Key factors in quality control of a high Cr content cast Ni-base superalloy K4648

K4648 possesses the highest Cr content in applied cast Ni-base superalloys and has been applied to manufacture the integral castings of advanced gas turbines in China. However, the large blocky primary alpha phase in alloy had detrimental effect on the impact ductility, and the alloy severely reacted with the silica-based ceramic core during the solidification process. The investigation results indicate that the primary alpha phase precipitated in the late solidification at 1190 degrees C. The Vickers microhardness value of alpha phase is 6.3 GPa at the load of 0.2 N. The primary alpha phase transforms to M23C6 carbides after solid solution treatment at 1180 degrees C. The cracked alpha or transformed M23C6 can be observed at the fracture surface and longitudinal section of the impact specimens. Moreover, the severe reaction exists in the metal/ceramic interface. The reaction products consist mainly of the oxides which elements come from both alloy and silica-based ceramic core. For high Cr content superalloys, the solidified parameters should be controlled properly to inhibit primary alpha phase precipitation and more stable alumina-based core is recommended.