The ASTM F-75 cobalt base alloy is widely used as a surgical implant material, due to its high corrosion and wear resistance and biocompatibility. The alloy is usually produced by investment casting method. In this work, effects of mold preheating temperatures, type of mold materials and pouring temperature on the as-cast microstructure and wear behavior of the alloy were studied. The investment cast specimens were solutionized for 1 and 4 h at 1220 °C followed by isothermally aging for 7 and 10 h at 850 °C. Samples were characterized by optical microscopy, scanning electron microscopy, X-ray diffraction, differential thermal analysis, macro hardness and wear test. Effects of the casting parameters on as-cast microstructure and wear behavior of the alloy was also simulated by using ProCAST software. The size of secondary dendrite arm spacing in as-cast microstructure was measured and compared to the simulated results. The SDAS, grain size, volume fraction of carbides, ratio of the volume fraction of blocky carbides to that of eutectic carbides in the microstructure were slightly decreased when the type of mold was slightly changed, although the wear behavior and hardness of the alloy were not changed in this condition. The predicted results were in good agreement with the measured ones. In fact, the cooling capability of molds A and B was in the same order. According to the result of this model, several mold preheating temperature such as 550, 700, 850 and 1000 °C was selected. The results showed that increasing the mold preheating temperature led to considerable change of the volume fraction of carbides in grain boundaries, increasing of average grain size from 50 to 150 µm and SDAS and improving of wear behavior of the alloy. The results showed that change of pouring temperature from 1430 to 1490 °C led to the coarsening of dendrites and increasing of SDAS. The solution heat treatment resulted in removing of eutectic carbides, appropriate distribution of blocky carbides and grain growth. The main structural change observed as a result of aging was the face-centered-cubic (Fcc) to hexagonal close-packed (Hcp) martensitic transformation that led to improvement of wear behavior. The volume fraction of the transformed hcp martensite was a function of time and temperature. According to the experimental and modeling results, investment casting of the F-75 alloy using mold A at mold preheating temperature of 1000 °C and pouring temperature of 1470 °C were determined as the appropriate casting conditions. Keywords : Investment cast, Co alloy, Microstructure, Wear behavior
