Anisotropic threshold stress intensity factor,K IH and crack growth rate in delayed hydride cracking of Zr-2.5Nb pressure tubes

The objectives of this study are to systematically investigate the delayed hybride cracking (DHC) velocity and the threshold-stress intensity factor, K _IH, of a Zr-2.5Nb pressure tube as a function of orientation and elucidate the cause of this anistropic DHC behavior. The DHC velocity as a function of orientation was determined using flattened cantilever beam specimens with 60 ppm H while the threshold-stress intensity factor K _IH, was evaluated as a function of orientation on the curved compactension (CT) and cantilever-beam (CB) specimens charged with hydrogen to 200 ppm H. To infer a difference in a stress gradient ahead of the crack tip as a function of orientation, tensile tests were conducted at temperatures ranging from room temperature (RT) to 560°C using small tensile specimens of 2-mm-gage length taken from three directions of the tube. A textural change was investigated by comparing the inverse pole figures before and after DHC while the pole figures were constructed to find out the growth pattern of the DHC crack as a function of orientation. Faster DHC velocity and lower K _IH were obtained over temperatures of 170 °C to 270 °C, when the DHC crack grew in the longitudinal direction of the Zr-2.5Nb pressure tube. The strain hardening after yielding and the extent of the textural change accompanied by DHC were higher in the longitudinal direction of the tube, suggesting a higher stress gradient ahead of the crack tip. Thus, the anisotropic DHC behavior of a Zr-2.5Nb pressure tube is discussed based on the stress gradient ahead of the crack tip governed by strain-hardening rate after yielding and a change in texture accompanied by DHC, and the distribution of the hydride habit planes. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Effect of heat treatment on delayed hydride cracking in Zr-2.5 Wt Pct Nb

The effect of heat treatments on delayed hydride cracking (DHC) in Zr-2.5 wt pct Nb has been studied. Crack velocities were measured in hydrided specimens, which were cooled from solution-treatment temperatures at different rates by water-quenching, oil-quenching, liquid-nitrogen quenching, and furnace cooling. The resulting hydride size, morphology, and distributions were examined by optical metallography. It was found that fast cooling rates, which produce very finely dispersed hydrides, result in higher crack growth rates and a stronger dependence of the crack velocity on the applied-stress intensity factor. Also, the incubation period before cracking commences was found to be relatively short for specimens with fine hydrides, whereas specimens with coarse hydrides required considerably longer incubation periods. These results can be explained by rapid growth of preexisting hydrides within the crack-tip plastic zone. In addition, different solution temperatures were used to investigate the effect of the continuity of the grain-boundary phase(β-phase) on the crack velocity. Transmission electron microscopy was used to examine the structure of this grain-boundary phase. It was found that for heat treatments, which destroyed theβ-phase continuity, the crack velocity was significantly reduced, as would be expected from the theory of enhanced diffusion through grain boundaries.     

Embrittlement of heat-treated Zr-2.5 wt% Nb pressure tubes

Abstract

Fracture studies were made on two batches of SGHWR pressure tubing. One of the batches had a normal grain size and structure, while the other was coarse grained due to overheating during heat treatment. The aim was to determine the critical crack length of the tubes under reactor operating conditions, and the effect on the critical lengths of embrittlement by hydrogen absorption and neutron irradiation damage. Tubes were measured directly where possible, while supplementary data were obtained from tests on small specimens, enabling the effects of embrittlement to be predicted. The coarse-grained tube was found to be much more susceptible to hydrogen and irradiation embrittlement than the fine-grained tube, and it is expected that the critical length would be reduced from an initial 4 in. to about 1.5 in. at the end of life. In contrast the critical length of the fine-grained material would only be reduced from 5 in. to approximately 3.5 in.

