Nb Pressure Tube Alloy Research Articles

Influence of temperature on threshold stress for reorientation of hydrides and residual stress variation across thickness of Zr–2.5Nb alloy pressure tube

Threshold stress, σ th, for reorientation of hydrides in cold worked and stress-relieved (CWSR) Zr–2.5Nb pressure tube material was determined in the temperature range of 523–673 K. Using tapered gage tensile specimen, mean value of σ th was experimentally determined by two methods, half thickness method and area compensation method. The difference between local values of σ th measured across the thickness of the tube and the mean σ th values yielded the residual stress variation across the tube thickness. It was observed that both the mean threshold stress and residual stress decrease with increase in reorientation temperature. Also, the maximum value of residual stresses was observed near the midsection of the tube.

Flow behaviour of a modified Zr–2.5wt%Nb pressure tube alloy

Anisotropy in flow behavior of cold worked and stress-relieved Zr–2.5wt%Nb pressure tube alloy was characterized by performing tensile tests in the temperature range of 298–823K and under a constant nominal strain-rate of ∼3×10−4s−1. Results showed that both yield and ultimate tensile strengths of this alloy decreased gradually with increasing test temperatures, with a rapid fall in strengths above a temperature of 623K. The tensile ductility did not change appreciably up to 673K but increased rapidly above this temperature. A strain energy model was developed to explain the occurrence of dynamic strain ageing (DSA). Analysis of the true stress–strain behavior suggested that the flow hardening or softening behavior of this alloy can be expressed in the form of a constitutive relationship comprising of a constant, work-hardening term and a term representing flow softening due to recovery.

Experimental observations of void growth in the Zr–2.5Nb pressure tube alloy

Voiding during tensile loading of Zr–2.5Nb pressure tube alloy has been experimentally observed at two temperatures. Tensile specimens cut from the wall of a pre-service pressure tube were tested to various degrees of strain and then sectioned to allow internal voids to be examined. A point counting procedure was used to determine the void volume fraction at various levels of strain. Scanning electron microscopy was completed to determine the degree of voiding, the number and size of voids and the void morphology. Observed voids generally remain small while the overall void fraction increased with increased strain. This suggests that the void formation process in this material is dominated by extensive void nucleation, with little growth of the nucleated voids before void coalescence and final fracture.

Terminal solid solubility of hydrogen in Zr-alloy pressure tube materials

In this work, terminal solid solubility (TSS) of hydrogen for Zr-alloy pressure tube materials was determined corresponding to end of hydride dissolution (TSSD) and start of hydride precipitation (TSSP) using the dilatometry technique. For this, Zircaloy-2 and Zr–2.5Nb pressure tube alloy coupons were gaseously charged with controlled amount of hydrogen in the range 10–100 μg/g. Change in length of cylindrical specimens of length ∼10 mm and diameter ∼3.5 mm machined from the coupons were measured as a function of temperature using a dilatometer. The samples were heated at 2 °C/min to 430 °C, held for 30 min at 430 °C and cooled back to the ambient temperature at 2 °C/min. The transition temperatures corresponding to the end of dissolution of hydrides during heating and beginning of precipitation of hydrides during cooling in these alloys were determined from thermal strain ( e) versus temperature ( T), average slope (of e versus T plot) versus T and differential thermal strain versus T plots. The enthalpies of hydride dissolution and precipitation for Zircaloy-2 pressure tube material were found to be 30–34.5 and 25.9–26.3 kJ/mol, respectively, whereas the corresponding enthalpies for Zr–2.5Nb pressure tube material were found to be 35.44 and 17.2–22.8 kJ/mol, respectively. This difference in the enthalpies between TSSD and TSSP is explained in terms of the different roles played by the components of strain energy associated with the elastic and plastic deformation in the matrix and precipitate, as a result of hydride accommodation in this alloy during heating and cooling process.

Delayed hydride cracking in Zr–2.5Nb pressure tube material: influence of fabrication routes

Abstract The delayed hydride cracking (DHC) velocity was measured for Zr–2.5Nb alloy pressure tubes manufactured from cold-worked and stress-relieved (CWSR) route and quenched and aged (Q&A) route at 162, 203, and 250 °C. It was observed that the DHC velocity in the pressure tubes produced from Q&A route is about half to one-third of that observed in the tubes produced from CWSR route. The lower DHC velocity in Q&A tubes compared to that in the CWSR tubes have been rationalized in terms of the difference in strength and effective hydrogen diffusivity compared to the tubes manufactured from CWSR route. A modification to Dutton–Puls model of DHC is suggested by proposing alternate expression for the elastic stress-field ahead of a sharp crack for an elasto-plastic material.

