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Formation of Bainite in Steels
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
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 [en]
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: urn:nbn:se:kth:diva-207596ISBN: 978-91-7729-391-0 OAI: oai:DiVA.org:kth-207596DiVA, id: diva2:1097218
Public defence
2017-06-14, B2, Brinellvägen 23, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20170523

Available from: 2017-05-23 Created: 2017-05-22 Last updated: 2017-05-23Bibliographically approved
List of papers
1. Morphology of Proeutectoid Ferrite
Open this publication in new window or tab >>Morphology of Proeutectoid Ferrite
2017 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 48A, no 3, p. 1425-1443Article in journal (Refereed) Published
Abstract [en]

The morphology of grain boundary nucleated ferrite particles in iron alloys with 0.3 mass pct carbon has been classified according to the presence of facets. Several kinds of particles extend into both grains of austenite and have facets to both. It is proposed that they all belong to a continuous series of shapes. Ferrite plates can nucleate directly on the grain boundary but can also develop from edges on many kinds of particles. Feathery structures of parallel plates on both sides of a grain boundary can thus form. In sections, parallel to their main growth direction, plates have been seen to extend the whole way from the nucleation site at the grain boundary and to the growth front. This happens in the whole temperature range studied from 973 K to 673 K (700 A degrees C to 400 A degrees C). The plates thus grow continuously and not by subunits stopping at limited length and continuing the growth by new ones nucleating. Sometimes, the plates have ridges and in oblique sections they could be mistaken for the start of new plates. No morphological signs were observed indicating a transition between Widmanstatten ferrite and bainitic ferrite. It is proposed that there is only one kind of acicular ferrite. (C) The Author(s) 2016. This article is published with open access at Springerlink.com

Place, publisher, year, edition, pages
Springer, 2017
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-204065 (URN)10.1007/s11661-016-3903-y (DOI)000394214300042 ()2-s2.0-85008477493 (Scopus ID)
Note

QC 20170330

Available from: 2017-03-30 Created: 2017-03-30 Last updated: 2017-06-29Bibliographically approved
2. Second Stage of Upper Bainite in a 0.3 Mass Pct C Steel
Open this publication in new window or tab >>Second Stage of Upper Bainite in a 0.3 Mass Pct C Steel
2017 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 48A, no 3, p. 1444-1458Article in journal (Refereed) Published
Abstract [en]

Upper bainite forms in at least two stages, the formation of parallel plates of ferrite and the transformation of the interspaces to a mixture of cementite and ferrite. The first stage was examined in a preceding metallographic study of the formation of ferrite in hypoeutectoid steels and the second stage, which is initiated by the occurrence of cementite in the interspaces, is the subject of the present study. The alloy from the preceding study will also be used here. The band of austenite in the interspaces between parallel plates is generally transformed by a degenerate eutectoid transformation when this band is thin. When it is thicker, it will transform by a more cooperative growth mechanism and result in a eutectoid colony, often with cementite platelets. A series of sketches are presented which illustrate in detail how the second stage of upper bainite progresses according to the present observations. The cooperative manner did not increase as the temperature was lowered because the tendency to form plates of ferrite was still increasing at lower temperatures, making the interspaces too narrow for the cooperative reaction to dominate over the formation of fine plates of ferrite. (C) The Author(s) 2016. This article is published with open access at Springerlink.com

Place, publisher, year, edition, pages
Springer, 2017
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-204066 (URN)10.1007/s11661-016-3902-z (DOI)000394214300043 ()2-s2.0-85008511996 (Scopus ID)
Note

QC 20170330

Available from: 2017-03-30 Created: 2017-03-30 Last updated: 2017-06-29Bibliographically approved
3. C-curves for lengthening of Widmanstätten and bainitic ferrite
Open this publication in new window or tab >>C-curves for lengthening of Widmanstätten and bainitic ferrite
(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).

Keywords
C-curve; lengthening; Widmanstätten; bainite; acicular
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-207640 (URN)
Note

QC 20170523

Available from: 2017-05-23 Created: 2017-05-23 Last updated: 2017-05-23Bibliographically approved
4. Morphology of Upper and Lower Bainite in a 0.7 Mass Pct C Steel
Open this publication in new window or tab >>Morphology of Upper and Lower Bainite in a 0.7 Mass Pct C Steel
(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
Morphology, Upper bainite, lower bainite, Fe-C, Steels.
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-207642 (URN)
Note

QC 20170523

Available from: 2017-05-23 Created: 2017-05-23 Last updated: 2017-05-23Bibliographically approved
5. Widening of Laths in Bainite
Open this publication in new window or tab >>Widening of Laths in Bainite
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Units of bainite in Fe-C alloys from the upper temperature range inherit their shape from Widmanstätten plates of ferrite which are lath-like. The thickness increases by long-range diffusion of carbon and the length by short range diffusion of carbon from the advancing edge of the tip. Both have been studied extensively and are fairly well understood. Widening growth seems to have been much neglected but a study of some aspects of widening is now presented. The present report is the last one in a series of four morphological studies of bainite, isothermally formed in Fe-C alloys with 0.3 or 0.7 mass pct carbon, mainly in the upper temperature range.  It contains a number of morphological observations made on cross sections of packets of bainite. They elucidated a number of interesting questions about bainite and resulted in some proposals. The ferrite plates in a packet are nucleated as a group on a grain boundary, not each one separately on the side of a prior plate. Lengthening occurs by advancement of a short edge that is formed in close contact to the grain boundary. Widening of laths does not start spontaneously. It is initiated by a modification of the structure of the long edge of the lath. When it then moves, the lattice of the new ferrite is rotated relative to the ferrite formed by lengthening and the habit plane is different. In a section through the length direction it is difficult to recognize what part of ferrite has formed by widening growth. Furthermore, it is proposed that the individual plates in a microstructure, previously used to illustrate subunits formed by repeated nucleation, were nucleated on a hidden grain boundary.  

Keywords
Bainite, Widening, Morphology, Fe-C, Steels
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-207738 (URN)
Note

QC 20170523

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

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