Abstract. 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• 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• 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 µm 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.
AD730 TM Ni-based superalloy specimens in solution-treated conditions were linear friction welded. Then, post-weld heat treatment (PWHT), consisting of γ´ sub-solvus solution treatments followed by aging, was conducted on the linear friction welded samples. High temperature creep tests were performed on the as-welded and PWHTed joints at two different temperatures: 700°C under 600 and 750 MPa stress levels, and 850°C under 100 and 200 MPa stresses. The creep resistance of the PWHTed joints was higher than that of the as-welded samples. The PWHTed joints exhibited better ductility than that of the base material at 850°C, while they showed slightly lower creep life at 700°C in comparison to the base metal. Microstructure examination showed that cracks initiated at the interface of oxidized particles at 700°C. The decrease in creep resistance of the AD730 TM Ni-based superalloy at 850°C was related to a combination of the formation of precipitate-free zones (PFZ) in the vicinity of the grain boundaries (GBs) and microcracking assisted by oxidation. The Larson-Miller Parameter (LMP) was used to correlate the creep strength, temperature and time to failure for the as-welded and PWHTed samples. LMP values varied between 21.5×10 3 and 24.5×10 3. It was found that in the investigated temperature range, the PWHTed AD730 TM has similar creep characteristics as Udimet TM 720 Li and Inconel 738LC at low values of LMP and better creep properties than those of the Inconel 617 alloy at higher LMP values.
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