High Temperature Mechanical Behavior of Zr-2,5 % Nb Alloy

Transcription

1 Transactions of the 7 th International Conference on Structural Mechanics in Reactor Technology (SMiRT 7) Prague, Czech Republic, August 7 22, 2003 Paper # C0- High Temperature Mechanical Behavior of Zr-2, % Nb Alloy BS Rodchenkov ), AN Semenov 2) ) ENES, POB788, Moscow, 00, Russia 2) RDIPE, POB788, Moscow, 00, Russia ABSTRACT Zr-2, % Nb alloy are widely used as structural material for pressure tubes of channel nuclear power reactors with heavy pressured water (CANDU-PHW) and boiler water (RBMK, Fugen) Tensile and creep behavior of Zr-2, % Nb alloy significantly depend on heat treatment To satisfy design requirements to reactor pressure tubes Zr-2, % Nb alloy have been used after different heat treatments: α-annealing (reactor RBMK-00), water or gas quenching and aging (reactor RBMK-00 and Fugen), cold-work and stress-relief (reactor CANDU) An experimental program has been carried out to research resistance to plastic flow and rupture of Zr-2, % Nb alloy at the temperatures C Tests were carried out in uniaxial tension at a constant cross-head motion from -2 to -4 s - and under uniaxial load (creep rupture) Character of flow and rupture stress temperature dependence was complex Three range of temperature were observed where the rate of the strength reduction was differ significantly Extremely large elongation are observed in the C range and are manifestations of superplasticity The temperature of maximum elongation was equal ~ C Increase of strain rate from ~ -4 s - to -2 s - did not changed the temperature ranges of superplasticity observation but resulted to decrease of elongation values Creep tests were carried out in the C range It was observed that creep curves at the temperatures more than C have not hardening stage and creep rate is increased up to failure time The failure elongation is diminished from increase of initial stress value Creep tests results (creep rates and time upto failure) have been described satisfactorily by equations of power theory of creep and creep rupture KEY WORDS: zirconium alloy, tensile tests, creep, strength, elongation, rupture, creep rate, superplasticity, INTRODUCTION Fuel channels of RBMK reactors are cooling by boiling light water with parameters: the temperature ( ) 0 C and pressure (7 8) MPa Pressure tubes of fuel channels have external diameter 88 mm and wall thickness 4 mm Annealed Zr-2, % Nb alloy are used for pressure tubes [] The strength of zirconium alloys at normal working condition were examined detaily in [2] and influence of the working parameters including irradiation and corrosion effects were shown in [3, 4] Prognosis of accidents is particular interest because of large attention to question of NPP safety Creep and accumulation of structural damage (pores, microcracks, et) of fuel channel materials is necessary take account during calculation of its deformation and resistance to rupture The value of deformation as result of high temperature creep can reach to some tens of percents after very short time This can result in considerable changes geometric form of construction and at least to rupture Therefore investigation of resistance to deformation and rupture for Zr-2, % Nb alloy dependence from temperature and stress for accident condition is very actual problem EXPERIMENTAL PROCEDURES AND MATERIALS The base material used in this study was industrial grade Zr-2, % Nb alloy Tensile and creep specimens were machined from pressure tubes in both the longitudinal and transverse direction Pressure tubes having outer diameter 88 mm and wall thickness 4 mm were manufactured using cold work and final α-annealing The specimens have cylindrical work part with diameter 3 mm and nominal gauge length of 0 mm A standard Instron test machine fitted with a furnace and a high-temperature capsule was employed for the uniaxial tension tests After assembly the capsule was filled in high-purity argon, thereafter the argon was maintained in the system at a slight positive pressure during all time testing The furnace temperature was controlled by standard thermocouple, specimens were held at test temperature for minutes before straining The temperature gradient along the gauge length was maintained at less than 3 0 C The specimens were tested in uniaxial tension at a constant cross head motion of from 3-4 to,7-2 s - and temperatures from 20 to C 2 3 specimens were tested at the each test temperature The load-displacement data were converted to standard engineering stress and strain values (UTS, YS, TEL, RA) using the original cross-sectional area and original gauge length

