Austenitic Cladding Research Articles

Numerical Investigations on the Residual Stress Field in a Cladded Plate Due to the Cladding Process

The inner surface of reactor pressure vessels is protected against corrosion by an austenitic cladding. Generally, the cladding is welded on the ferritic base metal with two layers to avoid cracks under the cladding and to improve the microstructure of the cladding material. On the other hand, due to the cladding process and the difference of the thermal expansion coefficient of the austenitic cladding and the ferritic base material, residual stresses are acting over the wall thickness. This residual stress field is important for the integrity assessment of the component. For the determination of the residual stress field, plates of a reactor pressure vessel steel were cladded by submerged arc welding and heat treated, representative of the real component. The numerical simulation was performed with the finite element code SYSWELD. The heat source of the model representing the welding process was calibrated against the temperature profiles measured during welding. In the analysis, the temperature-dependent material properties, as well as the transformation behaviour of the ferritic base metal, were taken into account. The calculated residual stresses show tensile stresses in the cladding followed by compressive stresses in the base metal that are in agreement with measurements with X-ray diffraction technique. After post-weld heat treatment the residual stresses in the ferritic base metal are reduced significantly.

Development of eco-friendly surface modification process for 316L austenitic stainless steel weld cladding

Austenitic stainless steels are used for industrial applications due to their improved corrosion resistance properties. Their poor tribological behaviour is a barrier for their wider application under corrosion–wear conditions taking place between rolling and sliding contacts. The present work has been conducted to develop a low temperature liquid nitriding process for austenitic stainless steel claddings to impart better resistance to corrosion–wear. This work presents the results of the corrosion and wear tests performed on liquid nitrided AISI 316L stainless steel claddings. The wear tests were conducted with a Ducom pin on disc machine under rolling–sliding conditions. Surface and subsurface were studied by an optical microscope. The hardness and depth of the modified layer were measured in the sectioned samples using a microhardness tester. The optical metallographic microstructures of the liquid nitrided specimens were revealed using Marbles reagent. The electrochemical behaviour of the liquid nitrided specimens was investigated with a dc anodic potentiodynamic polarisation technique.

Evaluation of Creep Damage and Diametral Strain of Fast Reactor MOX Fuel Pins Irradiated to High Burnups

In determining lifetime criteria of fast reactor fuel pins, creep damage due to fission gas pressure was evaluated on mixed-oxide fuel pins with austenitic stainless steel cladding irradiated to high burnups. The degree of creep damage of these fuel pins was expressed as cumulative damage fractions (CDFs), which were defined so that cladding breaching occurs when the CDF exceeds 1.0. The obtained CDFs for typical high-temperature fuel pins were on the order of 10−4–10−2 at the end of irradiation, indicating that these fuel pins had large safety margins against breaching due to creep damage. In order to investigate the factors that govern the lifetime of fuel pins, pin diametral increase and CDF were predicted under extended burnup conditions, and then were compared with their limit values. The predicted pin diametral increase reached its limit value earlier than did the CDF because of a significant increase in the cladding void swelling, suggesting that the lifetimes of fuel pins with austenitic stainless steel cladding are practically governed by the diametral increase rather than by the creep damage.

Behaviour of sub-clad and through-clad cracks under consideration of the residual stress field

Aim of the present study is an assessment of the behaviour of cracks in the ferritic base metal of components supplied with an austenitic welded cladding under special consideration of the residual stress field caused by the welding and heat treatment processes. For this purpose, an experimental and numerical investigation has been performed. The experimental study consisted of two component tests at low temperature using large-scale specimens with sub-clad and surface cracks. Subsequently, the experiments were analyzed numerically where the residual stress field was determined in an advanced numerical simulation of the welding and heat treatment processes. Based on the results, a fracture mechanics assessment of the ferritic and austenitic material zones on crack initiation and arrest was performed. The experimental and numerical results reveal that fracture was initiated in the ferritic range whereas the austenitic cladding remained intact even in the case of a limited crack extension in the base metal underneath the cladding.

