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Direct Observation that Bainite can Grow Below M-S
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.ORCID iD: 0000-0002-7656-9733
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.ORCID iD: 0000-0003-1102-4342
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2012 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 43A, no 13, 4984-4988 p.Article in journal (Refereed) Published
Abstract [en]

In situ simultaneous synchrotron X-ray diffraction and laser scanning confocal microscopy have confirmed that bainite in steels can grow below the martensite start temperature. This observation suggests that the formation curves for bainite in time-temperature-transformation diagrams should be extended below the martensite start temperature. Furthermore, the implication of this observation on the growth mechanism of bainitic ferrite is discussed.

Place, publisher, year, edition, pages
2012. Vol. 43A, no 13, 4984-4988 p.
Keyword [en]
Bainitic ferrite, Growth mechanisms, Laser scanning confocal microscopy, Martensite start temperature, Synchrotron x ray diffraction, Time temperature transformation diagrams
National Category
Other Natural Sciences
Identifiers
URN: urn:nbn:se:kth:diva-104592DOI: 10.1007/s11661-012-1342-yISI: 000311511300008Scopus ID: 2-s2.0-84870397030OAI: oai:DiVA.org:kth-104592DiVA: diva2:565182
Funder
Vinnova
Note

QC 20130107

Available from: 2012-11-06 Created: 2012-11-06 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Does Bainite form with or without diffusion?: The experimental and theoretical evidence
Open this publication in new window or tab >>Does Bainite form with or without diffusion?: The experimental and theoretical evidence
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With the increased interest in bainitic steels, fundamental understanding of the bainite transformationis of major importance. Unfortunately, the research on bainite has been hampered by an oldcontroversy on its formation mechanism. Over the years two quite different theories have developedclaiming to describe the bainite transformation i.e. the diffusionless and the diffusion controlledtheory. In this thesis, attention is directed towards fundamental understanding of the bainitetransformation and both experimental and theoretical approaches are used in order to reveal its truenatureIn the first part of this thesis the symmetry in the Fe-C phase diagram is studied. It is based on ametallographic mapping of microstructures using light optical microscopy and scanning electronmicroscopy in a high carbon steel. The mapping revealed symmetries both with respect to temperatureand carbon content and an acicular eutectoid with cementite as the leading phase was found andidentified as inverse bainite. By accepting that all the eutectoid microstructures forms by diffusion ofcarbon, one may explain the existence of symmetries in the Fe-C phase diagram. Additional supportof its existence is obtained from an observation of symmetries in an alloyed steel. From the performedwork it was concluded that the existence of symmetries among the eutectoid microstructures fromaustenite supports the idea that bainite forms by a diffusion controlled transformation.In the second part the growth of bainite is considered. An experimental study using laser scanningconfocal microscopy was performed and growth rates of the transformation products from austenite ina high carbon, high chromium steel was analysed. The growth rate measurements reveals the kineticrelation between Widmanstätten cementite and the acicular eutectoid previously identified as inversebainite which confirms its existence and the conclusions drawn in the first part. In addition, in-situobservations of bainite formation below Ms provide additional support for the diffusion controlledtheory for bainite formation.The final part of the work is a study of the critical conditions for the formation of acicular ferrite.Based on experimental information found in the literature a thermodynamic analysis is performed inview of the two theories. The results demonstrate that the governing process for Fe-C alloys cannot bediffusionless but both kinds of processes can formally be used for predicting Bs temperatures for Fe-Calloys.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. viii, 46 p.
Keyword
Carbon steels, Bainite, Bainitic ferrite, Widmanstätten ferrite, Microstructure, Microscopy, Thermodynamic modeling
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-121344 (URN)978-91-7501-755-6 (ISBN)
Public defence
2013-05-24, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
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Supervisors
Note

QC 20130503

Available from: 2013-05-03 Created: 2013-04-29 Last updated: 2013-05-03Bibliographically approved

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Borgenstam, Annika

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