2 Standard test machine type AIMA -2 was used for creep rupture tests Time heating to test temperature was not more than 20 minutes; time holding at the test temperature before loading was equal 3 minutes To avoid the oxidation the specimens were blown on by flow of high-purity argon during all time heating and testing Creep tests were carried out at temperatures C and stress 80 MPa RESULTS Tensile tests the temperature dependence of strength and ductility parameters of α-annealed alloy over the C range for three strain rates are presented at the figures (a, b) Three range of temperature can distinguish at temperature dependence of UTS and YS for standard rate tensile test 3,3-4 s - : ~( C), where low monotonous decrease of strength are observed, ~ ( C), where decrease rate of strength considerably grow and at beast ( C) in which, rate decrease of strength is again very small Increase of strain rate does not influence at qualitative change in general character of temperature dependence and increase only absolute values of UTS and YS and some decrease the difference between those Some deflection from monotonous character of YS change in range ~ C and C were observed if the strain rate was equal ~ 3-4 s - Figure b shows the temperature dependence of the total elongation The extremely large elongation observed in the range C are manifestations of superplasticity Maximum value of elongation was equal ~ 20 % at the temperature ~ C Increasing strain rate did not changed temperature range of superplasticity but resulted in decreasing of maximum value elongation Considerable change of deformation character was observed in this temperature range The superplastic specimen was characterized by an extremely small uniform elongation, quite large necking elongation and hence a large total elongation Before failure superplastic specimens exhibited no visible neck in gauge section Rather the specimen deforms along the entire gauge length Figure 2a,b compares UTS, YS and TEL of α-annealed Zr-2, % Nb alloy for longitudinal and transverse direction of pressure tube Alloy was more strength in transverse than in longitudinal direction The range temperature of superplasticity was the same in transverse and longitudinal direction, but absolute values of elongation were some higher in longitudinal direction, fig 2b Difference of tensile properties in transverse and longitudinal direction became insuffcant at the temperatures > C Two type of creep curves were observed The creep curves at temperatures lower than C had hardening stage, where creep rate gradually decreased (curve, fig 3) At temperatures higher C the creep curves had not hardening stage and creep rate increased up to failure time (curves 2, 3, fig 3) The value failure elongation was diminished with increasing of level initial stress Indicated peculiarity of creep curves at the temperatures higher C can explain using energy variant of the creep and strength theory [] Equations of energy variant creep and strength theory with introduction of concept on specific dissipation energy A * and structural parameter, reflecting accumulating of damages ω are of the form: ξ c = k 0 exp(-q c /RT) σ n c /(- ω) ; * с óс î ù& = () A where σ - stress, ξ c logarithmic creep strain rate, k 0 coefficient to be derived from the test results, Q c energy of creep activation, R Boltzmann constant, ω parameter of damages accumulation (ω = 0 at t = 0 and ω = in failure time), A * specific dissipation energy (A * = σ 0 ε *, where σ 0 initial stress and ε * elongation in failure time) Value of parameters k 0, n and Q c were determined using regression analysis of experimental date, and are shown in table Table Creep parameter of α-annealed Zr-2% Nb alloy Temperature, 0 C n k o, MPa -n/s Q c, KG/K ,67, ,32 4, 6 39 Comparison of experimental and calculated creep curves are shown at fig 4 2

3 00 00 V test = /sec V test = /sec V test = /sec 0 YS, UTS, MPa TEL, % 0 V test =333 V test =333 V test = /sec 3-3 /sec 3-2 /sec UTS YS a) b) Fig Zr-2,%Nb alloy temperature dependence of yield strength (YS), ultimate strength (UTS) (a) and total elongation (TEL) (b) for different test rates 00 YS in longitudinal direction YS in transverse direction 00 longitudinal direction transverse direction UTS in longitudinal direction YS, UTS, MPa 0 UTS in transverse direction TEL, % a) b) Fig 2 Zr-2,%Nb alloy temperature dependence of yield strength (YS), ultimate strength (UTS) (a) and total elongation (TEL) (b) for longitudinal and transverse directions 3

4 ε, % 2 ε, % t, min Fig 3 Temperature influence on creep curves of Zr-2%Nb alloy ( 0 C, C, C) t, min Fig 4 Creep curves of Zr-2%Nb alloy (experimental curves is shown by firm lines, calculated curves is shown by dash lines): T=800 C at σ = 20 MPa, 2 T=60 C at σ = 60 MPa, 3 T=600 C at σ = 72, MPa, 4 T=70 C at σ = 9,3 MPa, T=700 C at σ = 20 MPa, DISCUSSION In present study the superplasticity in Zr-2,% Nb observed in range C Results of present work some differ from received before H Rosinger and AEUnger [6] have been observed superplasticity in cold-worked Zr-2,% Nb alloy for pressure tubes of CANDU fuel channels in range C Garde etal [7, 8] have been observed it in Zircaloy-2 and Zircaloy-4 at two temperatures, 70 0 C and 0 0 C Bocek et al [9] has also been observed superplasticity at C for Zircaloy-4 tested in an air atmosphere The alloy Zr-2,% Nb containing no additional oxygen according [] consists of a stable hexagonal-closepacked α-zr phase up to C and a high-temperature stable body-centered-cubic phase of β-zr above C In the intermediate temperature range the Zr-2,% Nb alloy consists of a duplex phase of α and β Zr The two-phase region for as-received Zr-2,% Nb alloy for CANDU pressure tubes containing approximately 0,2-0, wt % oxygen, extends from 62 0 C to C [6] Superplasticity is generally identified with necking resistance and extraordinary elongation In superplastic materials there is no localized necking, but rather a reduction in area along the whole length of the test specimen The superplasticity effect is usually dependent on the strain rate, on the testing temperature, on the grain size and on the compositions Studying of the literature [ 6] indicates, that on two type of superplasticity can observe: * structural (micro grain) superplasticity, which is related to a single phase or two-phase fine-grained material deformed at temperature above 0,T m, where T m is the absolute melting point * environmental superplasticity, which results from anisotropic dimensional changes that occurs when a material is thermally cycled through the transformation temperature under a small load Evidence for structural superplasticity has been found in Zr and its alloys [6 3] Lee and Backofen [3] found superplasticity in Zircaloy-4 in the temperature range where the α- and β phases coexist At temperature where superplasticity elongation are observed the equilibrium structure consists of a 4

