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Morphology of Upper and Lower Bainite in a 0.7 Mass Pct C Steel
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.ORCID iD: 0000-0003-2766-976X
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
(English)Manuscript (preprint) (Other academic)
Abstract [en]

There has been an on-going discussion on the difference in formation mechanisms of upper and lower bainite. Various suggestions have been supported by reference to observed morphologies and illustrated with idealized sketches of morphologies. In order to obtain a better basis for discussions about the difference in mechanism, the morphology of bainite in an Fe-C alloy with 0.7 mass pct carbon was now studied in some detail from 823 K (550 °C) to 548 K (275 °C) at temperature intervals of 50 K or less. The work focused on bainite seen to start from a grain boundary in the plane of polish and showing an advancing tip in the remaining austenite.  The results indicate that there is no essential difference with temperature regarding the ferritic skeleton of feathery bainite. The second stage of bainite formation, which involves the formation of both ferrite and cementite, was regarded as a eutectoid transformation and the resulting morphologies were analyzed in terms of two modes, degenerate and cooperative eutectoid transformation. There was no sharp difference between upper and lower bainite. Ways to define the difference were discussed.

Keywords [en]
Morphology, Upper bainite, lower bainite, Fe-C, Steels.
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-207642OAI: oai:DiVA.org:kth-207642DiVA, id: diva2:1097851
Note

QC 20170523

Available from: 2017-05-23 Created: 2017-05-23 Last updated: 2017-05-23Bibliographically approved
In thesis
1. Formation of Bainite in Steels
Open this publication in new window or tab >>Formation of Bainite in Steels
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A systematic survey of morphology of bainite and proeutectoid ferrite was carried out in order to validate some old thoughts of bainite transformation mechanism. It is confirmed that there is no morphological evidence supporting a sharp change neither between Widmanstätten ferrite and the ferritic component of upper bainite, nor between upper and lower bainite. Both Widmanstätten ferrite and upper bainite start with precipitation of ferrite plates at a grain boundary while lower bainite starts with intragranular nucleation. In case of grain boundary nucleation, a group of parallel plates with same crystallographic orientation to the parent austenite grain forms. This process is followed by a second stage of decomposition of the austenitic interspace, which remained in between the primary ferrite plates. At high temperature, the austenitic interspace would either retain as thin slabs or transform into pearlite through a nodule originated from a grain boundary. At lower temperature, cementite precipitation starts to be possible and initiates simultaneous growth of ferrite. Generally, there are two modes of such eutectoid reactions operating in the second stage, i.e. a degenerate and a cooperative mode, which would lead to typical upper and lower bainite, respectively, in definition of carbides morphology. Both upper and lower bainite according to this definition are observed in a wide temperature range. A sharp temperature between the upper and lower bainite structures thus exists only when the definition is based on their nucleation sites, i.e. grain boundary nucleation for upper bainite and intragranular nucleation for lower bainite. Supposing that the first stage is a diffusionless process it should have a high growth rate to prevent carbon diffusion. This is not supported by lengthening rate obtained in current study as well as data from literature for Fe-C alloys. Finally, it is shown that the “subunits” play no role in the lengthening process of bainite.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. p. 54
Keywords
Fe-C alloys, Bainitic transformation, Proeutectoid ferrite, Upper bainite, Lower bainite, Morphology, Steels.
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-207596 (URN)978-91-7729-391-0 (ISBN)
Public defence
2017-06-14, B2, Brinellvägen 23, Stockholm, 10:00 (English)
Opponent
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Note

QC 20170523

Available from: 2017-05-23 Created: 2017-05-22 Last updated: 2017-05-23Bibliographically approved

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