Development of Ni-Base Disk Alloy for Large-Size Gas Turbines by Improving Macrosegregation Property of Alloy 718

Alloy 718, a high-strength Ni-base superalloy, is used for the gas turbine disks of aircraft engines and thermal power generation plants. However, macrosegregation defects often form in larger ingots, making it difficult to apply Alloy 718 to large-size industrial gas turbine disks. In order to improve the macrosegregation properties of Alloy 718, the chemical composition was modified using calculated phase diagrams. The macrosegregation defect type of Alloy 718 is a sinking type. According to the difference in density, ∆ρ, calculated using the Scheil-Gulliver equation, it is found that W, which contributes to the same solid solution strengthening as Mo, could strongly enhance floating-type macrosegregation defects. Therefore, it is possible to suppress sinking-type macrosegregation defects by W addition. A new high-strength Ni-base superalloy (named FX550) for large-size gas turbine disks has been developed by substituting W for Mo on the basis of Alloy 718. The microstructure, mechanical properties, and macrosegregation properties of a forging were investigated. According to the experimental results, the microstructure, microstructure stability, and tensile strength of FX550 were confirmed as being the same as those of Alloy 718. The macrosegregation properties of FX550 were also similar to those of Alloy 706, which is used for large-size gas turbine disks. This paper presents an alloy design method for improving the macrosegregation properties of Alloy 718. In addition, the trial status of a large-size disk is described.