Résumé

Des essais de rupture ont été faits sur deux lots de tubes de force SGHWR. Léun de ces lots avait une taille de grain et une structure normales tandis que le second avait de gros grains dûs à une surchauffe lors des traitements thermiques. Le but de cette étude était de déterminer la longueur critique des fissures des tubes dans les conditions d'opération et l'effet sur cette longueur critique de la fragilisation par l'hydrogène adsorbé et par irradiation aux neutrons. Quand ce fut possible les essais furent effectués sur les tubes et des données supplémentaires furent obtenues par des essays sur petits échantillons, ceci permettant de prédire les effets de la fragilisation. Le tube à gros grains était plus sensible à la fragilisation par hydrogène et par irradiation et il est preévu que la longueur critique sera réduite de 4 à 1.5 pouces a la fin de la vie utile. Pour le tube à grain fin par contre cette longueur ne serait réduite que de 5 à 3.5 pouces environ.     

The effects of stress,temperature and hydrogen content on hydride-induced crack growth in Zr-2.5 Pct Nb

The velocity of hydride induced subcritical crack growth in Zr-2.5 pct Nb has been determined using the potential drop method for measuring crack extension. A revised picture of the two-stage, crack velocity-stress intensity relationship has been obtained with a threshold stress intensity of 6 MPa·m^1/2, independent of temperature. A consistent temperature dependence of the crack velocity has been determined for hydrided material but the velocity measurements in as-received material are unexpectedly high. A previous theoretical model has been improved. The improved model has provided a useful basis for explaining some of the present data which could not be rationalized in terms of the previous model. Criteria for the stepwise crack propagation behavior are discussed.     

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. presently on leave from BARC, is Postdoctoral Fellow, Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH-45221.     

Computation of diametral deformation of irradiated Zr-2.5 Nb pressure tubes from RAPS-2

The pressure tubes in Pressurised Heavy Water Reactors (PHWRs) are subjected to high stresses, temperatures and fast neutron fluxes which cause changes in dimensions and material properties. To ensure the safe, reliable and economic performance of the reactor, it is important that these changes are known and that the rate of change can be predicted and demonstrated to remain within the design basis. The diametral expansion of pressure tubes due to irradiation creep and growth is an important property that may limit the useful life of the tube. A semiempirical predictive model has been developed to calculate the irradiation creep (diametral) and irradiation growth (diametral) contributions and total diametral change in irradiated pressure tubes. The model can account for variations arising from tube orientation. The sixteen numbers of Zr-2.5Nb pressure tubes (PTs) of RAPS-2 selected had undergone 7.15 Effective Full Power Years (EFPY) of reactor operation. The diametral changes at discrete locations along the length of the pressure tubes are estimated by using the model developed. The maximum of (along the tube length) measured internal diameter data of those fifteen pressure tubes have been compared with those obtained from the model.     

Metallurgical properties of heat-treated Zr-2.5 wt% Nb pressure tubes irradiated under power reactor conditions

Abstract

Three heat-treated pressure tubes of Zr-2.5 wt % Nb were irradiated as components of Chalk River reactor loops under conditions similar to those in Canadian power reactors of pressurized heavy water (CANDU-PHW) and boiling light water (CANDU-BLW) type. Two other tubes, one of which had been artificially hydrided to about 200 ppm H2, were irradiated unstressed in air at about 300°C.

Post-irradiation examination of the tubes shows that they can withstand severe through-wall defects: the critical crack length exceeds 43 mm at a hoop stress of 179 MN/m2 at 20° and 300°C. Mechanical strength was increased by neutron irradiation: eliminating autoclaving offers the advantage of lower rates of oxidation and hydrogen pickup.

Résumé

Trois tubes de force en alliage Zr-2.5 % pds Nb, ayant subi des traitements thermiques ont été irradiés comme composants de boucles dans le réacteur à Chalk River. Les conditions ressemblaient à celles dans les réacteurs canadiens à eau lourde sous pression (CANDU-PHW) et à eau légère bouillante (CANDU-BLW). Deux autres tubes, l'un ayant subi un traitement d'hydruration jusqu'à 200 ppm de H₂ ont été irradiés sans contrainte sous air à 300°C.