5 large fraction of a α-zr phase and a small fraction of the β-zr phase The β-zr phase is softer and more ductile for Zircaloy because the equilibrium concentration of oxygen in the β-phase is considerably less than that in the α-zr phase Also the body-centric-cubic β-phase has a more number of slip systems than the hexogonal-close-packed α- phase The maximum superplasticity observes in a temperature region in which a large fraction of the relatively stronger α-phase coexists with a small fraction of the softer β-phase which is present as network at the α-grain boundaries Results at present work for α-annealed Zr-2,% Nb alloy a goad correlate with such point of view H Rosinger and A Unger [6] observed more high temperature superplasticity in Zr-2,% Nb alloy where equilibrium structure consists of a large fraction of β-phase and a small fraction of α-phase They supposed that the α-phase is again harder than the β-phase The harder α-phase acts as second phase particles in the softer β-phase matrix and does not take part in the deformation of β-phase According our point of view here α-grains are more ductility and the main mechanisms of deformation in this case is grain-boundary sliding This point of view a good correlate with data of Nuttal [2] Analysis of specimens appearance and values of elongation after creep rupture tests were shown absence of superplasticity in these condition The same results were received in [6] for cold-worked Zr-2, % Nb alloy CONCLUSIONS This investigations had assessed the effect of temperature and strain rate on tensile properties of α-annealed Zr-2,% Nb alloy It has been found that: * The flow stress decreases with increasing temperature * The flow stress is strongly strain-rate dependent increasing with increasing strain rate * The same strain rate annealed Zr-2,% Nb alloy softer than cold-worked Zircaloy and Zr-2,% Nb alloy * Superplasticity is observed in the low temperature range of the duplex phase region for α-annealed Zr-2,% Nb alloy * Creep at range C satisfactorily is described equations of energy variant creep and strength theory REFERENCES Никулина АВ, Решетников НГ и др Technology of manufacture of pressure tubes from Zr-2, % Nb alloy for RBMK reactor Сб ВАНТ, с Материаловедение и новые материалы, 990, 2 (36), с 46-4 (in russian) 2 Ривкин ЕЮ, Родченков БС, Филатов ВМ Прочность сплавов циркония М, Атомиздат, 974 (in russian) 3 Родченков БС, Ривкин ЕЮ и др Strength of fuel channel pressure tubes Сб ВАНТ, с Материаловедение и новые материалы, 990, 2, с4 4 (in russian) 4 Платонов ПА и др Creep of RBMK reactor fuel channel from Zr-2, % Nb alloy Сб ВАНТ, с Материаловедение и новые материалы, 990, 2, с (in russian) Работнов ЮН, Милейко СТ Кратковременная ползучесть М, Наука, 970 (in russian) 6 YE Rosinger, AE Unger The Superplastic and Strain-Rate Dependent Plastic Flow of Zr-2 % Nb in the 873 to 373 K Temperature Range, AECL-648, AM Garde etal Uniaxial Tensile Properties of Zircaloy Containing Oxygen: Summary Report Argone National Laboratory Report ANL-77-30, AM Garde etal Micrograin Superplasticity in Zircaloy at 80 0 C J Nucl Mater, 62, 976, 26 9 M Bocek etal Superplasticity of Zircaloy-4, Proc of Sixth International Symposium Zirconium in the Nuclear Industry ASTM STP 633, 977 D Douglas Metallurgy of Zirconium, 97 JJ Kearnes et al Effect of Alpha/Beta Phase Constitution on Superplasticity and strength of Zircaloy-4 J Nucl Mater 6, 976, 69 2 K Nuttal Superplasticity in the Zr-2 % Nb alloy, Scripta Met,, 976, 83 3 D Lee, WA Backofen Superplasticity of Some Titanium and Zirconium Alloys Trans TSM-AIME, 239, 967, 34 4 RN Jonson, Superplasticity, Met Rev, 46, 970, RH Ashby, RA Verrall Diffusion Accomodated Flow and Superplasticity Acta Met, 2, 973, 49 6 HE Rosinger et al An examination of the anisotropic characteristics of CANDU Pressure Tubes at K, Report WNRE-38, 980