한국주조공학회

The effects of a heat treatment on the carbide formation behavior and mechanical properties of the cobalt-based superalloy X-45 were investigated here. Coarse primary carbides formed in the interdendritic region in the as-cast specimen, along with the precipitation of fine secondary carbides in the vicinity of the primary carbides. Most of the carbides formed in the interdendritic region were dissolved into the matrix by a solution treatment at 1274℃. Solutionizing at 1150℃ led to the dissolution of some carbides at the grain boundaries, though this also caused the precipitation of fine carbides in the vicinity of coarse primary carbides. A solution treatment followed by an aging treatment at 927°C led to the precipitation of fine secondary carbides in the interdendritic region. Very fine carbides were precipitated in the dendritic region by an aging heat treatment at 927°C and 982°C without a solution treatment. The hardness value of the alloy solutionized at 1150°C was somewhat higher than that in the as- cast condition; however, various aging treatments did not strongly influence the hardness value. The specimens as-cast and aged at 927°C showed the highest hardness values, though they were not significantly affected by the aging time. The specimens aged only at 982°C showed outstanding tensile and creep properties. Thermal exposure at high temperatures for 8000 hours led to the precipitation of carbide at the center of the dendrite region and an improvement of the creep rupture lifetimes.

Co-based superalloy, heat treatment, carbide, tensile, creep