X (i.e., nminally Ni-22Cr-18Fe-9M in mass %), while cntents f specific minr elements are ptimized: Mn and Si are adjusted in the ptimum ranges and Al, Ti and C are reduced t the pssible lwest levels^. The material was supplied in the frm f bars f 15 mm in diameter. The slutin treatment temperature f the bars was 1180 C and the grain size was ASTM N.3-4. Specimens f 6 mm in diameter and 30 mm in gauge length were used fr creep tests. 2.2 Test methds Creep tests were carried ut in helium envirnment designated as JAERI Type B helium, which was ne f the experimental specificatins f the simulated HTGR primary clant envirnments. Impurity cmpsitin f JAERI Type B helium is shwn in Table 2. The chemical characteristic f this impure helium envirnment is lw xidizing and slight carburizing fr Hastelly XR. Single specimen type uniaxial creep machines' 7 ' designed fr a helium envirnment were used. Special care was exercised in aviding undesirable perturbatin f the lcal impurity cmpsitin at the test sectin due either t degassing frm r t reactin with the machine cmpnents. Creep tests in the helium envirnment were carried ut at 800,900 and looo'c, and under 6.9 t 98.1 MPa. During the test, the temperature was mnitred by platinum/ platinum-rhdium thermcuples attached alng the specimen gauge sectin. Micrstructure bservatin was made fr ruptured specimens using ptical micrscpe, scanning electrn micrscpe and EPMA

4 3. Results and discussin 3.1 Creep rupture prperties Figure 1 shws the relatinship between stress and time t rupture fr Hastelly XR in JAERI Type B helium. The slid lines are the regressin curves btained frm analysis in terms f Larsn-Miller parameter, and the brken lines the design allwable creep-rupture stress (S R ) f the Design Allwable Limits^. The design allwable creep rupture stress was determined frm the minimum creep rupture time which was ne tenth f the mean values btained by applying the time-temperature parameter (TTP) methd t available creep rupture data in air and in simulated HTGR helium^. The strength level f the creep-rupture data including results f creep tests up t 48,557.5h is abve S R at each test temperature, and the lng-term data abve 10,000 h shw n significant degradatin in creep rupture lives. The TTP methd is ften used t analyze creep-rupture data and t predict rupture life and stress^8-9 ). In the present study, the fllwing parameters were investigated. Larsn-Miller: LMP=T(C+lg t R ), (1) Orr-Sherby-Drn: OSDP=lg t R - Q/ T, (2) where t R is time t rupture, T the abslute temperature, and C and Q parameter cnstants. Equatin f(), which represents stress dependence f TTP, is apprximated using plynmials f lgarithmic f stress, a, as f(a)=b 0 +b 1 lg(a)+b 2 (lg()) b k (lg(a)) k, (3) where b Q, b^,b 2,, b^ are regressin cnstants. Parameter cnstants in equatins (1)-(3) are ptimized t minimize standard errr f estimate (SEE) f lg t R. SEE=Jl(Yi-Yi) 2 /(n d -n p -k-1), (4) where Yi is a measured value f lg t R, Yi an estimated value f lg t R n d the number f -341-

5 JAERl-Cnf data pints, n p the number f parameter cnstant in TTP and k the degree f equatin (3). The ptimizatin prcedure was carried ut fllwing the study n standardizatin f creep-rupture data evaluatin f metals ( 9 ). Table 3 shws results f TTP analysis fr Hastelly XR. It is fund that the degree f plynmials is 2 fr LMP and OSDP t have gd apprximatin. Furthermre, the applicatin f LMP results in better fit than that f OSDP since the SEE value f LMP is smaller than that f OSDP. On this basis, the results btained frm analysis in terms f LMP are shwn in Fig.1. These regressin curves fit experimental results cntaining lng-term data. The final Larsn-Miller equatin is shwn as fllws: T( lgt R ) = lg() (lg(a)) 2, (5) The relatinship between reductin f area and time t rupture at 800, 900 and 1000X: is shwn in Fig.2. While rupture ductility generally decreases with increasing rupture time, the results shwn in Fig.2 have a tendency f leveling ff in the decrease f rupture ductility after a few thusand hurs. 3.2 Creep curve and steady-state creep rate Creep curves in lng-term tests are shwn in Fig.3. It was reprted that nrmal type creep curves cnsisting f transient, steady-state and accelerated stages were bserved at 800^ and that, fr Hastelly XR at looox), sme irregular creep curves were ften bserved ('. In the present case, while lng-term creep curves at 800 and 900^ are nrmal type curves as shwn in Fig.3, the creep curve at 1000 C is the irregular creep curve where the transient stage is hardly recgnized. Figure 4 shws the relatinship between the steady-state creep rate and stress. The creep rate at 3% strain was adpted as the steady-state creep rate when the steady-state regin was nt recgnized clearly in the irregular creep curves. The relatinship between the steady-state creep rate, E S, and stress, a, is generally expressed as a pwer law, ften called Nrtn's Law

