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JWJ > Volume 33(5); 2015 > Article
Journal of Welding and Joining 2015 October;33(5) :26-30.
Published online October 30, 2015.
doi:https://doi.org/10.5781/JWJ.2015.33.5.26
Fatigue Life of the Repair TIG Welded Hastelloy X Superalloy
Restu SIHOTANG*, Sang-Kyu CHOI**, Sung-Sang PARK***, Eung-Ryul BAEK***
*Research and Development Center, KOS Limited, Yangsan 626-230, Korea
**HYUNDAI BNG STEEL, Changwon 642-370, Korea
***School of Materials Science and Engineering, Yeungnam Univ, Gyeongbuk 712-749, Korea
Correspondence: Eung-Ryul BAEK  Email: erbaek@yu.ac.kr
  Published online October 30, 2015
ABSTRACT
Hastelloy X in this study was applied in jet engine F-15 air fighter as shroud to isolate the engine from outer skin. After 15 years operation at elevated temperature the mechanical properties decreased gradually due to the precipitation of continues second phases in the grain boundaries and precipitated inside the grain. The crack happened at the edge of the shroud due to the thermal and mechanical stress from jet engine. Selective TEM analysis found that the grain boundaries consist of M23C6 carbide, M6 Ccarbide and small percentage of sigma(σ) phase. Furthermore, it was confirmed the nano size of σ and miu (μ) phase inside the grain.
In this study, it was investigated the microstructure of the degraded shroud component and HAZ of repair welded shroud. In the HAZ, it was observed the dissolution of the M23C6 carbides and smaller precipitates, the migration of the undissolved larger M23C6 carbide and M6 Ccarbide. It is also observed the liquation due to the simply melt of the segregated precipitates in the grain boundaries. Interestingly, the segregated second phases which simply melt in the grain boundaries more easily happened at higher heat input welding condition. High temperature tensile test was done at 300℃, 700℃ and 900℃. It was obtained that the toughness of welded sample is lower compare to the non-welded sample.
The solution heat treatment at 1170℃ for 5 minutes was suggested to obtain a better mechanical properties of the shroud. The high cycle fatigue number of the repair welded shroud shows a much lower compare to the shroud. In addition, the high cycle fatigue number at room temperature after solution heat treatment was almost double compare to the before solution heat treatment under 420-500MPa stress amplitude. However, the high cycle fatigue number of repaired welded sample was shown a much lower compare to the non- welded shroud and solution treated shroud. One of the main reasons to decrease the tensile strength and the high cycle fatigue properties of the repair welded shroud is the formation of the liquid phase in HAZ.
Keywords: Superalloy | Hastelloy X | Grain boundary liquation | High cycle fatigue test
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