Contribution to the replacement of cobalt-free hardfacing coating by laser cladding in fast neutron reactors

Abstract

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.The following part of the article is dedicated to the presentation of the process parameter search. 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 phases with varying shape and composition, depending on the process parameter, have been evidenced.Finally, the presentation concludes by summarizing the links between the microstructure of the Colmonoy® 52, the process parameter and a selection of the parameter set for further tribology (pin-on-disk) and corrosion (liquid sodium medium) tests.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...