Les examens ultérieurs ont montré que les tubes peuvent supporter des défauts sévères qui traversent les parois: la longueur critique de fissure est supérieure à 43 mm pour une contrainte de 179 MN/m² a 20° et a 300°C. Une irradiation aux neutrons augmentent la résistance mécanique. Les taux d'oxydation et de dissolution d'hydrogène se voient diminuer si les tubes ne sont pas traités antérieurement à l'autoc1ave.     

Development of texture and structure in Zr-2.5 wt% Nb extruded tubes

Abstract

Factors which affect the texture and microstructure of Zr-2.5 wt % Nb extruded tubes have been investigated. The major factor that determines th~ texture is the way the metal flows, which is controlled by the die shape, the extrusion ratio and the lubricant. The microstructure is controlled by the temperature during extrusion and the cooling rate afterwards.

Résumé

Les auteurs ont examiné les facteurs influençant la texture et la microstructure de tubes de Zr-2.5 % Nb filés à la presse. La texture surtout fixée par l'écoulement du métal; cet écoulement est contrôle par la forme de la filière, le rapport de filage et le lubrifiant. La température de filage et le taux de refroidissement subséquent flxant la microstructure.     

Effects of crack tip stress states and hydride-matrix interaction stresses on delayed hydride cracking

A model of slow crack propagation based on the delayed hydride cracking (DHC) mechanism in hydride-forming alloys has been critically examined and evaluated to take account of recent experimental and theoretical advances in the understanding of hydride fracture and terminal solid solubility (TSS). The model predicts that the DHC velocity is a sensitive function of the hydrogen concentration induced in the bulk of the material as a result of the direction of approach to test temperature. For test temperatures approached from below, factors such as the hydridematrix accommodation energies, the stress state at the crack tip, and the value of the yield stress have a strong effect on the DHC arrest temperature in the technologically interesting temperature range of 400 to 600 K. A fracture criterion is explored based on the need to achieve a critical hydride length in the plastic zone at the crack tip. A necessary condition for DHC is that the crack tip hydride must grow to this critical length. An approximate estimate is made for the steady-state growth limit of the crack tip hydride as a function of the direction of approach to temperature and the crack tip stress state. For temperatures approached from below, growth of the crack tip hydride is limited to just outside the plastic zone boundary at low temperature, gradually receding toward and inside the plastic zone boundary with increasing temperature. At lowK _I values, this limits the crack tip hydride lengths to below their critical values for fracture. This could be one condition forK _IH . For test temperatures approaches from above, the growth limit is significantly increased, and the sensitivities to the above parameters become less evident.     

Anisotropy of yielding in a Zr-2.5Nb pressure tube material

The anisotropy of yielding, as measured by the ratio of yield stress in the axial and transverse directions of Zr-2.5Nb pressure tubes used in Canada Deuterium Uranium (CANDU) nuclear reactors, was determined experimentally by testing samples in uniaxial tension. The yield anisotropy was measured in uniaxial tension in samples obtained from the three directions of a Zr-2.5Nb plate and in shear, by testing in torsion “mini” pressure tubes from the same material. From these experiments, the temperature and strain-rate dependence of the yield stress and the dependence of the anisotropy of yielding on temperature were also determined. It is shown that the yield anisotropy of pressure tube material is constant for temperatures up to about 800 K and that the strain-rate sensitivity is also constant up to about 700 K and is equal to ∼0.02. In addition, the activation energy (Q) of this material was estimated by using the temperature dependence and rate sensitivity of the yield stress. It was found to be of the same order of magnitude as that determined earlier by other investigators. A polycrystalline, nonlinear self-consistent model that takes into account the crystallographic texture of the material was used to derive the values of the critical resolved shear stress (CRSS) which are consistent with prismatic, basal, and pyramidal glide and the values of the Hill’s plastic anisotropy coefficients which are consistent with the observed anisotropy of yielding. The model provided an estimate of the complete stress tensor, describing yielding of a Zr-2.5Nb pressure tube material.     

