Effect of Fe addition on microstructures and mechanical properties of Ni- and C-free Co-Cr-Mo alloys

A series of Co-Cr-Mo alloys with Fe contents ranging from 5 to 20 mass% were prepared in order to study the effect of Fe addition on the microstructures and mechanical properties (tensile strength, yield strength, elongation, and Rockwell C scale hardness) of Ni- and C-free Co-Cr-Mo alloys. Fe was added with the composition of Co-29Cr-6Mo-xFe (where x = 0, 5, 10, and 20). Alloys were heat-treated at 1230°C for 3 h and 1250°C for 5h. These heat treatment conditions were decided as considering pre-heat treatment followed by hot-forging process. An optical microscope and an XRD were used to determine the microstructure and identify the crystal structure. The volume ratio of γ phase to ε phase increases not only with heat treatment temperature and time but also with Fe content. The tensile strength, yield strength, and elongation are significantly improved by Fe addition. However, the tendency toward σ phase formation becomes higher as the Fe content increases, causing a deterioration of mechanical properties. From the viewpoint of mechanical properties, the amount of Fe addition should be less than 10 mass% in pre-heated condition of Co-29Cr-6Mo alloys.