Stress-reorientation of hydrides and hydride embrittlement of Zr–2.5 wt% Nb pressure tube alloy

Hydrogen in excess of the terminal solid solubility precipitates out as a brittle hydride phase in zirconium alloys. The hydrides acquire platelet shaped morphology due to their accommodation in the matrix and can cause severe embrittlement, especially when these are oriented normal to the tensile stress axis. The precipitation of hydride platelets normal to the tensile stress when cooled under stress from a solution-annealing temperature is commonly referred to as ‘stress-reorientation’. The stress-reorientation is associated with a threshold stress below which no reorientation is observed. In this work, stress-reorientation of hydrides was investigated for unirradiated, cold worked and stress-relieved Zr–2.5 wt% Nb pressure tube material for a reorientation temperature of 423–723 K. The effect of the reoriented hydrides on the tensile properties of the Zr–2.5 wt% Nb pressure tube alloy was evaluated in the temperature range of 298–573 K.

Zr-2.5Nb 합금의 크리프 물성 측정을 위한 SP 크리프 시험의 적용성에 대한 연구

The pressure tubes made of cold-worked Zr-2.5Nb alloy are subjected to creep deformation during service period resulting in changes to their geometry such as longitudinal elongation, diameter increase and sagging. To evaluate integrity of them, information on the material creep property of the serviced tubes is essential. As one of the methods with which the creep property is directly measured from the serviced components, small punch(SP) creep testing has been considered as a substitute for the conventional uniaxial creep testing. In this study, applicability of the SP creep testing to Zr-2.5Nb pressure tube alloy was studied particularly by measuring the power law creep constants, A, n. The SP creep test has been successfully applied for other high temperature materials which have isotropic behavior. Since the Zr-2.5Nb alloy has anisotropic property, applicability of the SP creep testing can be limited. Uniaxial creep tests and small punch creep tests were conducted with Zr-2.5Nb pressure tube alloy along with finite element analyses. Creep constants obtained by each test method are compared. It was argued that the SP creep test result gave results reflecting material properties of both directions. But the equations derived in the previous study for isotropic materials need to be modified. Discussions were made for future research directions for application of the SP creep testing to Zr-2.5Nb tube alloy.

Stress field estimation around a semi-constrained inclusion and its validation by interpreting hydride platelet orientation around a blister in a Zr–2.5Nb alloy

A methodology for estimating the stress field in the matrix surrounding a semi-constrained inclusion is proposed. The stress free misfit volume associated with the inclusion was partitioned into semi-constrained and constrained components. The objective was achieved by carrying out a series of sectioning, cutting and imposition of constrained misfit displacement on an imaginary notch of geometry identical to that of the inclusion–matrix contour. The stress field was computed using elastic finite element analysis for plane stress condition. The misfit displacement model thus developed was applied to compute the stress field around a hydride blister grown under controlled thermal boundary condition in Zr–2.5Nb pressure tube (PT) alloy and was validated by interpreting the hydride platelet orientation in the matrix surrounding the blister. Finally, an assessment of the structural integrity of the Zr–2.5Nb PT material containing hydride blister was made in view of this analysis.

The delayed hydride cracking avoidance manoeuvre for Zr-2.5 Nb pressure tube alloy

In this work, the effectiveness of using a temperature manoeuvre to reduce the susceptibility of Zr-2.5 wt% Nb pressure tube alloy to delayed hydride cracking (DHC) is studied. The temperature manoeuvre consists of lowering the temperature by at least 30°C to a subcooling temperature when the specimen is under load, and heating up to the original temperature. The DHC suppressed by this temperature manoeuvre can usually be restored by a subsequent temperature decrease, which can be used as a measurement of the effectiveness of the temperature manoeuvre. Factors that affect the effectiveness of the temperature manoeuvre are studied. It is found that stress intensity factor has the strongest effect, and that the tensile and yield strengths of the pressure tube alloy also have a significant effect. Some experiments were carried out simultaneously in two laboratories, and reasonably consistent results were obtained.

Effect of strain rate and test temperature on superplasticity of a Zr–2.5 wt% Nb alloy

Tensile specimens of a Zr–2.5 wt% Nb pressure tube alloy were deformed at strain rates ( ε ̇ ) of 3.2 × 10 −4–2.9 × 10 −2 s −1 and test temperatures ( T) of 625–800°C to investigate the nature of stress–strain curves and the optimum condition for superplasticity. The maximum ductility of 1384% was obtained at T=700°C and intermediate strain rate of 3.4 × 10 −3 s −1. The stress–strain curves were found to exhibit the dominance of flow hardening, especially in the early part of deformation, followed by flow softening. The variation in ductility and flow stress as a function of test condition is explained by the observed microstructural evolution and fracture behavior.

