Laser cladding and wear testing of nickel base hardfacing materials: Influence of process parameters

In fast neutron reactors, some parts can be subjected to displacements between each other (as movable parts for example). On these parts, the contact areas usually need a hardfacing coating. The standard hardfacing alloy is a cobalt-base alloy (for example Stellite®6). Unfortunately, in the primary coolant circuit and on wear conditions, cobalt can be released. Under neutron flux, the stable59Co can be transmuted into60Co by radioactive capture of neutrons and, therefore, can contaminate the primary circuit. Therefore, it is desired to replace this cobalt based hardfacing alloy by a cobalt-free one. Previous presentations have shown the potential interest of some nickel base materials as Colmonoy® alloy. In parallel, laser cladding has been identified as a deposition process that could increase the performances of the hardfacing materials compared to the standard process (Plasma Transferred Arc Welding). In all the study, the base material is the stainless steel 316LN. In the first section of this article, the authors present previous results related to the selection of hardfacing materials and their evaluation in comparable tribology conditions. Then, Tribaloy® 700, another nickel based alloy that has been poorly investigated, is presented and evaluated. This nickel base has a completely different microstructure, and its tribological behavior related to the variation of the microstructure is not well known. First, the authors present the features of the selected materials. Then, the authors present various property characterization results obtained by changing several process parameters. The quality of the clad is considered, and the process window providing a good clad is determined (no crack, only a few porosities, etc.). The variation of the microstructure is analyzed, and solidification paths are proposed regarding the process parameters. Wear tests are performed on typical wear conditions. The movement is linear. Argon is used for the protection of the sample against oxidation. Tests are carried out at 200 °C. Wear tests are analyzed, and wear mechanisms are correlated with the microstructure of the material

Nanosecond laser surface treatment of steels. Different applications in the fields of nuclear industry

International audienceAmong the numerous laser applications, laser surface melting by using a pulsed-laser is an innovative technology in the field of surface treatments. This technique presents many advantages. It only modifies the surface properties by keeping the mechanical properties of the bulk. It requires neither addition of other compounds nor contact, so it is quite economical and it does not pollute the material. It allows the treatment of complex shapes into closed spaces with difficult access. The laser can work in autonomy that present an interest in the fields of nuclear decontamination.This treatment consists in focusing a nanopulsed laser beam on the surface of the material, leading to the rather immediate melting of the surface through a micron depth, immediately followed by an ultra-fast solidification occurring with cooling rate up to 1010 K/s.By using different techniques of analysis, we showed that the combination of these processes leads to various modifications of surface properties. By combining with dexterity the different laser parameters, it is possible to functionalize the surface or to improve the native properties.Glow discharge optical emission spectrometry (GDOES), XPS and TEM were used to establish the segregation of chemical elements and the growth of a new oxide layer with new properties.XRD with grazing incidence was employed to identify the change of crystallographic structure, SEM was performed to promote the diminution of the surface defects and the microstructure.Applications in fields of nuclear will be presented, especially in terms of pitting corrosion of stainless steel used in secondary circuit, and protection against the nickel release of heat exchanger tubes in the primary circuit coolant

Growth of micrometric oxide layers for the study of metallic surfaces decontamination by laser

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Influence of the processing parameters on the final properties of powder-bed laser additively oxide dispersion strengthened (ODS) Fe-14Cr steel parts

International audienceOxide Dispersion Strengthened (ODS) ferritic steels typically contain a fine dispersion of nanosized Y-Ti-O precipitates, leading to an improvement of creep properties and neutron swelling resistance. These alloys are usually manufactured by different successive steps: mechanical alloying, outgassing, hot extrusion and cold working. Mechanical alloying aims at the dissolution of Y and Ti atoms into the ferritic matrix. This process leads to the precipitation and growth of fine Y-Ti-O oxide dispersoids during the heat treatments and the consolidation by hot isostatic pressing and/or by hot extrusion. Considering the limitations regarding the final shapes complexity of components obtained by this traditional fabrication route, the evaluation and development of alternative production methods are currently studied in order to increase the widespread use of ODS alloys. In the frame of assessing the potentialities of additive manufacturing to manufacture ODS complex parts, a Fe14Cr1W + 0.3percent Y2O3 + 0.3percent TiH2 milled powder is consolidated by Selective Laser Melting (SLM). The influence of processing parameters (scanning speed, scanning strategy, laser power, etc…) on the final microstructures as well as the final densities are studied. For this purpose, several microstructural techniques (scanning electron microscopy, electron backscattered diffraction & transmission electron microscopy) are coupled in order to analyze the cross-sections. First results are quite promising since density of more than 98percent could be achieved with a non-optimized powder. As expected, processing parameters strongly influence the microstructural evolution, especially the grains size and the precipitates’ density. The influence of powder properties, such as particles size distribution and flowability, on the final properties are also studied and presented in details. The objective of this work is to demonstrate how process parameters tailor the microstructure of such alloys and so final mechanical properties

Influence of the powder characteristics on the final properties of powder-bed laser additively oxide dispersion strengthened (ODS) Fe-14Cr steel parts

