CONTRIBUTION TO THE REPLACEMENT OF COBALT-FREE HARDFACING COATING BY LASER CLADDING IN FAST NEUTRON REACTORS

摘要

In fast neutron reactors, some parts can be submittedto displacements between each other (as movableparts for example). On these parts, the contact areasusually need a hardfacing coating. The standardhardfacing alloy is a cobalt-base alloy (as, forexample Stellite~®6). Unfortunately, in the primarycoolant circuit and on wear conditions, cobalt can bereleased. Under neutron flux, the ~(59)Co, stable, can betransmuted into ~(60)Co by radioactive capture ofneutrons and, therefore, can contaminate theprimary circuit. Therefore, it is desired to replacethis cobalt based hardfacing alloy by a cobalt-freeone.First we sum up the existing results on cobalt-freehardfacing materials. Several types of materials suchas nickel-base alloys, iron-base alloys and ceramicsown interesting properties. However, it has beenevidenced that there are few and incomplete worksthat attempt to understand the links between thematerial composition, the microstructure of thematerial obtained by the deposition process and itsset of parameters, and the tribo-corrosion behaviorof the coating.Consequently, it has been decided to tackle thisissue by selecting a set of possible cobalt-freehardfacing materials and using laser cladding. Thematerial selected as first candidate is the Colmonoy~®alloy (Colmonoy~® 52 for laser cladding). In theindustrial application, the alloy will be deposited onthe stainless steel.The following part of the article is dedicated to thepresentation of the process parameter search. Thecobalt-free hardfacing alloy is provided in powder.It is expected that laser metal deposition can providea controlled dilution of the substrate, a lowdeformation and a fine microstructure by a finecontrol of the energy deposition. In addition, thelaser metal deposition process offers a very largerange of parameter values.The experimental setup is presented. The first cladsobtained have exhibited porosities, cracks anddeformation. The other campaigns of experimentshave been focused of avoiding/suppressing thesedefects by changing the process parameters. Inparallel, the evolution of the composition of the cladand its microstructure are analyzed. Hard phaseswith varying shape and composition, depending onthe process parameter, have been evidenced.Finally, the presentation concludes by summarizingthe links between the microstructure of theColmonoy~® 52 , the process parameter and aselection of the parameter set for further tribology(pin-on-disk) and corrosion (liquid sodium medium)tests.