Modelling and analysis of delta ferrite content in claddings deposited by flux cored arc welding using a neural network

Measurement of delta ferrite in cladding gives important insight into the future mechanical and corrosion resistant behaviour of the cladded structures. The amount of delta ferrite formed during cladding is influenced by process parameters such as welding speed, welding current, and nozzle-to-plate distance. Therefore, it is essential to predict the effect of these parameters on the formation of delta ferrite. This article discusses the development of an artificial neural network model to predict the delta ferrite content in austenitic stainless-steel claddings deposited by the flux cored arc welding process. A novel approach of using the design of experiments to collect data to train the network has been adopted in this investigation. The study revealed that the delta ferrite content can be predicted more accurately using the neural networks with a minimum number of experiments. The results also indicated that welding current and speed have a significant influence on the amount of ferrite and the interaction effects of these parameters play a major role in determining ferrite in the claddings.

Investigation of Fatigue Crack Growth on Material for Reactor Pressure Vessel by Acoustic Emission

Contribution contains an application of acoustic emission method during instability and growth of fatigue crack in reactor pressure vessel specimen loaded by means of bending. Material of specimen was typical material of pressure water reactor vessel covered by austenitic cladding as a protection layer against corrosion. Specimen had shape of prism with defect in the middle where fatigue crack was created very close to austenitic cladding. Loading of specimen was done in special loading machine with defined speed of loading resp. increased force. Investigation of instability and propagation of inertial crack was observed by acoustic emission technique, potential drop method, by using of straingauges and by high-speed camera. Acoustic emission sensors were located from both sides of specimen. By this experimental technique was possible to observe "pop-in" of crack during loading. Different types of specimen with different mode of aging were investigated. Results of acoustic emission activity are described graphical curves in relation to individual stages of loading.

Production and Properties of Nano-scale Oxide Dispersion Strengthened (ODS) 9Cr Martensitic Steel Claddings

The 9Cr-ODS martensitic steel claddings were developed by cold-rolling and subsequent heat-treatment. The standard chemical composition is Fe-0.13C-9Cr-2W-0.2Ti-0.35Y 2 O 3 . The substantially elongated grains formed by cold-rolling turned out to be into equi-axied grains by ferrite to austenite phase transformation at the final heat-treatment. The produced claddings have the tempered martensitic structure and excess oxygen of 0.060 mass%. The superior tensile and creep rupture strength were shown in the produced cladding, compared with conventional ferritic (PNC-FMS) and even austenitic (PNC316) claddings at higher temperature and extended time. The strength improvement is attributed to finely distributed nano-scale complex oxide. The coarser ferrite grains produced by slow cooling make further improvement in the tensile and creep rupture strength beyond those of tempered martensite at high temperature and longer testing time. The higher excess oxygen content of 0.137 mass% prevents fine distribution of the oxide particles that lead to inferior high temperature tensile and creep strength.

Tensile yield load solutions for centre cracked bilayer (clad) plates with and without repair welds

Bilayer metallic structural components in the form of clad plates are often used owing to their superior environmental and mechanical properties. During the service life of these components, cracks or cracklike defects may be developed in the clad layer and may grow into the substrate. To repair the damaged regions, local repair welding techniques are often used. The presence of this repair welded part may affect the deformation behaviour, and hence yield load level, of the structure. Therefore, in the present paper, new analytical tensile yield load solutions for centre cracked clad steel plates (austenitic cladding on a ferritic substrate) with a repair weld have been developed and these provide good agreement with experimental results. Additionally, a previously reported tensile yield load expression for a centre cracked clad plate (without a repair weld) has also been verified using the experimental results.

Properties of Plutonium Dioxide as Nuclear Fuel

Generalized structural, thermodynamic, thermophysical, and strength characteristics as well as data on the dimensional stability, compatibility, and fragment gas release under irradiation of plutonium dioxide, used as a compact nuclear fuel for fuel elements in fast research reactors, are presented. Reliable operation of reactors with fuel elements based on plutonium dioxide confirmed that the properties of the fuel ceramic of fuel elements with austenitic stainless steel cladding have been determined with a high degree of reliability. 7 figures, 6 tables, 23 references.

Load-carrying capacity and crack resistance of a cladding by the Sigma-oscillating wire technique

If cracks are postulated in the ferritic base material beneath the austenitic cladding, their initiation and propagation under hypothetical loading cases is influenced by the load carrying capacity of the cladding. The toughness of the KKS-RPV cladding was assessed by means of elastic-plastic fracture mechanics methods. Sub-sized tensile and bend specimens were fabricated by reconstitution technique from broken halves of standard ISO-V Charpy specimens, representing crack extension in radial and circumferential direction. They were tested and evaluated and further analyzed with the Gurson model. For temperatures relevant to the loss of coolant and upset conditions analyzed, a sufficient toughness of the cladding in terms of J and CTOD resistance curves could be shown.

