EFFECT OF HEAT TREATMENT AND ALLOYING ELEMENTS ON PRECIPITATION AND SURFACE BEHAVIOR OF Co-Cr-Mo ALLOYS

The microstructures of as-cast and heat-treated Co-Cr-Mo with addition of C, Si and Mn have been investigated with a focus on phase and dissolution of precipitates. The heat treatment temperatures and heating periods employed ranged from 1448 to 1548 K and 0 to 43.2 ks, respectively. The precipitates observed in the as-cast and heat-treated alloys were carbides (M23C6, eta-phase, and pi-phase) and an intermetallic compound of chi-phase. Chi-phase was detected in the sample with lowest carbon content of this study, 0.15 wt%. The addition of Si seemed to increase the heating time for complete precipitate dissolution because of the effects of Si on the promotion of pi-phase formation at high temperatures and the increased carbon activity in the Co matrix. After polarization test in simulated body fluid to the single phasecontained specimens, it showed that chi-phase drastically decreased the corrosion resistance of the alloy. The alloys with chi-phase and M23C6 precipitates have lower corrosion resistance than those with pi-phase.