International audienceAdditive manufacturing processes are promising technologies, currently considered as new opportunities to optimize metallic components production routes, especially in aerospace, automotive, medical and energy industries. The development of the application fields of these technologies involves an increase in the number of possible printed materials. In order to become a robust and reliable way of production of metallic functional components, mastering these technologies must still address challenges. In this framework, the study of the defects impact on final properties of designed components is essential.To assess the potentialities of additive manufacturing in nuclear industry, ODS Fe-14Cr steels are produced by selective laser melting (SLM). ODS steels are studied due to their improved resistance under neutron irradiation thanks to a fine dispersion of nanosized Y-Ti-O precipitates. Such materials are produced by a first step mechanical alloying. The resulting powder is characterized by a non-spherical shape and are coarser than powders typically used in SLM equipment. The analyzes such as composition, density, particles size distribution, flowability and morphology are performed on this powder. The milled powder is then used to produce ODS steel parts as raw material or after some modifications such as sieving or annealing.The objective of this work is to study the impact of the powder characteristics on the final material properties. As expected, powder characteristics strongly influence the final density of solidified parts. The choice of the thickness layer is also an important parameter that has to be related with the particle size distribution of the powder. The optimization of processing parameters (scanning speed, scanning strategy, laser power, etc…) and powder characteristics lead to a significant improvement of final material properties. In this context, last results regarding ODS steels additive manufacturing study will be presented and new insights for industrial fabrication will be given

Application of the laser pyrolysis to the synthesis of SiC, TiC and ZrC pre-ceramics nanopowders

International audienceRefractory carbide nanostructured ceramics appear to be promising materials for high temperature applications requiring hard materials such as nuclear energy industry. Such carbide materials are usually obtained with micrometric sizes from the high temperature carboreduction of an oxide phase in a raw mixture of C black and titania or zirconia. TiC and ZrC nanopowders were produced from an intimate mixture of oxide nanograins with free C synthesized by laser pyrolysis from the decomposition of a liquid precursor. The temperature and the duration of the thermal treatment leading to the carburization were decreased, allowing the preservation of the nanoscaled size of the starting grains. A solution of titanium isopropoxide was laser-pyrolysed with ethylene as sensitizer in order to synthesize Ti/C/O powders. These powders were composed of crystalline TiO2 nanograins mixed with C. Annealing under argon enabled the formation of TiC through the carburization of TiO2 by free C. The final TiC mean grain size was about 80 nm. Zr/O/C powders were prepared from a solution of zirconium butoxide and were composed of ZrO2 crystalline nanograins and free C. The same thermal treatment as for TiC, but at higher temperature, showed the formation of crystalline ZrC with a final mean grain size of about 40 nm. These two liquid routes of nanoparticles synthesis are also compared to the very efficient gaseous route of SiC nanopowders synthesis from a mixture of silane and acetylene

Laser additive manufacturing applied to nuclear components repair and Co-based materials replacement in friction areas

International audienceLaser additive manufacturing is a recent technology that turns a CAD modelled object into a physical one, layer by layer, by addition of projected melted metallic powders. The process is compatible with a vast array of metals and complex geometries and provides a good metallurgical quality. The potential of this process for nuclear industry materials has been assessed on two applications, nuclear components repair and friction resistant coatings using Ni-based alloys instead of Co-based alloys. For generation II&III reactors, repairing presents an advantage over part replacement in terms of cost and delay. Indeed, a number of nuclear components with complex geometries are unique and the difficulty to reproduce them in a timely manner with standard shaping methods results in a more global unavailability. We report about repair of defects on Stellite®6 parts using laser cladding technology.In fast neutron reactors, parts subjected to wear conditions are usually made of cobalt-based alloys. Cobalt may however be released and activated into 60Co, thus contaminating the primary circuit. Hence the motivation to use cobalt-free alloys. Also, the laser cladding process can increase the performances of nickel-based hardfacing materials compared to the standard PTAW coating process.In both cases we have observed occurrences of crack formation. A correlation between process parameters, structural properties of the projected materials and crack formation was found. We describe how the process parameters control the inherent extensive thermal cycling and consequently the properties of the final material, and we propose a methodology to avoid crack formation. The great flexibility of the laser projection process opens the door to deposition with composition gradients that allows avoiding cracks by accommodating the inducing strain inside the material

Report of High Temperature Measurements with a Fabry-Perot Extensometer

Fabry-Perot (FP) sensors like other Fiber Optic (FO) sensors may be of particular interest for in pile experiments in MTR with little room available thanks to their compact size. Light weight also reduces gamma heating hence limiting the thermal effect. Different physical parameters such as temperature, strain, displacement, vibration, pressure, or refractive index may be sensed through the measurement of the optical path length difference in the cavity. We have developed a Fabry-Perot extensometer able to operate at high temperature (up to 400°C), under a high level of radiation (neutron and gamma flux). The measurement based on interferometry is largely insensitive to radiation induced attenuation (RIA) thanks to the wavelength encoding of the useful signal, but for such high fluence as encountered in a reactor core, a special rad-hard fiber is needed. Operating in the wavelength domain around 1ím remains preferable to minimize the impact of irradiation. Moreover, fast neutron radiation is expected to change the structure of the fiber and possibly others materials in the transducer. Then, we revised the basic scheme of Extrinsic Fabry-Perot Interferometer (EFPI) so that the effects of compaction remain limited. Tests under mixed neutron and gamma irradiation permitted to verify the general behavior and particularly the low drift with radiation induced compaction (RIC). Also, two types of tests have been conducted to verify the accuracy at high temperature. The first ones are “measurements” of thermal dilatation of materials: the sensor is fixed on a sample and knowing its thermal expansion, it is possible to predict the measurement expected from the optical sensor when the temperature is increased from low to high temperature. The comparison between the predicted and experimental outputs informs on how the sensor is accurate. The second types are tests on a tensile test bench operating at high temperature. The Fabry-Perot measurements are compared, in the elastic domain, with the expected strain given by the Young modulus of the material, and also on a larger strain domain, with the measurements of a high temperature axial extensometer. Both types of tests are presented and commented