Unusual SEM Image Contrast from Corrosion Oxide Films of Zr-2.5%Nb Pressure Tube Alloy

Abstract Zr-2.5wt%Nb alloy is used as the pressure tube material in CANDU reactors. The pressure tube microstructure consists primarily of α-Zr (< l%Nb) grains elongated in the axial direction of the pressure tube and typically 0.3-0.5 μm thick in the radial direction. The α-Zr grains are partially surrounded by a thin layer of Nb-rich β-Zr phase (≈ 20%Nb). During corrosion, α-Zr is converted primarily to monoclinic ZrO2 with minor amounts of tetragonal ZrO2. Oxidation of α-Zr is complicated by the metastable nature of the phase below 863K. A detailed understanding of the nature of oxidized β-Zr remains a topic of on-going investigation, although some information have recently become available. Present SEM observations shed some lights on the charging characteristics of the oxidized β-Zr compared with ZrO2. Oxide films on pre-polished surfaces perpendicular to the radial direction of the pressure tube were grown in steam at 673K in an autoclave; the treatment corresponds to the last stage of the pressure tube production.

Ｘ線材料強度 Ｚｒ‐２．５％ＮＢ合金における集合組織と弾性定数の中性子回析による測定

The cystallographic texture and elastic constants have been measured on heat-treated Zr-2.5%Nb alloy pressure tubes by neutron diffraction. The texture of heat-treated Zr-2.5%Nb alloy is not so strongly oriented compared with that of non-heat-treated Zr-Nb alloy. However, the pole figures show that the appropriate diffraction planes for the axial direction strain are 10·0, 00·2 or 10·1 diffractions, those for the hoop direction strain are 00·2, 10·1 or 11·0 diffractions, and those for the radial direction strain are 11·0 or 10·2 diffractions. The diffraction plane dependency of elastic constants almost agrees with that obtained from Kroner's single cristal model. However, the diffracttion plane dependency of elastic constants is not so strong. From the residual stress measurements by the neutron diffraction, the values of Young's modulus for 10·0, 10·1 and 00·2 diffractions are found to be 96GPa, 95GPa and 104GPa, respectively, and Poisson's ratios are 0.34, 0.36 and 0.32, respectively. Finally, the residual stress evaluation method for the material with strongly oriented texture is proposed.

A Study of Superplasticity in a Zr-2.5wt%Nb pressure Tube Alloy

A Study of Superplasticity in a Zr-2.5wt%Nb pressure Tube Alloy

Evolution of microstructure during fabrication of Zr-2.5 Wt pct Nb alloy pressure tubes

Microstructural changes occurring during the fabrication of Zr-2.5 pct Nb alloy pressure tubes by a modified route, involving hot extrusion followed by two pilgering operations with an intermediate annealing step, have been examined in detail. In the conventional fabrication route, the hot extrusion step is followed by a single cold drawing operation in which the cold work to the extent of 25 pct is imparted to the material for achieving the required mechanical properties. Tensile properties obtained at each stage of fabrication have been evaluated and compared between the two processes. The main aim of this work has been to produce a microstructure and texture which are known to yield a lower irradiation growth. Additionally, suitable annealing conditions have been optimized for the intermediate annealing which annihilates the cold work introduced by the first cold pilgering operation without disturbing the two-phase elongated microstructure. This elongated α+ β I microstructure is required for obtaining the desired level of strength at 310 °C. The final microstructure and the crystallographic texture of the finished pressure tube have been compared with those reported for the conventionally processed material.

The distribution of Nb and Fe in a Zr-2.5 wt% Nb alloy, before and after irradiation

The influence of irradiation (3.58 × 10 24 n/ m 2; E > 1 MeV at 583 K for 52 days) has been compared to the effects of thermal exposure alone (52 days at 583 K) on the redistribution of Nb and Fe in an as-extruded Zr-2.5 wt% Nb pressure tube alloy. The major change associated with the thermal treatment is a decomposition of the retained β-Zr phase, to form ω. As the ω-phase grows, it rejects Nb and Fe to the surrounding β-Zr. No significant changes were noted in the Nb and Fe levels segregated at α-α grain boundaries on heat treatment. Similar effects were observed in the irradiated samples, but in addition the retained β-phase is depleted of Fe. At the same time Fe is detected in the α-matrix of the irradiated specimen. Although analytical electron microscopy in a dedicated STEM instrument does not lead to reliable estimates of the absolute concentration of Fe in the irradiated sample, a series of experiments are described which show that Fe is present in the α-matrix of the alloy.

