Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Surface Integrity of Broached Inconel 718 and Influence of Thermal Exposure
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
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: urn:nbn:se:liu:diva-111062DOI: 10.3384/lic.diva-111062Local ID: LIU-TEK-LIC-2014:108ISBN: 978-91-7519-249-9 (print)OAI: oai:DiVA.org:liu-111062DiVA: diva2:752960
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
List of papers
1. Surface Integrity and Structural Stability of Broached Inconel 718 at High Temperatures
Open this publication in new window or tab >>Surface Integrity and Structural Stability of Broached Inconel 718 at High Temperatures
Show others...
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
Keyword
Structural integrity, Broaching, Inconel 718, Plastic deformation, Residual stresses, Thermal exposure, Superalloy
National Category
Materials Engineering Other Materials Engineering
Identifiers
urn:nbn:se:liu:diva-111056 (URN)10.1007/s11661-016-3515-6 (DOI)000377434700041 ()
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
2. Effect of thermal exposure on microstructure and nano-hardness of broached Inconel 718
Open this publication in new window or tab >>Effect of thermal exposure on microstructure and nano-hardness of broached Inconel 718
Show others...
2014 (English)In: MATEC Web of Conferences Vol. 14 (2014) EUROSUPERALLOYS 2014 – 2nd European Symposium on Superalloys and their Applications: Session 8: Recrystallization and Grain Growth / [ed] J.Y. Guédou and J. Choné, Les Ulis, France: E D P Sciences , 2014, 08002-p.1-08002-p.6 p.Conference paper, Published paper (Refereed)
Abstract [en]

Inconel 718 is a high strength, heat resistant superalloy that is used extensively for components in hot sections of gas turbine engines. This paper presents an experimental study on the thermal stability of broached Inconel 718 in terms of microstructure and nano-hardness. The broaching process used in this study is similar to that used in gas turbine industries for machining fir-tree root fixings on turbine discs. Severe plastic deformation was found under the broached surface. The plastic deformation induces a work-hardened layer in the subsurface region with a thickness of ∼50 μm. Thermal exposure was conducted at two temperatures, 550 C and 650 C respectively, for 300 h. Recrystallization occurs in the surface layer during thermal exposure at 550 C and α-Cr precipitates as a consequence of the growth of recrystallized δ phases. More recrystallized grains with a larger size form in the surface layer and the α-Cr not only precipitates in the surface layer, but also in the sub-surface region when the thermal exposure temperature goes up to 650 C. The thermal exposure leads to an increase in nano-hardness both in the work-hardened layer and in the bulk material due to the coarsening of the main strengthening phase γ′′.

Place, publisher, year, edition, pages
Les Ulis, France: E D P Sciences, 2014
Series
MATEC Web of Conferences, ISSN 2261-236X ; Volume 14
Keyword
Structural integrity, Broaching, Inconel 718, Plastic deformation, Residual stresses, Thermal exposure, Superalloy
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-111058 (URN)10.1051/matecconf/20141408002 (DOI)000351930400024 ()2-s2.0-84906910411 (Scopus ID)
Conference
2nd European Symposium on Superalloys and Their Applications, May 12-16, 2014, French Riviera, Giens, France
Available from: 2014-10-06 Created: 2014-10-06 Last updated: 2016-09-07Bibliographically approved
3. Analysis of Thermal Effect on Residual Stresses of Broached Inconel 718
Open this publication in new window or tab >>Analysis of Thermal Effect on Residual Stresses of Broached Inconel 718
Show others...
2014 (English)Conference paper, Published paper (Refereed)
Abstract [en]

Inconel 718 is a nickel based superalloy that is widely used as a turbine disc material in gas turbine industries. This study details the effect of thermal exposure on the residual stresses produced when broaching Inconel 718. The chosen parameters for broaching in this study are similar to those used when manufacturing turbine discs. The broaching operation produced a high level of tensile residual stresses at the broached surface. A layer with tensile residual stresses was formed in the sub-surface region, followed by a layer several times thicker with compressive residual stresses. Thermal exposure was conducted at 550 °C. The depth distributions of residual stresses after thermal exposure are presented and discussed in this paper. Complete relaxation of the surface tensile residual stresses was observed after 30 h thermal exposure, whereas the 3000 h thermal exposure influenced both the surface and sub-surface residual stress states.

Place, publisher, year, edition, pages
Trans Tech Publications Inc., 2014
Series
Advanced Materials Research, ISSN 1022-6680 ; 996
Keyword
Broaching, Incone 718, Residual Stresses, Thermal Exposure, Recrystallization
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-111057 (URN)10.4028/www.scientific.net/AMR.996.574 (DOI)000348282100093 ()
Conference
ECRS9 6-10 July, Troyes, Frankrike
Available from: 2014-10-06 Created: 2014-10-06 Last updated: 2015-02-20Bibliographically approved
4. Damage analysis of a retired gas turbine disc
Open this publication in new window or tab >>Damage analysis of a retired gas turbine disc
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
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-111060 (URN)000364592900047 ()978-1-119-02794-2 (ISBN)
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

Open Access in DiVA

Surface Integrity of Broached Inconel 718 and Influence of Thermal Exposure(1085 kB)1306 downloads
File information
File name FULLTEXT01.pdfFile size 1085 kBChecksum SHA-512
b572f7556ca4bbd31cccfa9b1dcff802e3bf23fb49619ea1b26986ceb593e6a0be41adc2a07b83a642b3942590c91c390096ae8538ff426f1112cfc0c40e8b93
Type fulltextMimetype application/pdf
omslag(141 kB)34 downloads
File information
File name COVER01.pdfFile size 141 kBChecksum SHA-512
11588fd865570e845408bb1819c79ebe07d06700f26fbae658ebb2ab06d865405757ad5dec6afb17329c4da738d853fe57640751a3bfeaaa0868b4d282e68f91
Type coverMimetype application/pdf

Other links

Publisher's full text

Authority records BETA

Chen, Zhe

Search in DiVA

By author/editor
Chen, Zhe
By organisation
Engineering MaterialsThe Institute of Technology
Materials Engineering

Search outside of DiVA

GoogleGoogle Scholar
Total: 1306 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

doi
isbn
urn-nbn

Altmetric score

doi
isbn
urn-nbn
Total: 1101 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf