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Surface Integrity and Structural Stability of Broached Inconel 718 at High Temperatures
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.
Siemens Industrial Turbomachinery AB, Finspång, Sweden.
Siemens Industrial Turbomachinery AB, Finspång, Sweden.
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2016 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 47A, no 7, 3664-3676 p.Article in journal (Refereed) Published
Abstract [en]

The current study focused on the surface integrity issues associated with broaching of Inconel 718 and the structural stability of the broached specimen at high temperatures, mainly involving the microstructural changes and residual stress relaxation. The broaching operation was performed using similar cutting conditions as that used in turbo machinery industries for machining fir-tree root fixings on turbine disks. Thermal exposure was conducted at 723 K, 823 K, and 923 K (450 A degrees C, 550 A degrees C, and 650 A degrees C) for 30, 300, and 3000 hours, respectively. Surface cavities and debris dragging, sub-surface cracks, high intensity of plastic deformation, as well as the generation of tensile residual stresses were identified to be the main issues in surface integrity for the broached Inconel 718. When a subsequent heating was applied, surface recrystallization and alpha-Cr precipitation occurred beneath the broached surface depending on the applied temperature and exposure time. The plastic deformation induced by the broaching is responsible for these microstructural changes. The surface tension was completely relaxed in a short time at the temperature where surface recrystallization occurred. The tensile layer on the sub-surface, however, exhibited a much higher resistance to the stress relief annealing. Oxidation is inevitable at high temperatures. The study found that the surface recrystallization could promote the local Cr diffusion on the broached surface.

Place, publisher, year, edition, pages
Springer, 2016. Vol. 47A, no 7, 3664-3676 p.
Keyword [en]
Structural integrity, Broaching, Inconel 718, Plastic deformation, Residual stresses, Thermal exposure, Superalloy
National Category
Materials Engineering Other Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-111056DOI: 10.1007/s11661-016-3515-6ISI: 000377434700041OAI: oai:DiVA.org:liu-111056DiVA: diva2:752916
Funder
Linköpings universitet, 2009-00971
Note

The original title of this article when published in manuscript form was Structural integrity of broached Inconel 718 subjected to thermal exposure.

Available from: 2014-10-06 Created: 2014-10-06 Last updated: 2017-12-05Bibliographically 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
2. Surface Integrity and Fatigue Performance of Nickel-based Superalloys
Open this publication in new window or tab >>Surface Integrity and Fatigue Performance of Nickel-based Superalloys
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Due to global warming, the demand for more efficient gas turbines has increased. A way to achieve this, is by increasing the operating temperature of the gas turbine. Therefore, nickel-based superalloys have been developed to withstand these extreme temperatures and loads, especially in the hot sections. Today, the way of operating land-based gas turbines is changing. Instead of running for long periods of time, the operation is becoming more flexible, with ever-increasing cyclic loads and number of start and stop cycles. To handle the increased stress and cycles, component resistance to fatigue failures need to be improved.

Surface integrity is critical to fatigue performance, since fatigue cracks normally initiate at a surface. Machining changes the surface integrity which can result in worse fatigue resistance. The work presented in this Ph.D. thesis was conducted in collaboration with Siemens Industrial Turbomachinery AB in Finspång, Sweden. Surface integrity changes which are induced during machining and their effects on fatigue performance have been studied on the alloy Inconel 718. Inconel 718 is a widely-used nickel-based superalloy for high temperature applications in modern gas turbines.

Broaching, milling, and wire electrical discharge machining, related to component manufacturing in turbo machinery industries, were included in this study. Machining induced surface defects provide preferential sites for fatigue crack initiation which influence the fatigue performance of the alloy. If compressive residual stresses are induced during machining, they benefit the fatigue life by retarding fatigue crack initiation away from surface regions. Shot peening was performed on machined Inconel 718, by which high compressive residual stresses are deliberately induced. It results in enhanced fatigue performance.

The high temperatures in gas turbines generally deteriorate the surface integrity. Recrystallization often occurs in the highly deformed surface layer. Microstructural degradation, in the form of α-Cr precipitates, have also been frequently observed in the deformed surface and sub-surface microstructure. Oxidation at elevated temperatures degrades the surface integrity and thereby also the fatigue performance. Fatigue cracks are preferably initiated at oxidized surface carbides, if thermal exposure has been made prior to the test. It is even worse when high temperatures relax the beneficial surface compression induced by shot-peening and thereby lowering the fatigue resistance.

Machinability of a newly developed nickel-based superalloy, AD 730TM, and the surface integrity induced during turning have also been studied in this thesis project. AD 730TM is a candidate for turbine disc applications with an operating temperature above 650 °C. At such high temperatures, Inconel 718 is no longer stable and its mechanical properties start to degrade.

To summarize, the results from this thesis work show the importance of understanding surface integrity effects for fatigue applications, especially in harsh environments. Moreover, the knowledge gained through this work could be used for surface enhancement of turbine components which are subjected to a high risk of fatigue failure. These will contribute to more efficient and flexible power generation by gas turbines.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2017. 72 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1821
National Category
Manufacturing, Surface and Joining Technology Other Materials Engineering Tribology Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:liu:diva-134124 (URN)10.3384/diss.diva-134124 (DOI)9789176856000 (ISBN)
Public defence
2017-02-17, ACAS,Hus A, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2017-01-24 Created: 2017-01-24 Last updated: 2017-01-25Bibliographically approved

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Chen, ZhePeng, Ru LinJohansson, Sten

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