Microstructural changes in laser hardened Zr-2.5Nb alloy

Laser surface treatment of metals is a process in which heating or melting of the surface of the metal by laser irradiation is followed by a rapid cooling via self-quenching. This process can produce metallurgical structures with improved wear and corrosion resistance. For zirconium alloys, laser melting (glazing) has been used to modify the microstructure of the surface layers of Zircaloy-2 and -4 tubing, and Zr-2.5Nb pressure tube alloy (1-4) without affecting the bulk properties. A supersaturated [alpha][prime]-martensitic structure was produced by the laser-glazing process. In addition, laser glazing produced a rough surface finish, which was attributed to rippling effects during the rapid solidification. In the present work the authors have investigated laser surface heating because such a treatment is expected to produce a similar microstructure as laser glazing, but with a better surface finish. Laser irradiation could raise the specimen's surface temperature into the [beta]-phase region and the subsequent rapid cooling should be sufficient to achieve the martensitic phase transformation, without requiring surface melting. The microstructure produced by laser surface heating was characterized by transmission electron microscopy. The depth profiles of the laser surface heated layers were obtained using microindentation. This paper reports the results obtained in this investigationmore » of microstructural changes produced in the surface layers of Zr-2.5Nb alloy by laser surface heating.« less

Investigation of zirconium hydrides and hydrogen solubility limit in Zr-2.5 Nb alloy by small angle neutron scattering

In zirconium and its alloys, hydride phases form as needle-shaped ([gamma]-phase) or plate-shaped ([delta]-phase) precipitates when the terminal solid solubility (TSS) of hydrogen in the metal is exceeded. The hydrogen TSS in zirconium is almost negligible at room temperature, but at 620 K the solubility limit increases to about 1 at%. Zr-2.5 Nb alloy pressure tubes are the structural components in CANDU-PHW reactors and, if hydrides are present, the materials may be embrittled. A precise knowledge of the hydrogen TSS is required for assessing the fitness-for-service of Zr-2.5 Nb pressure tubes. However, in TSS measurements a hysteresis behavior is observed where the temperatures attributed to hydride dissolution and precipitation are different. This hysteresis occurs even in the absence of applied stress and may contribute to a variation in TSS values. To evaluate the feasibility of SANS for future experiments that will study zirconium hydride precipitation and dissolution, Zr-2.5 Nb samples containing several distributions of hydrides were tested at both room temperature and up to 600 K.

Microstructural and microchemical studies of Zr-2.5Nb pressure tube alloy

The microstructure and distribution of Nb and Fe in a standard Zr-2.5Nb pressure tube alloy has been studied by analytical electron microscopy before and after irradiation (~ 5 × 10 25 n/ m 2; E > 1 MeV at 553 K). Irradiation produces pronounced changes in both the dislocation substructure and the chemical composition of the α and β-phases. Both 〈 a〉 and 〈 c〉-component defects are observed after irradiation; 〈 c〉-component screw dislocations are split after irradiation and are decorated by β Nb particles. There is copious precipitation of β Nb in the α-phase. Fe segregation is found at α-α subboundaries and in the β-phase prior to irradiation. After irradiation increased levels of Fe are observed at α-α subboundaries, while the β-phase is depleted of Fe.

A study of the distribution of Nb and Fe in two-phase Zr-2.5 wt% Nb alloys

An analytical transmission electron microscopy study of two-phase (α-β) structures in a Zr-2.5 wt% Nb pressure tube alloy was used to follow the distribution of Nb and Fe as a function of alloy heat treatment and tube processing. The presence of Fe (~ 0.1 wt%) modifies the (α-β) phase equilibria as Fe is a β-stabilizing element. Significant segregation of Fe to β Zr and β Nb structures was measured and shown to be in good agreement with the prediction of a graphical method used to find the phase boundaries in the ternary diagram (at the Zr corner) from the binary data. Segregation of Fe at α-α subboundaries was found in the as-extruded and cold-worked plus stress-relieved treatments. The Fe profiles, measured across subboundaries, indicated a strong segregation of Fe to the core or near-core regions of the dislocations comprising the boundary structure.

Mitigation of degradation mechanisms affecting CANDU pressure tubes

Abstract The Zr-2.5Nb alloy pressure tube provides the primary containment for the coolant in the CANDU reactor. Because these tubes contain the individual fuel strings and pass through the center of the reactor, they are exposed to the full fast neutron flux of up to 4.0 × 1017 neutrons per square meter per second (E > 1 MeV). Any demonstration of the integrity of these tubes relies on a leak-before-break (LBB) assessment, which, in turn, depends on the fracture properties of the tube. These properties become degraded with service due to both the gradual pickup of hydrogen isotopes and the reduction of fracture toughness due to irradiation. This paper gives an overview of the relevant fracture properties used in the LBB scenario and the effect of service conditions on their degradation. The particular properties discussed are the susceptibility to sub-critical crack growth by delayed hydride cracking, and the reduction in fracture toughness which reduces the critical crack length. Methods of protecting the tubes against fracture and improving their properties are discussed.