20th International Forgemasters Meeting - IFM 2017

Currently ferritic heat resistant steels are mainly used for large industrial gas turbine disks because large disks made of ferritic heat resistant steels can be produced easily. On the other hand, it is well known that Ni based supperalloys such as Alloy706, Alloy718, Waspaloy, etc., on have much higher strength at elevated temperatures than ferritic heat resistant steels. Alloy718 has very high strength at high temperature but it contains a large amount of alloying elements and it is difficult to produce its large ingots because it has tendency to cause freckle defects. It is easier to produce large ingots of Alloy706 than Alloy718 but Alloy706 has lower strength at high temperature than Alloy718. Therefore a high strength Ni based superalloy which has the same strength at high temperature as Alloy718 and of which large ingots can be produced easily is required for large gas turbine disks in order to increase the operating temperatures of gas turbines and power generation efficiency. It is necessary to reduce the tendency to form freckle segregations of Alloy718 in order to produce a large ingot of a high strength Ni based superalloy such as Alloy718. A new Ni based superalloy with the same strength as Alloy718 and lower tendency to form freckle segregations, FX550, was developed by modifying the alloy compositions of Alloy718 to minimize the liquid density difference between condensed and average chemical compositions at a co-existing temperature of solid and liquid. Previously a large ingot of FX550 without freckle segregations was produced by VIM-ESR-VAR process and a gas turbine disk with a diameter of 1.5m class was produced1). In this study, larger disk with a diameter of 1.9m class was tried to be produced. In the same way as the previous trial, in order to design disk forging process and predict of the microstructure, computer simulation was used. Finally, a trial production of FX550 were demonstrated based on computer simulation results.