Mechanical properties and development of supersolvus heat treated new nickel base superalloy AD730™

The enhancement of efficiency in power generation gas turbine requires the development of new superalloys capable of withstanding higher temperatures. The development of AD730 (TM) superalloy was achieved to provide to this new cast & wrought (C&W) superalloy a higher combination between mechanical properties, microstructural stability and cost than that of other C&W superalloys with a temperature capability up to 750 degrees C. Supersolvus heat-treatment of AD730 (TM) was studied to improve the creep properties of fine grain AD730 (TM) superalloy which were not high enough to reach the foreseen conditions of future power generation gas turbine disks. Firstly, the grain growth was studied to select the supersolvus temperature 1120 degrees C and to obtain a homogeneous coarse grain microstructure. Then, various supersolvus heat-treatments with different cycles were tested and applied on a forged pancake with a section representative of power generation gas turbine disk. The average grain size was evaluated to be close to 200 mu m for all heat-treatments. Tensile, creep, fatigue and fatigue crack growth tests were performed to compare the various heat-treatments. FEG-SEM examinations were also realized to discuss the relationships between heat-treatment, intragranular gamma prime precipitation and mechanical properties. Finally, a comparison made with other supersolvus heat treated C&W superalloys shows that AD730 (TM) properties obtained with coarse grain microstructure are at the expected level and enable applications for power generation gas turbine discs.