6 JAERl-Cnf e s = A n, (6) where n is the stress expnent and A a cnstant. The stress expnent, n, btained by applying the least squares methd t data at each temperature is at 800T:, at 900^ and at loocc, respectively. The n value at each temperature des nt change within this experimental cnditin even if the stress decreases. It was reprted that n wuld be unity under the diffusinal creep mechanism and that n wuld be greater than 3 under the dislcatin creep mechanism^'. The creep prcess f Hastelly XR within this experimental cnditin is interpreted t be dminated by the dislcatin creep mechanism since n values are abve 3. Generally n value is abut 5 fr slutinstrengthened allys, and it is ften abve 5 fr precipitatin-strengthened allys^2'. Hastelly XR is strengthened mainly by slutin and additinally by precipitatin. The facts that the n value is abve 5 at 800^) and that it decreases with increasing temperature are explained by the decrease in the functin f precipitatin strengthening due t the carsening f precipitates at high temperatures. An activatin energy fr creep, Qc can be derived frm a plt f lg e Q against (1/T) at cnstant stress. The Qc values btained were kj/ml. These values are larger than the activatin energy value, 285 kj/ml' 13 ', f lattice diffusin f Ni in Ni-20 at %Cr ally. By these appraches, we can say that dislcatin creep with n values f abut 5 and with Qc values larger than the activatin energy fr lattice diffusin f Ni in Ni-20 at %Cr ally is predminant under cnditins applied in this study. The fllwing Mnkman-Grant equatin is useful t perfrm the predictin f life r residual-life f high-temperature cmpnent. lg t R = c-mlg e s, (7) where c and m are cnstants. Figure 5 shws the relatinship between the time t rupture and the steady-state creep rate fr Hastelly XR. The regressin curves at each temperature r at all temperatures are als shwn in this figure. Creep rupture lives fr Hastelly XR can be predicted using the Mnkman-Grant equatin

7 3.3 Carbn analysis and micrstructure bservatin Since decarburizatin and carburizatin have appreciable effects n creep behaviur ), carbn analysis was carried ut fr ruptured specimens. Furthermre, micrstructure bservatin fr surface and crss sectins f ruptured specimens was perfrmed. Figure 6 shws results f carbn analysis. Carbn cntent was analyzed fr sectin including rupture prtin where there are many cracks and fr 8-mm-diameter sectin where there is little creep defrmatin. Carbn cntent in the frmer is high as shwn in Fig.6. This tendency becmes significant with higher temperatures and lnger times. On the ther hand, there is nly a little carburizatin in 8-mm-diameter sectin even after lng-term expsure t the impure helium. It can be pinted ut that carbn Intrusin during the steady-state creep stage was limited t a negligible level. N specimen experienced decarburizatin which caused significant decrease in rupture lives( 14 ). Carburizatin during creep in simulated HTGR helium did nt substantially degrade creep prperties f Hastelly XR. The fllwing characteristic f micrstructures f specimens ruptured in lng-term creep tests can be described frm Fig. 7. The depth f the surface crack was abut urn at 800 and 900^, and abut 200 xm at 1000 C. It was fund that creep fracture was caused by the grwth and link-up f cavities nucleated at sites f precipitates at grain bundaries, grwing nearly perpendicular t the stress axis. Grain bundaries perpendicular t the stress axis are under tensile stress. Precipitates at grain bundaries under tensile stress carsened as shwn in Fig.7. In rder t btain infrmatin n precipitates, EPMA analysis was carried ut. Figure 8 shws the results f EPMA analysis fr the specimen ruptured at 900^ in 48,587.5 h. It is clear that bright precipitates c-exist with dark precipitates in bth secndary and backscattered electrn images. The bright precipitates have high mlybdenum, silicn and cmparatively lw chrmium cntents, while the dark precipitates have high chrmium, carbn and mlybdenum, and lw silicn cntents. Table 4 shws quantitative analysis f precipitates and matrix by EPMA. The dark phase is Cr-rich carbide presumed t be -344-

