Available online at ScienceDirect. 20th European Conference on Fracture (ECF20)

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1 Available online at wwwsciencedirectco ScienceDirect Procedia Materials Science 3 ( 2014 ) th European Conference on Fracture (ECF20) Analysis of creep behavior of for use of VHTR syste Woo-Gon Ki a, *, Jae-Young Park b, IMW Ekaputra b, Min-Hwan Ki a, Yong-Wan Ki a a Korea Atoic Energy Research Institute (KAERI), Daedeokdaero, Yuseong-gu, Daejeon , Republic of Korea a Pukyong National University, 365 Shinsunro, Na-gu, Busan , Republic of Korea Abstract is the one of the leading candidate aterials for interediate heat exchangers (IHX) application of a Very High Teperature Reactor (VHTR) syste for econoic production of electricity and hydrogen Creep rupture data for were obtained fro a series of creep tests with different applied stresses at, and On the basis of a systeatic analysis of the experiental data, creep behaviour for was analyzed using various creep relationships and laws, such as Norton s power law, Monkan-Grant Relationships (MGR), Modified Monkan-Grant Relationships (MMGR), creep daage tolerance factor, and Zener-Holloon Paraeter (Z), and the creep constants used in each equation were reasonably deterined The creep curve of was soewhat different fro those of typical heat-resistance steels, and it did not exhibit a textbook creep curve The secondary creep stage is unclear, and it appeared to exhibit a predoinantly accelerated creep rate fro the start of a tertiary creep stage in short tie ranges The creep deforation was doinantly developed fro the foration and growth of cavity in creep daage process The MMGR appeared to be ore narrowed in data scattering than the MGR, and it followed well a straight line of 10 as =097 In the plot of the Z paraeter vs stress, it obeyed a straight line with a slope of n =587 regardless of the three different teperatures It would be inferred that the sae creep echanis was operative within the present stress and teperature ranges, and the creep daage tolerance factor of was found to be Published The Authors by Elsevier Published Ltd Open by Elsevier access under Ltd CC BY-NC-ND license Selection Selection and and peer-review peer-review under under responsibility responsibility of the of Norwegian the Norwegian University University of Science of Science and Technology and Technology (NTNU), Departent (NTNU), Departent of of Structural Engineering Structural Engineering Keywords: Creep; ; VHTR; Monkan-Grant Relationship; Creep daage tolerance factor; Zenor-Holloan paraeter * Corresponding author Tel: ; fax: E-ail address: wgki@kaerirekr Published by Elsevier Ltd Open access under CC BY-NC-ND license Selection and peer-review under responsibility of the Norwegian University of Science and Technology (NTNU), Departent of Structural Engineering doi: /jspro

2 1286 Woo-Gon Ki et al / Procedia Materials Science 3 ( 2014 ) Introduction A very high teperature reactor (VHTR) is one of the ost proising Generation-IV reactor types for the econoic production of electricity and hydrogen Its ajor coponents are the reactor internals, reactor pressure vessel (RPV), piping, hot gas ducts (HGD), and interediate heat exchangers (IHX), as shown in Fig 1 The IHX aong these is a key coponent, and is a prie candidate aterial owing to its superior creep resistance above 800 o C copared with other potential superalloys [Cook (1984), Dewson and Li (2005), Ki et al (2013), Ki et al (2010)] At present, the Korea Atoic Energy Research Institute (KAERI) is developing a nuclear hydrogen developent and deonstration (NHDD) plan of a capacity with 200 MW th in the theral and core outlet teperature of It has been carried out in a governent funded project called Developent of Key Technologies for Nuclear Hydrogen since 2006 [Chang et al (2007), Ki et al (2013)] Since the VHTR syste is designed for a life span of 60 years operating at and 7 MPa in heliu ipurities, creep behavior is considered as one of the ost iportant echanical properties Of the existing alloys, nickel-based is the leading candidate for use in the next-generation nuclear plant (NGNP) heat exchangers because it has the highest creep strength of solid solution alloys under consideration for teperature above However, until now, the ASME design code for was not developed for design use Material works to coplete the ASME code case developent are ongoing according to a next-generation nuclear plant research and developent plan Through this plan, a new Alloy 617 Code Case is planned to be approved by 2017 To do so, a nuber of creep data and creep constants are required to coplete the new Code Case, and creep behavior should also be investigated fro a systeatical analysis with creep teperature and stress conditions of wide ranges In the present investigation, creep properties of are obtained through a series of creep tests under different applied stresses at three teperature ranges of 850, 900, and at the Korea Atoic Energy Research Institute (KAERI) An analysis of creep rupture behavior is perfored using various creep relations and laws, and the constants used in the creep equations are deterined on the basis of the creep experiental data, and the results are given and discussed Noenclature Constants in power-law for Monkan-Grant constant Modified Monkan-Grant constant Modulus elasticity (GPa) Slope constants in Monkan-Grant and odified Monkan-Grant relationships Creep exponent in power-law for Activation Energy (J/ol) Universal gas constant (83J/ol K) T Absolute teperature (K) Rupture tie (h) Z Zener-Holloan Paraeter Strain in creep failure (%) Miniu creep rate (1/s) Stress (MPa) Creep daage tolerance factor 2 Experiental Details Coercial grade nickel-based superalloy, (Inconel 617), was used in this study The aterial was a hot-rolled plate with a thickness of Creep test speciens were fabricated in cylindrical for with a 30 gauge length and 6 diaeter The gage section was parallel to the longitudinal rolling direction Circular

