The Effects of Composition and Heat Treatment on the Microstructure and Performance of Cobalt-based Alloys for Molten Zinc Applications
Authors:   , Escott Luis 
Published: September 2022

The purpose of this research was to identify and develop cobalt-based superalloys for journal bearing applications by improving the stability and service life of submerged galvanising pot hardware. To achieve this goal the mechanical properties and chemical resistance of four cast cobalt superalloys were investigated. The alloys were subjected to three heat treatments which included a high temperature annealing treatment, a solution treatment and additional age hardening, and a plasma nitriding treatment. The heat-treated alloys were examined and compared to untreated samples to identify any microstructural changes or improvements in material performance. Four commercially available cobalt-based superalloys were identified for analysis including three cast CoCrW alloys and one cast CoCrMo Tribaloy. It was identified that increasing the carbon and tungsten content, with the CoCrW alloys, increased the area fraction of chromium and tungsten carbide phases in addition to increasing the concentration of alloying elements retained in solid solution. Both factors contributed to increased material hardness. The substitution of tungsten with molybdenum and the absence of carbon, with the CoCrMo Tribaloy, replaced the formation of eutectic carbides with a primary Laves phase that dominated the Tribaloys microstructure. This provided the CoCrMo alloy with enhanced hardness properties compared to the CoCrW alloys. The untreated alloys were submerged in a molten zinc alloy bath containing 0.35wt.% Al in a series of static immersion tests where the alloys were submerged for times ranging between 1-4 weeks. All the materials reacted with the molten zinc bath resulting in the diffusion of bath species into the alloys microstructure as well as the leaching of alloying elements into the bath. With each alloy, molten metal ingress preferentially occurred in the cobalt rich solid solution phase whereas the carbide and Laves phases were more resistant to bath reactivity. The high area fraction of Laves phase and increased molybdenum concentrations retained in eutectic solid solution with the T-800 Tribaloy, provided the alloy with enhanced chemical resistance to the bath in comparison to the CoCrW alloys. The improved chemical resistance reduced the depth of diffusion and reduced the quantity of cobalt-aluminide formation at the alloys surface. A plasma nitriding treatment resulted in the formation of a nitrogen rich diffusion layer at the surface of each of the alloys. Increasing the treatment temperature or treatment time produced a thicker diffusion layer with increased concentrations of nitrogen. The formation a nitride layer significantly improved the surface hardness of each of the alloys where increased material hardness correlated with a thicker diffusion layer. The plasma treated samples were dip tested in a molten zinc bath containing 0.35wt.% Al. The formation of the nitride diffusion layer improved the long-term chemical performance of the alloys where reduced zinc and aluminium diffusion depths were recorded with the CoCrW alloys after 4-weeks of testing. The nitride treatment generally improved the surface integrity of the alloys where increased levels of alloying elements were retained within the matrix diffusion layer after prolonged bath exposure and each alloy experienced less material loss. There was also a noticeable reduction in the quantity of cobalt-aluminide phases at the alloy’s surfaces. An annealing heat treatment slightly changed the morphology of the alloy’s microstructure where phase precipitation within the solid solution phase occurred in addition to low levels of carbide dissolution. Small improvements in material hardness were recorded with the CoCrW alloys whereas the annealing treatment had detrimental effects on the hardness properties of the CoCrMo alloy. Using the nano-properties of the matrix to calculate the mean H/E ratio it was predicted that the wear performance of the matrix of the WT-4 and WT-12 alloys were slightly improved by the annealing process. A solution treatment significantly changed the microstructure morphology of the WT-6 and WT-12 alloys. The solution treatment resulted in extreme coarsening of the chromium and tungsten carbides with the WT-12 alloy and coarsening of the chromium carbides and the precipitation of tungsten rich phases with the WT-6 alloy. Low levels of carbide dissolution were also recorded. A drop in material hardness was recorded with both alloys after the solution treatment. Additional aging treatments did not significantly alter the alloys microstructures further but precipitation within the matrix occurred. This coincided with significant improvements in the hardness properties of both alloys. Calculating the mean H/E ratio of the matrix phase in both alloys estimated that no improvements in matrix wear resistance occurred with WT-6 alloy whereas an improvement with the WT-12 matrix was calculated.