Phase and morphology of carbides in ASTM F75 Co-Cr-Mo-C alloys formed at 1473 to 1623 K

The phase and morphology of precipitates in heat-treated Co-28Cr-6Mo-xC (x = 0.12, 0.15, 0.25, and 0.35mass%) alloys were investigated. The as-cast alloys were solution-treated in the temperature range of 1473 to 1623 K for 0 to 43.2 ks. Complete precipitate dissolution was observed in all four alloys, each of which had different carbon contents. The holding time for complete dissolution was greater for alloys with greater carbon content. The curve representing the boundary between the complete- and incomplete-dissolution conditions for each alloy is C shaped. Under the incomplete precipitate dissolution conditions of the Co-28Cr-6Mo-0.25C alloy, an M₂₃C₆ type carbide and a π-phase (M₂T₃X type carbide with β-Mn structure) were observed at 1548 to 1623 K, and starlike precipitates with a stripe pattern and with a dense appearance were both observed; the former comprised the M ₂₃C₆ type carbide + γ-phase, and the latter was the π-phase. In contrast, only a blocky-dense M₂₃C₆ type carbide was observed at 1473 to 1523 K.