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Effect of Cooling and Shot Peening on Residual Stresses and Fatigue Performance of Milled Inconel 718
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0002-8304-0221
Ytstruktur Arboga AB, Arboga, Sweden.
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2017 (English)In: Residual Stresses 2016 ICRS 10 / [ed] T.M. Holden, O. Muránsky, and L. Edwards, 2017, Vol. 2, 13-18 p.Conference paper, Published paper (Refereed)
Abstract [en]

The present study highlights the effect of cooling and post-machining surface treatment of shot peening on the residual stresses and corresponding fatigue life of milled superalloy Inconel 718. It was found that tensile residual stresses were created on the milled surface, regardless of the use of coolant, however, the wet milling operation led to a lower surface tension and a reduced thickness of the tensile layer. The shot peening performed on the dry-milled specimens completely annihilated the surface tensile residual tresses and introduced a high level of surface compression. A comparable fatigue life for the wet-milled specimens was obtained as compared with the specimens prepared by dry milling. This is very likely attributed to that the milling-induced surface damage with respect to cracked non-metallic inclusions is the predominant cause of the fatigue failure. The presence of the compressive layer induced by shot peening resulted in a significant increase of the fatigue life and strength, while the extent to which the lifetime was prolonged was decreased as the applied load was increased.

Place, publisher, year, edition, pages
2017. Vol. 2, 13-18 p.
Series
Materials Research Proceedings, ISSN 2474-395X
Keyword [en]
Residual stresses, surface integrity, milling, shot peening, superalloys, fatigue
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-132001DOI: 10.21741/9781945291173-3ISI: 000401041500003ISBN: 978-1-9452-9116-6 (print)OAI: oai:DiVA.org:liu-132001DiVA: diva2:1034910
Conference
10th International Conference on Residual Stresses ICRS-10, Sydney, Australia, 3-8 July 2016
Note

Funding agencies: Linkoping University [2009-00971]; Ytstruktur Arboga AB; Siemens Industrial Turbomachinery AB

Available from: 2016-10-13 Created: 2016-10-13 Last updated: 2017-06-13Bibliographically approved
In thesis
1. 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)
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Available from: 2017-01-24 Created: 2017-01-24 Last updated: 2017-01-25Bibliographically approved

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