3 Woo-Gon Ki et al / Procedia Materials Science 3 ( 2014 ) grooves were achined at both ends beyond the shoulder region of these speciens to attach a high-precision extensoeter for onitoring the elongation during creep testing The loading fraes used in the creep tests had a lever ar ratio of 20:1 A split furnace was used to heat the speciens of in dry air at 850, 900, and The pull rod and jig used for the creep tests were anufactured with a nickel-based superalloy aterial to sufficiently endure oxidation and theral degradation during creep Creep strain data with elapsed ties were taken autoatically through a PC K-type therocouples were used to onitor teperatures within the gage section of the speciens Creep strain-tie curves were obtained, and the value of the iniu creep strain rate was obtained by calculating the secondary creep stage fro the strain tie creep curves The creep properties of the rupture tie, iniu creep rate, rupture elongation, and reduction of area were obtained in dry air at 850, 900, and of The creep data were analyzed using the plots of various creep equations, and the constants in these creep equations are deterined on the basis of a systeatic analysis of the experiental data 3 Results and Discussion 31 Creep curves of The creep properties of such as the rupture tie, iniu creep rate, rupture elongation, and reduction of area were obtained at,, and The creep data were analyzed using the plots of various creep equations, and the constants in these creep equations are reasonably deterined on the basis of a systeatic analysis of the experiental data Fig 1 shows creep curves of typical heat-resistance steels The creep curve of sees to be soewhat different fro those of typical heat-resistance steels, such as 225Cr-1Mo steel, 9Cr-1Mo steel, and austenitic stainless steel (type 316LN) The creep curves of low and high-cr ferritic steels plot well distinct priary, secondary, and tertiary creep stages (called textbook creep curve ), but does not exhibit a textbook creep curve, as shown well in Figs 2 The secondary creep stage is unclear, and it sees to exhibit a predoinantly accelerated creep rate fro the start of a tertiary creep stage in short tie ranges 10 Noralized plastic strain (/) Cr-Mo, 540 o C 9Cr-1Mo, 600 o C Type 316SS, 650 o C, Fig 1 Major coponents of VHTR syste for nuclear hydrogen production Noralized tie(h) Fig 2 A coparison of the noralized creep curves for and typical heat resistance steels 32 Analysis of creep rupture data A plot of log stress vs log rupture tie showed teperature dependence in the creep strength tested at 850, 900, and A higher teperature results in a lower creep strength The creep rate and rupture tie can be given by stress dependence of the power law for, as follows

4 1288 Woo-Gon Ki et al / Procedia Materials Science 3 ( 2014 ) n A (1) n t r A (2) The iniu creep rate obeys the stress dependence of the power law for at,, and The slope is n 5-68, as shown in Fig 3 In addition, creep rupture tie obeys stress dependence of the power law for at,, and The slope is n 45-60, as shown in Fig 4 Material constants of A, A n, and n values are alost the sae regardless of the teperature changes, but the n and n values have a decrease with increase in teperature It eans that closer values of respective n and n had siilar creep deforation and fracture echaniss The four constants are suarized in Table1 Miniu creep rate (1/h) 001 1E-3 1E-4 1E-5 : y=298e-13x 510 : y=183e-13x 563 : y=198e-14x 687 1E Stress (MPa) Fig 3 Plot showing stress dependence of iniu creep rate at 850, 900, and of Tie to rupture (h) : y=928e11-x 517 : y=153e10-x 452 : y=311e11-x Stress (MPa) Fig 4 Plot showing stress dependence of rupture tie at 850, 900, and of Table1 Material constants of A n, A, n obtained for a power-law for at each teperature for Tep ( o C) A n A n x x x x x x Figs 5 and 6 show the plots of the Monkan-Grant Relationships (MGR) of Eq 3 and odified Monkan-Grant Relationships (MMGR) of Eq 4 [Phaniraj et al (2003)] The equations are given as follows, respectively log t log C (3) r r f MG log ( t / ) log C (4) MMG In the plot of MGR, the data show soe scattering at the three teperatures, and the slope on average is =087 On the other hand, the MMGR reduced the data scattering, and it follows well a straight line with about unity as =097 regardless of the teperature changes The MMGR is superior in creep data plots to the MGR In the MMG and MMGR, have values of C MG =052 and C MMG =0420 In addition, creep daage tolerance factor,, provides a easure of the susceptibility of a aterial to localized cracking at stress and strain concentrations It is considered to be a aterial perforance characteristic Based on a continuu creep daage echanics approach, is defined as the ratio of strain failure f to MGR, tr, ie, the secondary creep strain The equation can be given by