8 23g carbide^6'"'. The bright phase has high mlybdenum and cmparatively lw chrmium cntents. This bright phase enriched with M is presumed t be MgC type carbide ( 1? ). 4. Cnclusins Creep tests f Hastelly XR were carried ut in simulated HTGR helium up t abut 50,000 h at 800, 900 and 1000T:. The main results btained are as fllws: (1)The test results up t abut 50,000 h shwed n significant degradatin in creep prperties such as the rupture life, rupture ductility and the steady-state creep rate. Creep-rupture stress is substantially abve Spf the Design Allwable Limits. (2)The stress dependence f the steady-state creep rate is expressed in terms f the Nrtn equatin. The values f the stress expnent is 4.5 t 5.7. On this basis, it is judged that dminant creep prcess is dislcatin creep. (3) Rupture lives f Hastelly XR can be estimated with sufficient accuracy using a Larsn-Miller parameter. The relatinship between the steady-state creep rate and time t rupture is expressed in terms f the Mnkman-Grant equatin. (4)Sme appreciable carburizatin was recgnized in the specimens ruptured after creep tests in simulated HTGR helium. Carburizatin during creep in the helium did nt substantially degrade creep prperties f Hastelly-XR. (5)Creep fracture was caused by the grwth and link-up f cavities nucleated at sites f precipitates at grain bundaries, grwing nearly perpendicular t the stress axis. There were tw phases, Cr-rich carbide presumed t be M 2 3C 6 and M-rich carbide presumed t be MgC in specimens after lng-term creep tests. Acknwledgements Authrs are very grateful t staffs f Material Perfrmance and Testing Labratry fr their assistance during this lng-term study

9 References (1)Japan Atmic Energy Research Institute : Present Status f HTGR Research and Develpment (1996). (2)HTTR Designing Labratry, Department f Fuels and Materials Research and Department f High-Temperature Engineering : JAERI-M (1989) [in Japanese]. (3)HADA, K, MOTOKI, Y, and BABA, O,: JAERI-M (1990) [in Japanese]. (4)KURATA,Y, OGAWA, Y. and KONDO, T. :Nucl. Technl... 66, 250(1984). (5)0GAWA, Y, KURATA, Y.SUZUKI, T., NAKAJIMA, H. and KONDO, T. : Nihn- Genshiryku-Gakkai Shi (J. At. Energy Sc. Japan), 36, 967 (1994) [in Japanese]. (6)SHINDO, M. and KONDO, T.: Prc. Cnf. n Gas-Cled Reactrs Tday, Bristl/UK, 1982(British Nuclear Energy Sciety) Vl.2, p.179. (7)0GAWA,Y and KONDO, T.: JAERI-M 8801 (1980) [in Japanese], (8)VISWANATHAN, R.: Damage Mechanism and Life Assessment f High-Temperature Cmpnents, ASM Internatinal, Metals Park, Ohi, (1989). (9)VAMAS Data Evaluatin Cmmittee : Study n Standardizatin f Creep-Rupture Data Evaluatin f Metals, The Irn and Steel Institute f Japan, (1994) [in Japanese]. (10)YOKOI, S., MONMA, Y, KONDO, T,., OGAWA, Y. and KURATA, Y : JAERI-M (1983) [in Japanese]. (11)FROST, H.J. and ASHBY, M.F. : Defrmatin Mechanism Maps, Pergamn Press, Lndn, (1982). (12)SIDEY, D. and WILSHIRE, B.: Met. Sci. J., 3, 56 (1969). (13)MONMA, K., SUDO, H. and OIKAWA, H. : Nihn-Kinzku-Gakkai Shi (J. Jpn. Inst. Met.), 28, 188 (1964) [in Japanese]. (14)KURATA, Y, OGAWA, Y. and NAKAJIMA, H. : Tetsu-t-Hagane (J. Irn Steel Inst. Jpn.), 74, 380 (1988) [in Japanese]. (15)idem.: ibid., 74, 2185 (1988) [in Japanese]. (16)SABOL, G.P. and STICKLER, R.: Phys. Stat. Sl., 35,11(1969). (17)TANABE, T, ABE, R, SAKAI, Y and OKADA, M. : Transactins ISIJ (J. Irn Steel Inst. Jpn.), 26, 968 (1986)

