Heat Treatment Optimization of a γ′-Strengthened Nickel-Based Superalloy Based on Central Composite Design

Thermomechanical processing of blades and disks can become challenging when the microstructure of the material consists of a high volume fraction of strengthening phases. In Rene 65, the precipitate volume fraction is 40 pct in the as-forged condition. The microstructure consists of a bimodal/trimodal γ′ precipitate distribution on the grain boundaries and intergranually. In order to control grain size and reduce hardness, primary γ′ should be present on the grain boundaries and the secondary and tertiary precipitates should be coarse. In this work, an attempt to optimize the heat treatments that coarsen those two populations by implementing tools from experimental design is attempted. Two sample categories were examined with different thermomechanical histories, one deformed and one deformed and annealed, in order to verify if the ultimate heat treatment can be applied at different processing steps. The results confirm that there is a definite effect of the original microstructure, since there were different heat treatments that were proven to be the most effective to reduce hardness for the two conditions. Microstructural analysis revealed precipitate splitting and coralloid microstructures were observed for the first time in this alloy. Two mechanisms are proposed for the reduction of hardness depending on the initial microstructure.