The role of heat treatment on creep rupture ductility and its underlying metallurgical mechanism of forged Ni-based superalloy
The role of heat treatment on creep rupture ductility and its underlying metallurgical mechanism of forged Ni-based superalloy
Friday, February 28, 2025: 11:45 AM
Indian Wells I (Grand Hyatt Indian Wells Resort)
Forged Ni-based superalloys have excellent high-temperature strength and corrosion resistance so that they are widely utilized for the key components of gas turbines and aeroengines. Although many previous studies have focused on the creep rupture strength of the superalloys, their creep rupture ductility is also the important issue since the actual structures have discontinuity of shapes which results in stress concentration. In this study, the effect of heat treatment conditions on creep rupture ductility of forged Ni-based superalloy was evaluated and its metallurgical mechanism was discussed.
Commercially available Ni-based superalloy Udimet 520 (Ni-12Co-19Cr-6Mo-1W-3Ti-2Al) was used for this study. After forging the as-received material, solution treatment at 1120℃ was conducted with various cooling rate. The material was further aged with two-steps at 850℃ and 760℃. Smooth round bar specimens were sampled from the heat treated material, and creep tested at 650℃×716 MPa.
Time-to-rupture was found to be longer as the cooling rate after solution treatment was higher. On the other hand, the lower elongation was obtained for the faster cooling rate. Elongation was reduced to 5% at relatively high cooling rate of 60℃/min compared to above 20% at about 20℃/min. Then, the two-step aging conditions were modified while keeping the solution treatment the same and the cooling rate at 60℃/min. Due to their modifications, the significant improvement of elongation from 5% to 19% was achieved by some two-step aging conditions without the degradation of time-to-rupture. These results were able to be explained by the difference of precipitation distributions, especially gamma-prime phase within grain and on grain boundary, and grain boundary M23C6 carbides.
The results found in this study can be the guideline for adopting suitable heat treatments of forged Ni-based superalloys for the sound operation of the equipment.
Commercially available Ni-based superalloy Udimet 520 (Ni-12Co-19Cr-6Mo-1W-3Ti-2Al) was used for this study. After forging the as-received material, solution treatment at 1120℃ was conducted with various cooling rate. The material was further aged with two-steps at 850℃ and 760℃. Smooth round bar specimens were sampled from the heat treated material, and creep tested at 650℃×716 MPa.
Time-to-rupture was found to be longer as the cooling rate after solution treatment was higher. On the other hand, the lower elongation was obtained for the faster cooling rate. Elongation was reduced to 5% at relatively high cooling rate of 60℃/min compared to above 20% at about 20℃/min. Then, the two-step aging conditions were modified while keeping the solution treatment the same and the cooling rate at 60℃/min. Due to their modifications, the significant improvement of elongation from 5% to 19% was achieved by some two-step aging conditions without the degradation of time-to-rupture. These results were able to be explained by the difference of precipitation distributions, especially gamma-prime phase within grain and on grain boundary, and grain boundary M23C6 carbides.
The results found in this study can be the guideline for adopting suitable heat treatments of forged Ni-based superalloys for the sound operation of the equipment.