EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Stellite are Co-based alloys used as hardfacing materials of key moving-mechanisms in the primary circuit of pressurized water reactors. They exhibit a Co-Cr-rich matrix and W and Cr carbides that confer exceptional tribological properties under sliding wear conditions that no Ni- or Fe-based alloys have been able to replicate. Counterpart to Stellite efficiency is that 59Co fragments are generated under friction then activate under neutron irradiation in radioactive 60Co that increase workers exposure to radiative fluxes. Here, an alloy-design strategy based on a combination of Calphad calculations and hardness predictions and on experimental characterization was employed to find Complex Concentrated Alloys (CCA) to replace Stellite. Thermodynamic equilibrium calculations using Thermo-Calc and the TCHEA3 database were performed on the Cr-Fe-Mn-Ni alloy system to determine its single-phase solid-solutions at given temperatures. Then, five solid-solution strengthening models from the literature were represented in the “Cr20-Fe-Mn-Ni at 1100°C” pseudo-ternary system to identify elemental strengthening tendencies. Compositions were selected based on their predicted hardness, produced by arc-melting, cold-rolled and heat-treated to produce recrystallized microstructures. Grain growth, tensile and Vickers hardness tests were performed to determine the intrinsic yield strength by Hall-Petch equation. Microstructural characterizations such as X-ray diffraction, BSE and chemical analysis of main and minor elements were performed (resp. ICP and GDMS). The models accuracy was evaluated to assess a more precise and reliable solid-solution strengthening forecast in the single-phased region of the Cr-Fe-Mn-Ni alloy system. Most interesting compositions were selected as “matrix” for the second step of the alloy-design strategy where carbides precipitation was studied. Ternary diagrams were drawn out of solidification microstructures following Scheil calculations, which edges are the “matrix”, the substitutional (ex: W) and the interstitial (ex: C) elements. Therefore, the nature and fraction of carbides are estimated for further alloy synthesis, which should allow determining the prediction accuracy.