Anisotropy in impact behavior of Zr-2.5Nb pressure tube alloy

Zr-2.5Nb alloy tubes in cold worked and stress relieved (CWSR) condition serve as pressure boundary for hot coolant in Indian Pressurized Heavy Water Reactor (IPHWR). Due to both microstructural and crystallographic anisotropy, the mechanical properties in general and fracture behavior in particular are anisotropic for this material/component. In this work impact behavior of the pressure tube material was characterized over a range of temperature by impact test using specimens with crack growth direction along axial and transverse directions of the tubes. It has been found that both temperature and orientation have strong influence on the absorbed impact energy.     

The influence of applied stress,crack length,and stress intensity factor on crack closure

Experimental and analytical evidence for the influence of applied stress, crack length, and stress intensity factor on crack closure is critically compared and evaluated. Fatigue crack opening behaviors are broadly catalogued into three classes. Class I comprises “near-threshold” behavior, where crack closure levels increase with decreasing stress intensity factor. In class II, the “stable” regime, the crack opening level is independent of the stress intensity factor and crack length but is influenced by the applied stress. Class III is characterized by the loss of elastic constraint accompanying extensive yielding at the crack tip or in the remaining ligament, especially with further crack growth. Here, the crack opening level decreases with increasing crack length until little or no closure occurs. These three different classes of closure behavior are extensively illustrated with both experimental data and the results of numerical closure simulations, particularly original finite element (FE) analyses. No single relationship between crack opening levels and the fundamental fatigue parameters is found to hold universally, due to the wide range of mechanisms which cause or influence closure.     

Hydride orientation and tensile properties of Zr-2.5 wt% Nb pressure tubing hydrided while internally pressurized

Abstract

An investigation was carried out to determine the orientation and the effect on tensile properties of hydrides precipitated in heat-treated Zr-2.5 wt% Nb tubing during the corrosion hydriding of internally pressurized samples. It was found that as the stress during hydriding was increased the hydrides gradually assumed a characteristic radial orientation nearly perpendicular to the hoop stress. The observed radial hydride orientation was influenced by prior β grain size, residual quenching stresses, cold-work strains and stresses applied while the pressure tube was being hydrided. Hoop stresses in excess of 20,000 psi were required to produce complete radial orientation of the hydrides.

Transverse tensile tests on the hydrided tubes were conducted using a reduced section ring technique. These tests revealed a low ductility at low temperatures with a transition in the range 60°–100°C for material with radially oriented hydrides. Low tensile ductility was observed at significantly higher temperatures when a massive hydride layer was present near the surface of the test samples.

Résumé

Dans des tubes en Zr-2.5 % poids Nb trempé et vieilli et soumis à la corrosion hydrogenante sous pression, une étude a été entreprise pour déterminer l'orientation des hydrures précipités et leurs effets sur les propriétés mecaniques en traction. Les résultants indiquent que lorsque la contrainte augmente pendant léhydrogenation, les hydrures acquièrent graduellement une orientation radiale caractéristique, à peu prés perpendiculaire à la contrainte circonférentielle. Cette orientation radiale des hydrures est influencée par la taille du grain β initial, les contraintes de trempe résiduelles, les déformations à froid et les contraintes appliquées lorsque le tube sous pression est hydrogéné. Des contraintes circonférentielles supérieures à 20,000 psi sont nécessaires pour produire une orientation parfaitement radiale des hydrures.

Des essais de traction en travers des tubes hydrogénés ont été réalisés en utilisant la technique de l'anneau à section réduite. Ces essais ont révélé une faible ductilité à basses températures avec une transition dans le domaine 60°–100°C pour le matériau à hydrures orientés radialement. Une faible ductilité en traction a été observée à des températures beaucoup plus hautes quand une couche massive d'hydrures existait près de la surface des échantillons.     

