Refurbishment of Creep Damage Using Re-Heat Process for Ni-Based Superalloy under Bending Load

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1 (Journal of the Society of Materials Science, Japan), Vol. 60, No. 3, pp , Mar Ni Refurbishment of Creep Damage Using Re-Heat Process for Ni-Based Superalloy under Bending Load by Masaru SEKIHARA, Kunihiro ICHIKAWA, Shinya IMANO, Yukio KAGIYA, Akihiro ITO and Kouji CHUUJOU Ni-based superalloys are commonly used in hot gas path parts such as turbine blades because of their higher strength under high temperature. Their high strength results from an intermetallic compound such as a cubical γ phase precipated in a γ phase. However, the γ phase tends to show coarsening and lafting. The re-heat process has been adopted to repair the creep damage in the Ni-based superalloys. The re-heat process can restore the γ phase by carrying out thermal cycles with an appropriate combination of temperature and duration. In this report, we focused on creep damage under the bending load. First, the creep constitutive law was developed to calculate creep strain more accurately. Then, we investigated the relationships between the effect of the re-heat process and creep crack initiation under the bending load through a mock-up creep test. The time-dependent hardening law was selected as a creep constitutive law. The calculated creep deformation showed relatively good correspondence with the results of uni-axial and mock-up creep tests. Also, creep crack initiation seemed to be controlled by the re-heat process at an appropriate stage. Re-heated specimens showed critical creep deformation, corresponding to creep crack initiation, about half that of virgin specimens. This seemed to be because of transgranular precipitation, not all of which can be removed using the re-heat process. In uni-axial creep tests, we found some critical value for cumulative creep strain, which causes harmful precipitation during the re-heat process. Therefore, we conclude that creep damage in Ni-based superalloys can be repaired effectively using the re-heat process before cumulative creep strain reaches a critical value that is restrained by the initiation of harmful precipitation or creep cracks. Key words : Gas turbine, Ni based super alloy, Creep constitutive law, Creep cracks, Bending load, Re-heat process 1 Ni Cubic γ (Ni 3Al) γ (NiAl) γ 1) Ni γ Fig. 1 / 2) Corcoruto Received Mar. 8, The Society of Materials Science, Japan Materials Res. Laboratory, Hitachi Ltd., Saiwai-Cho, Hitachi, Hitachi Works, Hitachi Ltd., Saiwai-Cho, Hitachi, Materials Res. Laboratory, Hitachi Ltd., Saiwai-Cho, Hitachi, Chubu Electric Power Co. Inc., Midori-Ku, Nagoya, Chubu Electric Power Co. Inc., Konan-Cho, Hekinan,

2 Ni 203 Fig. 1 Schematic model of reheat process effect. HIP (Hot Isostatic Pressing) 3) Ishii 0.5% HIP + 4) Yoshioka 3 3 γ γ 5) 2 HIP + 6) Cheruvu 5 Ni + 2 7) Miglietti 2 8) Lvova MC M 23C 6 9) Suzuki EBSD (Electron Backscatter Diffraction) SP (Small Punch) SPC (Small Punch Creep) 10) Kimura PWA γ/γ 11) James 3 γ MC TCP (Topologically Closed Packed) 1% ERF (Economic Repair Parameter) 12) 2 Table 1 Ni HIP 2 γ γ (Cubic) 1 γ Ni 3Ti 2 HIP γ 1 2 Fig. 2 (a) Table MPa 250MPa Fig. 2 (b) Fig. 3 T Table 1 Chemical composition (weight %). 9)

3 204 Fig. 4 Bending test schedule for each specimen. Fig. 2 Test specimen. (mm) Table 2 Tensile creep test conditions HIP Fig. 5 (1) T Arrenius σ t ε cr C1 C7 13) Table 3 (1) (1) Fig. 5 Table 3 (1) 3 2 Fig Table 3 Coefficients for creep strain equation. Fig. 3 Schematic model of bending test. T Fig ,000N 1

4 Ni 205 Fig. 5 Creep strain behavior during tensile loading. (Normalized by measured strain for 1,000 hr at 875 ) 2 5 Fig. 7 (1) ANSYS Fig. 3 6 Fig. 3 Fig % 2 46% 3 11% 4 29% 5 6% Fig Table Table HIP HIP HIP

