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C-curves for lengthening of Widmanstätten and bainitic ferrite
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.
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]

Widmanstätten ferrite and bainitic ferrite are both acicular and their lengthening rate in binary Fe-C alloys and low alloyed steels under isothermal conditions is studied by searching the literature and through new measurements. As a function of temperature it can be represented by a common curve for both kinds of acicular ferrite in contrast to the separate C-curves often presented in TTT diagrams. The curves for alloys with low carbon content show no obvious decrease in rate at low temperatures down to 623 K (350 °C). For alloys with higher carbon content than 0.5 mass%, the expected decrease of rate as function of temperature below a nose was observed. An attempt to explain the absence of a nose for low carbon contents by an increasing deviation from local equilibrium at high growth rates is presented. It is based on a very simple kinetic model, which predicts that the rates for Fe-C alloys with less than 0.3 mass% carbon are high enough to result in an increasing deviation from local equilibrium on decreasing temperature, starting at about 773 K (500oC).

Keyword [en]
C-curve; lengthening; Widmanstätten; bainite; acicular
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-207640OAI: oai:DiVA.org:kth-207640DiVA, id: diva2:1097850
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
Keyword
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)
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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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