Laser cladding and tribocorrosion testing of cobalt-free hardfacing coatings for fast neutron reactors

In fast neutron reactors, some parts can be submitted 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 (as, for example Stellite®6). Unfortunately, in the primary coolant circuit and on wear conditions, cobalt can be released. Under neutron flux, the 59Co, stable, can be transmuted into 60Co 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.First we sum up the existing results on cobalt-free hardfacing materials. Several types of materials such as nickel-base alloys, iron-base alloys and ceramics own interesting properties. However, it has been evidenced that there are few and incomplete works that attempt to understand the links between the material composition, the microstructure of the material obtained by the deposition process and its set of parameters, and the tribo-corrosion behavior of the coating.Consequently, it has been decided to tackle this issue by selecting a set of possible cobalt-free hardfacing materials and using laser cladding. The material selected as first candidate is the Colmonoy® alloy (Colmonoy® 52 for laser cladding). In the industrial application, the alloy will be deposited on the stainless steel 316LN.The cobalt-free hardfacing alloy is provided in powder. It is expected that laser metal deposition can provide a controlled dilution of the substrate, a low deformation and a fine microstructure by a fine control of the energy deposition. In addition, the laser metal deposition process offers a very large range of parameter values.The experimental setup is presented. The first clads obtained have exhibited porosities, cracks and deformation. The other campaigns of experiments have been focused of avoiding/suppressing these defects by changing the process parameters. In parallel, the evolution of the composition of the clad and its microstructure are analyzed. Hard eutectic phases with varying shape and composition, depending on the process parameter, have been evidenced and discussed.Laser cladding process performances are compared to nominal process (PTAW). Sample for corrosion under sodium and tribology are manufactured. Wear tests are made on pin-ion-disk device. 316L spheres are used on hardfacing alloys samples. Argon is used for protection of the sample against oxidation. Tests are made at room temperature and at 300°C. Results on cobalt based and cobalt free hardfacing alloys are presented. These two materials exhibit very different wear mechanism. Cobalt based materials are more resistant against wear when increasing the load or the temperature.In fast neutron reactors, some parts can be submitted 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 (as, for example Stellite®6). Unfortunately, in the primary coolant circuit and on wear conditions, cobalt can be released. Under neutron flux, the 59Co, stable, can be transmuted into 60Co 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.First we sum up the existing results on cobalt-free hardfacing materials. Several types of materials such as nickel-base alloys, iron-base alloys and ceramics own interesting properties. However, it has been evidenced that there are few and incomplete works that attempt to understand the links between the material composition, the microstructure of the material obtained by the deposition process and its.