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Near-surface residual stresses and microstructural changes after turning of a nickel-based superalloy
Linköping University, Department of Mechanical Engineering, Engineering Materials. Linköping University, The Institute of Technology.
2003 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Nickel-based superalloys are precipitation hardened alloys with complex compositions. They are used in aircraft engines and land-based gas turbines in load-bearing structural components that are exposed to high temperatures. Failure mechanisms in this environment are high and low cycle fatigue, creep, and corrosion.

During manufacturing, residual stresses are often introduced into the material due to inhomogeneous plastic deformations, both intentionally and unintentionally. One such manufacturing process is metal cutting, which introduces residual stresses in the surface layer. The stress state in the near-surface zone of components is of special interest as the surface often experiences peak loads and cracks have their starting point there.

In this thesis, near-surface residual stress distributions and microstructural changes are studied in the nickel-based superalloy Inconel 718 for two different turning operations, face grooving and facing. Process variables are in both cases cutting speed and feed that have been varied between (10 and 1200) m/min and (0.01 and 0.5) mm, respectively.

The first turning technique face grooving, which gives cutting conditions similar to orthogonal cutting, showed a clear dependency of the residual stresses on the cutting speed. The tensile stress at the surface, the maximum compressive stress below the surface, and the thickness of the affected layer increase with increasing cutting speed. The tensile stresses are constrained to a thin surface layer and compressive residual stresses below the surface dominate the depth profile of the residual stresses. Only at low cutting speed, residual stresses were largely avoided.

The second turning technique facing confirmed the dependency of the residual stresses on the cutting speed and revealed a similar dependency on the feed. Microstructural investigations of near-surface cross-sections by means of transmission electron microscopy showed a zone where the grains had undergone plastic deformation, indicated by slip bands. On top of this layer, a surface layer exists where the grain size has radically decreased to between 50 nm and 130 nm. The grain size of the nanocrystalline layer is fairly constant for a certain cutting speed and feed but depends on these cutting parameters. An increase in cutting speed and feed leads to larger grains in the nanocrystalline layer.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet , 2003. , 34 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1002
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-30052Local ID: 15512ISBN: 91-7373-601-5 (print)OAI: oai:DiVA.org:liu-30052DiVA: diva2:250873
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-11-28
List of papers
1. Residual Stresses in a Nickel-based Superalloy Introduced by Turning
Open this publication in new window or tab >>Residual Stresses in a Nickel-based Superalloy Introduced by Turning
2002 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 404-407, 173-178 p.Article in journal (Refereed) Published
Abstract [en]

Near-surface residual stress distributions in the nickel-based superalloy Inconel 718 that originate from the machining operation turning are studied. The turning process that is used in the experiments is face grooving which gives quasi-orthogonal cutting conditions. Cutting speed and feed have been varied to investigate their effects on the residual stress state. Tensile residual stresses with a maximum of 1300 MPa were found at the surface that turn rapidly into compressive residual stresses of up to -800 MPa. The depth distributions of the residual stresses are presented and discussed with respect to observations made by optical and transmission electron microscopy.

Keyword
residual stress, machining, superalloy, microstructure
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-24027 (URN)10.4028/www.scientific.net/MSF.404-407.173 (DOI)3582 (Local ID)3582 (Archive number)3582 (OAI)
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2017-12-13
2. Residual stress evolution and near-surface microstructure after turning of the nickel-based superalloy Inconel 718
Open this publication in new window or tab >>Residual stress evolution and near-surface microstructure after turning of the nickel-based superalloy Inconel 718
2005 (English)In: Zeitschrift für Metallkunde, ISSN 0044-3093, Vol. 96, no 4, 385-392 p.Article in journal (Refereed) Published
Abstract [en]

Turning experiments have been carried out with the cutting speed and feed as variables that were systematically varied between 10 m min−1 and 1200 m min−1, and 0.1 mm and 0.5 mm, respectively, while all other cutting parameters were held constant. The arising residual stress distributions are presented and the influence of the varied machining parameters is investigated. Compressive residual stresses dominate the depth profiles but are often accompanied by a thin tensile residual stress layer at the surface. Microstructural investigations of near-surface cross-sections by means of transmission electron microscopy showed a zone where the grains had undergone plastic deformation, indicated by slip bands. On top of this layer, a surface layer exists where the grain size has radically decreased to only 50 nm to 130 nm. The grain size of the nanocrystalline layer is fairly constant for a certain cutting speed and feed, but depends on these cutting parameters. An increase in cutting speed and feed leads to larger grains in the nanocrystalline layer.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-28422 (URN)13560 (Local ID)13560 (Archive number)13560 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2017-12-13

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