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 stable 59Co 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. 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.