5 206 Fig. 6 Images of specimen after bending tests. Fig. 7 Deflection behavior during bending tests. (Normalized by calculated initial deflection)

12075(p.202-209) 11.2.25 11:01 ページ 207 再生熱処理による Ni

6 12075(p ) :01 ページ 207 再生熱処理による Ni 基超合金の曲げ荷重下でのクリープ損傷回復 Table 4 Micro photographic image of specimen. 207

7 HIP HIP Fig T 3.5% Table 4 Fig. 9 T Fig. 10 CrMoV Fig. 10 Bending stress distribution in thickness direction. 14) Fig Fig. 12 Table 5 EBSP Fig. 8 Equivalent creep strain distribution after 500h. Fig. 9 Progress of bending stress at crack observed position. Fig. 11 Relationship between creep deflection and crack length. (Normalized by calculated initial deflection).

Ni 209 Fig. 12 Effect of cumulative creep strain on harmful precipitation. (Normalized by measured strain for 1,000hr at 875 ). Table 5 Observed images of grain boundary.

Okazaki, Micromechanics approach to the morphology of cellular microstructure in single crystal Ni-based superalloy, Journal of the Society of Materials Science, Japan, Vol.54, No.2, pp.

8 Ni 209 Fig. 12 Effect of cumulative creep strain on harmful precipitation. (Normalized by measured strain for 1,000hr at 875 ). Table 5 Observed images of grain boundary. (Electron Back Scattering Pattern) 4 Ni (1) Ni (2) (3) (4) 1 (5) Ni (6) HIP 1 ) M. Sakaguchi and M. Okazaki, Micromechanics approach to the morphology of cellular microstructure in single crystal Ni-based superalloy, Journal of the Society of Materials Science, Japan, Vol.54, No.2, pp (2005). 2 ) A. Kanaya, J. Kusumoto, H. Kitagawa, S. Imano, M. Sekihara and K. Ichikawa, The effect of reheat process on long term operation of gas turbine blades, Proceedings of the 57th JSMS Annual Meetings (2008). 3 ) S. Corcoruto and U. Guerreschi, Enabling technologies for advanced turbine component life extension, ASME GT (1999). 4 ) J. Ishii, Latest gas turbine repair and life extension technologies, The Thermal and Nuclear Power, Vol.53, No.7, pp (2002). 5 ) Y. Yoshioka, R. Ishii, D. Saito, K. Fujiyama and N. Okabe, Effect of refurbishment heat treatment on creep life of cast Ni-base superalloy IN738LC, Proceedings of the 71th JSME Spring Annual Meeting, Vol.930, No.63, pp (1993). 6 ) Y. Yoshioka, D. Saito, K. Ishibashi, J. Ishii, A. Izumi, Y. Aburatani, A. Itou, Y. Kagiya, H. Watanabe and S. Hyakudome, Life-refurbishment of service-degraded gas turbine buckets, ASME-GT (2004). 7 ) N. S. Cheruvu, V. P. Swaminathan and C. D. Kinney, Recovery of microstructure and mechanical properties of service run GTD-111 DS buckets, ASME-99-GT-425 (1999). 8 ) W. M. Miglietti, R. Curtis and J. Helm, Rejuvenation heat treatments and their role in the repair of IN738 turbine components, ASME-GT (2002). 9 ) E. Lvova and D. Norsworthy, Influence of service-induced microstructural changes on the aging kinetics of rejuvenated Nibased superalloy gas turbine blades, Journal of Materials Engineering and Performance, Vol.10, No.3, pp (2001). 10) Y. Suzuki, K. Ogawa and T. Shoji, Recovery of creep damages by means of re-heat treatments, The 45th Symposium on Strength of Materials at High Temperatures, pp (2007). 11) T. Kimura, Y. Koizumi, T. Yokogawa, M. Sakamoto, H. Harada and H. Imai, Rejuvenation of serviced turbines blades by Re-heat treatment, Annual Conference of the GTSJ, Vol.36, pp (2008). 12) A. James, Review of rejuvenation process for nickel base superalloys, Materials Science and Technology, Vol.17, No.5, pp (2001). 13) ANSYS V11.0 Element Reference, Ansys, incorporated, p.40 (2007). 14) M. Sekihara and S. Sakurai, Effect of stress relaxation behavior on creep crack initiation of CrMoV cast steel, The 41th Symposium on Strength of Materials at High Temperatures, pp (2003).

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