Damage mechanics analysis (Gurson model) and experimental verification of the behaviour of a crack in a weld-cladded component

During emergency core cooling situations, the criticality of hypothetical cracks located in the ferritic base material beneath the austenitic cladding is strongly influenced by the integrity of the cladding: if the cladding is intact, the crack tip loading is significantly reduced compared with the case where the cladding is assumed to be broken. In order to assess this situation, a three-point-bend specimen consisting of a ferritic block with an austenitic cladding and a semi-elliptical crack in the ferritic base material beneath the cladding was tested and evaluated. The global (i.e., load versus displacement curve) and local (i.e., ductile crack extension in ferrite and austenite) behaviour of this specimen was predicted by means of two- and three-dimensional finite-element analyses based on the Gurson model. The material characterization was done in terms of Gurson material parameters utilizing sub-sized tension tests with improved evaluation and fracture mechanics tests with precracked Charpy-type specimens. A good agreement between analyses and experiment could be achieved.

Development and Application of Micromechanical Material Models for the Characterization of Materials

The concept of the local approach is applied to investigate different damage mechanisms of structural steels by micromechanical material models. Static and dynamic fracture resistance curves for a neutron-embrittled weld material are derived from tensile and Charpy-type specimens with the modified Gurson model. The behaviour of a test piece with a crack in the ferritic base material beneath an austenitic cladding could well be predicted. A constitutive model for creep and creep rupture was developed, implemented in a finite element program and applied to tests on a 12 % Cr steel. The model is a combination of the viscoplastic Robinson model and a damage mechanics model. It is formulated with internal variables for isotropic and kinematic hardening and with a scalar damage parameter. The application of the model to modified C(T) specimens with different hole diameters shows a good agreement between the predictions and the experiments.

Estimating the tensile properties of an austenitic cladding tube steel following corrosion by uranium dioxide, simultated fission products and neutron exposure

As there seems to be a general lack of systematic data about mechanical properties of fuel rod cladding tubes following in-pile operations, the tensile properties (tensile strength, 0.2% offset yield strength, ultimate elongation and uniform elongation) were measured in specimens which had either only been corroded or irradiated, been both corroded and irradiated or were in the initial state. The tensile properties Z of the specimens which had been both corroded and irradiated were calculated from the tensile properties of the specimens which had only been corroded, those which had only been irradiated and those kept in their initial state, in accordance with this formula: Z( corroded + irradiated) = Z( corroded) · Z( irradiated) Z( initial state) The flat specimens (0.7 × 4 mm) were made of 1.4970 type austenitic cladding tube steel. As with breeder fuel rods, the corrosive medium used was a mix of hyperstoichiometric uranium dioxide and simulated fission products (Cs, I, Te). The strain rate, test temperature, depth of corrosion, simulated burnup, and state of the material parameters were varied systematically. The results of calculations were, on average, in good agreement with the measured results, although some measured values showed considerable variation due to corrosion.

Mechanisms of postweld intergranular underclad cracking

Forged components of ferritic steel are protected by a welded austenitic clad. intergranular cracking can take piace in the ferritic phase close to the ferritic austenitic interface. After developing a fabrication technique for these cracks, the formation conditions are studied. Auger electron spectroscopy investigations of specimens containing a real crack opened inside the vacuum chamber are used for interpretation. Sulphur segregations embrittle the grain boundaries which are separated by different dilatations of the clad and the base metal during the postweld heat treatment.

Combination of radiation and hydrogen damage of reactor pressure vessel materials

The hydrogen embrittlement of low-alloyed base steel, austenitic cladding and heat affected zone (HAZ) of a reactor pressure vessel was measured for both unirradiated and irradiated materials. The fracture toughness decreased with both hydrogen charging and neutron irradiation; the shift of the fracture toughness-temperature transient curve is influenced by both damage processes. The plastic zone in hydrogen-charged material becomes smaller. The total elongation of both CrMoV and CrNiMoV HAZ decreases with increasing hydrogen content. This influence is pronounced in the HAZ after a weld process without subsequent annealing, a total loss of plasticity being observed in this case. The properties of the austenitic layer are not influenced at comparable hydrogen contents.

