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Damage analysis of a retired gas turbine disc
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
Siemens Industrial Turbomachinery AB, Finspång, Sweden.
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
2014 (English)In: Proceedings of the 2014 Energy Materials Conference, John Wiley & Sons, 2014, 405-410 p.Conference paper, Published paper (Refereed)
Abstract [en]

Gas turbine discs operate mostly at high temperature gradients and are subjected to mechanical loads simultaneously. The high thermal and mechanical loads eventually could result in degradation and damages in disc material, thereby increasing the risk of disc failure. In this study, a damage analysis was performed in a retired gas turbine disc made of Inconel 718. Oxidation attack and microstructural degradation as the consequence of the high service temperature have been found to be the main damages that take place in the non-contact area of the retired disc. In the blade/disc contact area, fretting fatigue occurs, with a result that cracks initiate from the oxide/metal interface and propagate in the disc alloy parallel to the sliding direction of fretting, consequently reducing the stability and safety of the disc. Meantime, oxygen diffuses into the fretting fatigue cracks, thereby exacerbating the oxidation attack. A multi-layered scale with periodic formation of the Fe-oxide/spinel layer and the metallic layer is formed on the contact surface. In both contact and non-contact area, recrystallization and α-Cr precipitation take place in the surface layer of the disc alloy. The locations where α-Cr precipitates are commonly considered to be the natural sites for mechanical weakness.

Place, publisher, year, edition, pages
John Wiley & Sons, 2014. 405-410 p.
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-111060ISI: 000364592900047ISBN: 978-1-119-02794-2 (print)OAI: oai:DiVA.org:liu-111060DiVA: diva2:752943
Conference
Energy Materials 2014 TMS, November 4-6, 2014, Xi’an, China
Available from: 2014-10-06 Created: 2014-10-06 Last updated: 2015-12-07Bibliographically approved
In thesis
1. Surface Integrity of Broached Inconel 718 and Influence of Thermal Exposure
Open this publication in new window or tab >>Surface Integrity of Broached Inconel 718 and Influence of Thermal Exposure
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Inconel 718 is a nickel-based superalloy that is extensively used as a disc material in gas turbine engines. The service life of gas turbine discs is normally governed by the modes of material degradation and fatigue failure since they work mostly at high temperatures and are subjected to cyclic mechanical loadings. Fatigue failures often start with the initiation of cracks at the surface and the precise details of the failure process significantly depend on the surface conditions. In turbine disc production, one of the last manufacturing steps is to broach root fixings, commonly of fir-tree design, for blade mounting. It has always been a challenge when machining Inconel 718 due to its high strength retention at elevated temperatures, rapid work hardening, as well as low thermal conductivity. This usually leads to rapid tool wear, and consequently shorter tool life, and at the end to the deterioration of the surface integrity of the machined components.

The aim of this licentiate thesis is to increase our knowledge about the surface integrity, especially microstructure and residual stresses, of broached Inconel 718 and its stability under thermal exposure. This knowledge can later be used for analyzing the initiation and propagation of fatigue cracks in broached Inconel 718, particularly in the case of high temperature fatigue, thereby giving a better understanding of the failure mechanism of gas turbine discs from a fatigue point of view.

A broaching operation has been performed using similar cutting conditions as that used in turbo machinery industries for producing fir-tree root fixings. In addition, service damages were analyzed in a retired dis of Inconel 718. Surface defects, severe plastic deformation and generation of high tensile residual stresses have been found to be the main damages to the surface integrity caused by the broaching operation. The machining induced plastic deformation was found to accelerate the microstructural degradation beneath the broached surface when subjected to thermal exposure. The surface tensile residual stresses can be completely removed after short thermal exposure, the tensile layer in the sub-surface region, however, exhibited a high resistance to stress relief at high temperatures. The damage analysis on the retired disc indicated that presence of the highly deformed layer on the machined surface is negative for preventing the occurrence of fretting fatigue in turbine discs.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. 44 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1676
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-111062 (URN)10.3384/lic.diva-111062 (DOI)LIU-TEK-LIC-2014:108 (Local ID)978-91-7519-249-9 (ISBN)LIU-TEK-LIC-2014:108 (Archive number)LIU-TEK-LIC-2014:108 (OAI)
Presentation
2014-10-10, ACAS, A-building, Campus Valla, Linköpings universitet, 10:15 (English)
Opponent
Supervisors
Available from: 2014-10-06 Created: 2014-10-06 Last updated: 2014-10-06Bibliographically approved

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Chen, ZhePeng, Ru LinJohansson, Sten

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