Study on cobalt free hardfacing materials for wear resistance in sodium fast reactors

In this article, we present the ongoing study concerning the selection of cobalt free hardfacing materials for wear resistance in the French Sodium Fast Neutron Reactor ASTRID, currently under development. In the reactor, some parts can be submitted to sliding contact between each other. 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 neutrons irradiation and, therefore, can contaminate the primary circuit and will be an issue for deconstruction. Therefore, it is desired to replace this cobalt based hardfacing alloy by a cobalt-free one. First, we present the selection of some promising materials and processes selected from the bibliography and previous achieved works. Two processes are used for manufacturing the clads: Plasma Transferred Arc and the Laser Cladding. From the bibliography, different nickel base alloys have been selected. In the presentation, we consider our investigations made on Colmonoy 5. The cobalt base alloy Stellite 6 is evaluated as the reference. The microstructure of the Colmonoy 5 is compared for the two processes. Then, different properties of the clad are evaluated by mechanical, aging, and wear tests. In the project, two tribometers based on planar and linear sliding at high temperature have been set up: one under inert gas protection and one under liquid sodium at operating temperature. After the presentation of the tribometers, wear tests are presented and the wear behavior of the Colmonoy 5 deposits is discussed. Finally, conclusion is given on the quality of the material to be a substitute of cobalt base material.