5 Woo-Gon Ki et al / Procedia Materials Science 3 ( 2014 ) f t r (5) was investigated to take =240, as shown in Fig 7 showing the plot of average creep rate vs iniu creep rate of at 850- For =15-25, the creep daage echanis corresponds to cavitation, and for =15 or ore, it corresponds to icrostructural degradation such as a coarsening of the precipitates and/or dislocation substructural softening It is known that Grade 91 steel of typical heat resistance steel has 5 and ore [Phaniraj et al(2003)] Rupture tie (t r ), (h) C MG =0517 y=0517x -087 Rupture tie/ Strain to failure (t r / f ), (h) C MMG =0420 y=0420x E-6 1E-5 1E-4 1E Miniu creep rate ( ), (1/h) Fig 5 Plot showing the MGR of the creep data tested at 850, 900, and of Average creep rate ( r / t r ), (1/h) E-3 1E-4 = 240 y=240x 10 1E C 1E-6 1E-6 1E-5 1E-4 1E Miniu creep rate ( h Fig 7 Plot of average creep rate vs iniu creep rate tested at 850, 900, and of E-6 1E-5 1E-4 1E Miniu creep rate ( ), (1/h) Fig 6 Plot showing the MMGR of the creep data of tested at 850, 900, and of Z= exp(q/rt) 1E15 1E14 1E13 1E12 1E11 1E10 Z=230E33( /E) E9 5E-5 25E-4 5E E Fig 8 Plot of the Zener-Holloan Paraeter as a function of stress ( /E) at 850, 900, and of Fig 8 shows a plot of the Zener-Holloan Paraeter (Z) as a function of stress ( /E) of tested at 850, 900, and The creep strain rate is expressed as [Deter and Bacon (1988)]

6 1290 Woo-Gon Ki et al / Procedia Materials Science 3 ( 2014 ) n 1 A1 exp( Q/ RT) (6) Z exp( Q/ RT) (7) Z A n 1 1 where Q is the activation energy, A 1 and n 1 are the experientally deterined constants, and quantity Z is called the Zener-Holloon paraeter A straight line can be seen well with a slope of n =587 for regardless of the three different teperatures It can be inferred that the sae echanis is operative in the teperature range of the present creep conditions Fracture icrographs were also investigated fro creep rupture speciens Creep deforation of was doinantly developed fro the foration and growth of cavity in creep daage process The creep cracks were initiated by linking and incorporating with the cavities, and were then developed along the cavities fored on the grain boundary A failure occurred by the growth and interconnections of the cavities rather than a necking failure generated in the tertiary creep region Failure ode was doinant for a typical intergranular fracture The precipitates in creep ruptured speciens were fored coarsely on the grain boundaries and scattered throughout atrix Cylindrically-shaped coarse carbides of Cr-rich M 23 C 6, were ainly fored on the grain boundaries [Jang et al (2008)] Conclusions To investigate the creep rupture behavior of, a series of creep tests was perfored with different applied stresses at 850, 900, and Creep behaviors were analyzed using creep data various creep laws on the basis of experiental creep rupture data The creep curve of A617 was soewhat different with typical heatresistance steels, and the secondary creep was unclear, and the tertiary creep stage was started at the early stage of creep tie Creep failure was due to the generation, growth and linking of cavities The MMGR reduced the data scattering and presented a good straight line as =097 The MMGR showed a better plot than the MGR Creep daage tolerance factor ( ) for was found to be 24 This was in agreeent with aterials exhibiting typical cavitation daage In the plot of Z paraeter vs stress, a straight line was found for n =587 regardless of the three different teperatures It would be inferred that the sae creep echanis was operative within the current teperature ranges Acknowledgeents Author would appreciate KAERI, and this study was supported by Nuclear Research & Developent Progra of the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning (2012M2A8A ) References Chang, JH et al, 2007 A Study of a Nuclear Hydrogen Production Deonstration Plant, Nuclear Eng and Tech 39(2), 111 Cook, R, 1984 Creep properties of Inconel-617 in Air and Heliu at 800 to 1000 o C Nuclear Technology 66, 283 Deter, GE and Bacon, D, 1988 Mechanical Metallurgy McGraw-Hill Book Copany, London, pp306 Dewson, S, and Li, X, 2005 Selection Criteria for the High Teperature Reactor Interediate Heat Exchanger Proc of ICAPP 05 Seoul, Korea, Paper # 5333 Jang, C, Lee, D, and Ki, D, 2008 Oxidation Behavior of in Very High Teperature Air and Heliu Environents Int J of Press Vessels and Piping 85, 368 Ki, WG, Park, JY, Ekaputra, IMW, Hong, SD, Ki, SJ, and Ki, YW, 2013 Coparative Study on the High-teperature Tensile and Creep Properties of Base and Weld Metals Journal of Mechanical Science and Technology 27(8), 2331 Ki, WG,Yin, SN, Lee, GG, Ki, YW, and Ki, SJ, 2010 Creep Oxidation Behavior and Creep Strength Prediction for Int J of Press Vessels and Piping 87, 289 Phaniraj, C, Choudhary, BK, Bhanu Sankara Rao, K, and Rai B, 2003 Relationship between Tie to Reach Monkon-Grant Ductility and Rupture Life Scripta Materialia 48,1313 (8)