复合型疲劳试验与折线裂纹强度因子计算

本文用三点弯曲偏裂纹试样研究了Ⅰ—Ⅱ复合型裂纹的疲劳扩展。由于裂纹扩展呈折线形,所以首先研究了折线裂纹应力强度因子的计算方法,并对计算结果用有机玻璃进行了实验验证。从而可以对疲劳裂纹扩展的数据进行有效的整理,得以说明一些判据的可用性,并对有关问题进行讨论。     

Effect of threshold stress intensity on fracture mode transitions for hydrogen-assisted cracking in AISI 4340 steel

Fracture mode transition in hydrogen-assisted cracking (HAC) of AISI 4340 steel has been studied from an equilibrium aspect at room temperature with 8.6-mm-thick double cantilever beam (DCB) specimens. The threshold stress intensity,K _th , necessary for the occurrence of HAC and the corresponding fracture surface morphology have been determined as a function of hydrogen pressure and yield strength. The K _th increases with decrease in hydrogen pressure at a given yield strength and also with decrease in yield strength at a given hydrogen pressure. AsK _th increases, the corresponding HAC fracture mode changes from the intergranular (IG) and quasi-cleavage (QC) modes to the microvoid coalescence (MVC) mode. The experimental results indicate that the critical hydrogen concentration for crack extension in the IG mode is higher than that for crack extension in the MVC mode. The fracture mode transition with varying hydrogen pressure and yield strength is discussed by simultaneously considering the micromechanisms for HAC and the hydrogen pressure and yield strength dependencies ofK _th .     

Measurement and analysis of diametral deformation in irradiated Zr-2.5%Nb pressure tube

Internal diameter measurements have been carried out on pressure tube S-07 of KAPS II and fifteen RAPS II pressure tubes. These data have been analyzed to derive empirical correlation for predicting diameter change in pressure tube during irradiation. The correlations are derived for the diametral strain rates in terms of the stress, temperature and fluence for KAPS II and RAPS II pressure tubes which indicated essentially same functional dependences with a slight change in the prefactor (8.55 vs 12.35). This paper presents the details of the study carried out on modeling of diametral creep in the pressure tubes.     

The role of crack tip strain rate in the stress corrosion cracking of high strength steels in water

Creep tests have been performed on fracture mechanics specimens of as-quenched 4340 and 3.5NiCrMoV rotor steel to confirm the importance of crack tip strain rate in causing stress corrosion cracking. By allowing creep in a noncracking environment, dry air for the high strength steels tested, cracking did not occur when water, the corrosive solution, was later added to the system. Thus, it is possible to inhibit stress corrosion in spite of conditions otherwise conducive to crack growth. Conditions necessary to restart cracking were also tested. The importance of this result in terms of the mechanism of stress corrosion and difficulties in measuring K_ISCC is discussed. I. O. Smith, formerly Associate Professoraf with the Department of Mining and Metallurgical Engineering, University of Queensland     

The stress corrosion resistance and fatigue crack growth rate of a high strength martensitic stainless steel,AFC 77

The plane-strain stress corrosion thresholdK _Iscc and fatigue crack growth rate have been determined for a high strength martensitic stainless steel, AFC 77, in both conventionally processed and strain-aged conditions. TheK _Iscc (in 3.5 pct sodium chloride solution) is markedly affected by both the tempering temperature and the degree of strain aging. The highestK _Iscc of 105 ksi \(\sqrt {in} \) . was obtained by tempering at 500°F and the lowestK _Iscc of 10 ksi \(\sqrt {in} \) . by tempering at 1100°F. Retained austenite raisedK _Iscc at tempering temperatures up to 1000°F, which was the highest tempering temperature at which austenite could be maintained. Fatigue crack growth rates in both dry air (<10pct relative humidity) and 3.5 pct sodium chloride solution were at a maximum for material tempered at 700°F. Over the range of stress intensity studied, retained austenite reduced fatigue crack growth rate in salt solution but increased it in dry air.