Ultrasonic sizing of defects in the austenitic cladding of reactor pressure vessels

This paper reports the measurement procedure used for inspection of stainless steel cladding and sizing of flaws with inclined longitudinal transmitter-receiver probes. The special characteristics of these probes are discussed and a practical solution for calibration is presented. The results achieved in reactor pressure vessels inspections verify the applicability and accuracy of the technique described in this paper.

Effect of cladding residual stresses on [formula omitted] in a pressure vessel

An austenitic cladding layer is not normally taken into consideration in fracture mechanics calculations for reactor pressure vessels. Cladding residual stresses may, however, create additional straining of cracks. This paper describes some results of finite element calculations, which have been performed in order to establish the influence of cladding on the brittle fracture of a reactor pressure vessel.

A general theory of scattering by thin interface layers, plates, and shells.

In recent years there have been a number of attempts to model the effect of thin interface layers on the scattering and transmission of sound by bodies submerged in a continuous medium. Interface layers or coatings occur in numerous engineering acoustic applications. For example, when two solid bodies are bonded together, the adhesive or the imperfect fusion region will generally introduce an acoustic mismatch with both partners. Similarly, the use of austenitic cladding as a protection against corrosion creates an analogous problem, except that here the layer should be thought of as being fused to the surface of the material. In underwater and aeroacoustics the fluid–solid interactions with a shell or plate are often modeled using the Euler–Bernoulli thin plate theory. As far as the scattering problem here is concerned, the thin plate equation appears as an ‘‘effective boundary condition’’ relating the transverse displacement to the fluid pressure. Similarly, a thin interface layer between two solids is also modeled by some effective boundary condition. A typical approach is to assume that the layer is characterized by interfacial mass, spring, and damping constants. The latter are estimated by comparing the predictions of the model with the exact solutions to certain statical canonical problems. The difficulty with this approach is that, generally, it is not self-consistent. In other words if the spring constant is calculated so that the predictions of the model agree well with certain exact solutions at ‘‘normal incidence,’’ say, then it is usually not the case that they give agreement under different loading conditions. Similarly, in the fluid–solid interaction problem, the Euler–Bernoulli theory gives a correct approximation to the dispersion relation for leaky antisymmetric Rayleigh–Lamb waves in a fluid-loaded thin plate but incorrectly predicts the transmission and reflection of a plane wave by such a plate in the fluid! In this paper these problems are addressed by developing a rational approximation scheme for an arbitrary thin layer immersed in a contrasting matrix. a)The author is currently on leave from The Department of Mathematics, University of Manchester, Manchester M13 9PL, U.K.

Carbon redistribution between an austenitic cladding and a ferritic steel for pressure vessels of a nuclear reactor

In this work, carbon redistribution between an austenitic cladding and a ferritic-carbidic steel for pressure vessels of the Voronez water energy nuclear reactor is studied. The carbon concentration in the surroundings of the weld interface was determined by electron beam microanalysis. The method of line segment analysis, with the help of FeC standards, used in this work enabled the correct determination of the total carbon content and its dependence on the distance from the weld interface. The diffusion study was carried out in the temperature interval from 500 to 750°C, on experimental weldments prepared by resistance welding and on a submerged-arc strip weldment used in technical practice. The carbon diffusion in the weldments was described by a diffusion model assuming two separate carbon diffusion coefficients D − and D + of the left- and right-hand part of the weldement respectively. While D − = 2.7 × 10 −5 exp(−164/ RT)m 2 s −1 agrees well with the carbon diffusion coefficientin austenite, D + = 1.2 exp(−209/ RT) m 2 s −1 corresponds to the effective carbon diffusion coefficient in the ferrite-carbide mixture. It has been shown that the results obtained on the experimental weldments can be applied to the technical weldments and enable the prediction of carbon redistribution during post-weld heat treatment and application.

The enhancement of signal to noise ratio in an ultrasonic inspection of UOE austenitic clad pipe welds

The enhancement of signal to noise ratio in an ultrasonic inspection of UOE austenitic clad pipe welds