Low temperature plasma nitriding of a Co30Cr19Fe alloy for improving cavitation erosion resistance

Cobalt alloys are used when improved cavitation-erosion (CE) resistance is needed. Low temperature plasma nitriding - (LTPN) is known to greatly enhance the CE resistance of austenitic and duplex stainless steels, due to formation of a very hard, super-saturated fcc - phase, known as expanded austenite or S-phase. In this work, Low Temperature Plasma Nitriding of a non-standard Co-Cr alloy was carried out to explore the formation of an expanded S-phase hard layer and to assess its effect on the CE resistance of the Co-Cr alloy. The Co-Cr samples, containing α-fcc and ε-hcp solid solutions phases, were plasma nitrided at 350 °C and 400 °C for 20 h. The CE tests were carried out in a vibratory cavitation equipment according to ASTM G32-92. Microstructural and micromechanical characterization of the specimens indicated the formation of an expanded S-phase fcc layer, containing small amounts of CrN. Plasma nitriding at 400 °C and greater amounts of α-fcc volume fraction in the matrix led to thicker and harder (10.5 GPa) S-phase layers. The 400 °C nitrided samples exhibited higher CE resistances than the non-nitrided samples, with up to 267% greater incubation times and 5 times reduced wear rate. All Co30Cr19Fe samples showed higher CE resistances than AISI 304 and only the solution-treated sample showed lower CE resistance than Stellite 6. The results are discussed in terms of the mechanisms of material removal, during the initial stages of CE, which are controlled by plastic deformation, with formation of slip steps, grain boundaries protrusion and material removal from these protruded areas. Twin boundaries are preferably eroded. The increase in nitrogen content increases the elastic energy returned to the environment and decreases the amount of plastic energy absorbed by the alloy, at cavitation impact spots.