10 JAERl-Cnf Table 1 Chemical cmpsitin f Hastelly XR (mass%) Cr Fe C Si C Mn B Hastelly X (ASTM B435) 20.5~ ~ ~0.15 max ~2.5 max Hastelly XR Hastelly X (ASTM B 435) Hastelly XR M 8.0~ P max S W Al 0.03 Ti 0.02 Ni Remainder Remainder Table 2 Impurity levels f JAERI Type B helium (vl ppm) H 2 H 2 0 c 2 CH 4 200~ ' ~110 2~3 5~6 Table 3 Cmparisn f standard TTP (time-temperature parameter) fit fr rupture lives f Hastelly XR Degree f Plynmals SEE(standard errr f estimate) Larsn-Miller Parameter Orr-Sherby-Drn Parameter Table 4 Quantitative analysis f precipitates and matrix f Hastelly XR ruptured at 10OO'C and 6.9 MPa in 13,014 h (rnass%) Ni Cr Fe M Mn W Precipitate (Dark) Precipitate (Bright) Matrix

11 JAKRI-Cnf s 1 '., 800 C Hastelly XR GO *^"-.^ J1000 C Regressin curve "*"* C 900 C 1000 C s 0 A n J Time t rupture (h) Fig. 1 Stress vs. time t rupture fr Hastelly XR In JAERI Type B helium. The slid lines are the regressin curves btained frm applicatin f Larsn-Miller parameter, and the brken lines the design allwable creep-rupture stress (S R ) f the Design Allwable Limits ( 3 ). 100 ' '''"I. Hastelly XR I 40 4 > 1? 20 0 l 1 I III 800 C O "I 900 C Time t rupture (h) D A 1000 C D Iff Fig. 2 Reductin f area vs. time t rupture fr Hastelly XR in JAERI Type B helium

12 50 40 Hastelly XR _ JAERI Type B helium Q C, 13. 7MPa _ Time (h) Fig. 3 Lng-term creep curves f Hastelly XR in JAERI Type B helium Stress (MPa) Fig. 4 Relatinship between steady-state creep rate and stress fr Hastelly XR in JAERI Type B helium

13 10 5 : ' ' ' ' ' ' " 1 Hastelly XR \ 3 B i- r C : 900 C i c All : 800 C C A ^, 1000 C D i 10~ 3 10~ 2 Steady-state creep rate (%/h) - 10 Grant relatinship) fr Hastelly XR in JAERI Type B helium & 0.10 I 0.08 c I ZZ 1000'C Open : sectin including rupture prtin Slid : 8-mnrdiameter sectin O6 ZZZ As received r ) I t I I I i I 10' < Time t rupture (h) 10 5 Fig. 6 Results f carbn analysis n the specimens f Hastelly XR creep-ruptured in JAERI Type B helium. Fig. 5 Relatinship between time t rupture and steady-state creep rate (Mnkman

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15 cns P en "a ZJ a. (D b CC X (0 X c a> 01 > N nj c V m X ^ V-* r" x: c O T- c -352-