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Does Bainite form with or without diffusion?: The experimental and theoretical evidence
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
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 [en]
Carbon steels, Bainite, Bainitic ferrite, Widmanstätten ferrite, Microstructure, Microscopy, Thermodynamic modeling
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-121344ISBN: 978-91-7501-755-6 (print)OAI: oai:DiVA.org:kth-121344DiVA: diva2:618512
Public defence
2013-05-24, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20130503

Available from: 2013-05-03 Created: 2013-04-29 Last updated: 2013-05-03Bibliographically approved
List of papers
1. On the Symmetry Among the Diffusional Transformation Products of Austenite
Open this publication in new window or tab >>On the Symmetry Among the Diffusional Transformation Products of Austenite
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2011 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 42A, no 6, 1558-1574 p.Article in journal (Refereed) Published
Abstract [en]

Information on the diffusional transformation products of austenite in high-carbon steels is reviewed and supplemented with new microscopic studies. A comparison with transformation products in low-carbon steels indicates that there is a symmetry with pearlite in the middle, where ferrite and cementite are equal partners, and with acicular ferrite or cementite on each side. They both form with a surface relief, and at lower temperatures, each one is the leading phase in a eutectoid microstructure, bainite and inverse bainite, respectively. However, there is an asymmetry because at low temperatures bainite appears in high-carbon steels but inverse bainite never appears in low-carbon steels. At a constant high carbon content, there is another kind of symmetry, which is related to temperature. At intermediate temperatures the eutectoid reaction results in spherical nodules in which the cementite constituent originates from Widmanstatten plates. It turns spiky at both higher and lower temperatures with the leading phase in the spikes being cementite at higher temperatures and ferrite at lower temperatures. In the first kind of symmetry, there is an abrupt change among the three reaction products; in the second kind of symmetry, there is a gradual change. Accepting that all the eutectoid microstructures form by diffusion of carbon, one may explain the existence of both symmetries by the variation of the ratio of the supersaturations of ferrite and cementite with carbon content and with temperature.

Keyword
HYPEREUTECTOID STEELS; LOWER BAINITE; DECOMPOSITION; MORPHOLOGY; ALLOYS; CARBON
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-33964 (URN)10.1007/s11661-010-0539-1 (DOI)000290176100021 ()2-s2.0-79958786497 (Scopus ID)
Note
QC 20110531Available from: 2011-05-31 Created: 2011-05-23 Last updated: 2017-12-11Bibliographically approved
2. Eutectoid Transformations in 4.12 Mass Pct Cr 0.88 Mass Pct C Steel
Open this publication in new window or tab >>Eutectoid Transformations in 4.12 Mass Pct Cr 0.88 Mass Pct C Steel
2011 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 42A, no 13, 3941-3951 p.Article in journal (Refereed) Published
Abstract [en]

The sequence of eutectoid microstructures, obtained by lowering the temperature of the isothermal transformation, was studied in synthetic steel with 4.12 mass pct Cr 0.88 mass pct C. The results were compared with observations on plain carbon steels with 1.65 and 1.67 mass pct C. In both cases, the main features can be explained as an effect of a lowered temperature on the increasing supersaturation of cementite in austenite and an even stronger effect on that of ferrite. One distinction was a continuous change in the pearlite structure toward a more acicular structure. This structure is named acicular pearlite.

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-53393 (URN)10.1007/s11661-011-0794-9 (DOI)000297253100016 ()2-s2.0-83055172946 (Scopus ID)
Note
QC 20111228Available from: 2011-12-28 Created: 2011-12-28 Last updated: 2017-12-08Bibliographically approved
3. Kinetic Study of Transformations ofAustenite in a 4.12 mass% Cr 0.88 mass% C Steel
Open this publication in new window or tab >>Kinetic Study of Transformations ofAustenite in a 4.12 mass% Cr 0.88 mass% C Steel
(English)Manuscript (preprint) (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-121692 (URN)
Note

QS 2013

Available from: 2013-05-03 Created: 2013-05-03 Last updated: 2013-05-03Bibliographically approved
4. Direct Observation that Bainite can Grow Below M-S
Open this publication in new window or tab >>Direct Observation that Bainite can Grow Below M-S
Show others...
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.

Keyword
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:nbn:se:kth:diva-104592 (URN)10.1007/s11661-012-1342-y (DOI)000311511300008 ()2-s2.0-84870397030 (Scopus ID)
Funder
Vinnova
Note

QC 20130107

Available from: 2012-11-06 Created: 2012-11-06 Last updated: 2017-12-07Bibliographically approved
5. Thermodynamic Analysis of the Critical Condition for Acicular Ferrite
Open this publication in new window or tab >>Thermodynamic Analysis of the Critical Condition for Acicular Ferrite
(English)Article in journal (Other academic) Submitted
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-121705 (URN)
Note

QS 2013

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

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