The cyclic phase transformation

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Transcript of The cyclic phase transformation

The cyclic phase transformationSteels:
aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben,
voorzitter van het College voor Promoties,
in het openbaar te verdedigen op dinsdag 27 juni 2013 om 10:00 uur
door
Tianjin University, Tianjin, China
geboren te Anqing, China.
Samenstelling promotiecommissie:
Rector Magnificus voorzitter
Prof. dr. ir. S. van der Zwaag Technische Universiteit Delft, promotor
Prof. dr. G. Purdy McMaster University, Canada
Prof. dr. M. Militzer University of British Columbia, Canada
Prof. dr. J. Ågren KTH - Royal Institute of Technology, Sweden
Prof. dr. E. Gamsjager Leoben University, Austria
Prof. dr. Z. G. Yang Tsinghua University, China
Prof. dr. ir. E. Bruck Technische Universiteit Delft
Prof. dr. ir. R. Benedictus Technische Universiteit Delft, Reservelid
The research carried out in this thesis is financially funded by ArcelorMittal.
Copyright c© 2013 by Hao Chen
All rights reserved. No part of the material protected by this copyright notice may be
reproduced or utilized in any form or by any means, electronic or mechanical,
including photocopying, recording or by any information storage and retrieval
system, without the prior permission of the author.
Printed in The Netherlands by PrintPartners Ipskamp
isbn 978-94-6191-771-3
iv
Contents
2 The cyclic phase transformation concept 7
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3 The kinetics of cyclic phase transformations in a lean Fe-C-Mn alloy 27
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.3 Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.3.2 Paraequilibrium model . . . . . . . . . . . . . . . . . . . . . . . . 31
3.4 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.4.2 Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4 Analysis of the stagnant stage during cyclic phase transformations 47
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.3.1 Fe-C alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.3.2 Fe-C-Mn alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.3.4 Fe-C-Mn-M (M= Ni, Si, Co) alloys . . . . . . . . . . . . . . . . . . 58
4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5 Indirect evidence for the existence of an interfacial Mn Spike 63
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.3.1 Effect of Mn concentration . . . . . . . . . . . . . . . . . . . . . . 70
5.3.2 Effect of the number of cycles prior to final cooling . . . . . . . . 73
5.3.3 Creating 2 Mn spikes to create 2 growth retardation stages . . . . 79
5.3.4 Linking growth retardation to a physical location of Mn spikes . 82
5.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
7.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
7.3 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
7.3.2 Interface friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
7.4 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
7.5 Theoretical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
8 Transformation stasis during the isothermal bainitic ferrite formation in
Fe-C-X alloys 129
8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
8.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
A The effect of transformation path on stagnant stage 163
A.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
A.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
A.4 conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
B A mixed mode model with covering soft impingement effect 175
B.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
B.2 Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
B.2.2 The mixed-mode model . . . . . . . . . . . . . . . . . . . . . . . . 180
B.3 Numerical calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
B.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
1.1 Phase transformations in steels
While steel has a history covering serval thousands of years, it is still one of the
most important structural materials in practical applications nowadays. Like many
other materials, the mechanical properties of steel are determined by its microstructure
and composition. However, due to the versatility in its microstructure the mechanical
properties of steel are much more adjustable than those of other materials. The versatile
microstructures in steel are obtained via the transformation of the iron lattice from face
centered cubic (FCC) to body centered cubic (BCC). During the lattice transformation,
there is also redistribution of carbon or other alloying elements between these two iron
lattices, which also influences the mechanical properties. In order to precisely tune the
mechanical properties of steel, it is required to deeply understand the mechanism of
the FCC to BCC transformation in steel.
In metallurgy, the FCC iron is termed “Austenite ”, which is thermodynamically
stable at elevated temperatures and enriched in carbon. The temperature A3 above
which only the austenite is stable is determined by the composition of the steel, and for
common steel grades A3 is between 727 C and 912 C. During a typical heat treatment,
the steel is first heated up to a temperature higher than A3 for austenization, and then
cooled down for the FCC to BCC transformation. Upon cooling the morphology
2 Chapter 1. Introduction
and carbon content of the BCC iron formed can vary significantly. Two BCC iron
microstructures are of interest here: (i) Allotriomorphic ferrite. Allotriomorphic ferrite
grains are equiaxed, and mainly grow from the austenite grain boundaries at relatively
high temperatures. It is also called “grain boundary ferrite ”. In this thesis, the
allotriomorphic ferrite will be called “ferrite ”for simplicity. The transformation from
austenite to ferrite is a time-dependent reconstructive reaction which requires large
scale displacement of the iron and carbon atoms, and the carbon will be rejected by
ferrite and diffuse into austenite due to the low carbon solubility in ferrite. From
a thermodynamical point of view the substitutional alloying elements should also
diffuse between austenite and ferrite to minimize the Gibbs energy. However, from a
kinetical point of view the substitutional alloying elements can not take part in long
range diffusion during the transformation due to their low diffusivities. It is generally
accepted that the rate of austenite to ferrite transformation is controlled by carbon
diffusion, and the chemical driving force is only dissipated by the diffusion process [1].
However, some recent studies qualitatively indicate that the transformation rate is also
influenced by the interface mobility [2–6] and partitioning of substitutional alloying
elements [7–13]; (ii) Bainite (bainitic ferrite). Bainitic ferrite is BCC iron with an non-
polygonal microstructure that forms in steels upon cooling to medium temperatures.
The mechanism of bainitic transformation is still heavily disputed [14–33], and two
competitive views: (a) the mechanism of bainitic ferrite formation is the same as that of
ferrite although their morphologies are totally different [34–39]. During the formation
of a bainitic ferrite plate, it is perceived that the carbon has to diffuse away from bainitic
ferrite to austenite, while the substitutional alloying elements do not partition. The
growth rate of bainitic ferrite is only determined by carbon diffusion ; (b) the bainitic
transformation is considered to be a diffusionless process [17, 20, 40, 41]. During the
growth of a bainitic plate there is no need for carbon diffusion, but carbon diffusion
may take place after the growth.
Generally speaking, during phase transformations there are two processes: nucle-
ation and growth. The phase transformation starts by the nucleation of the new phase,
1.1. Phase transformations in steels 3
and then the newly developed phase interfaces migrate into the parent phases. Even
with the most modern techniques, the nucleation process can not be measured directly
and precisely, thus its mechanism is still not very clear. The growth stage of phase
transformations in steels has been studied widely both experimentally and theoreti-
cally [13, 20, 30, 34–36, 38, 39, 41–52]. Despite abundant effort that has been paid in the
past, some key questions, in particular the role of alloying elements on the kinetics
of the moving interfaces and on the transformation kinetics, are not yet fully solved.
In this thesis, the kinetics of austenite decomposition into ferrite at high temperatures
and that into bainitic ferrite at medium temperatures in low alloyed steels are of in-
terest, and effort will be paid to improve the understanding of growth mechanism of
these two transformations. It has been found in the literature that the austenite to
ferrite transformation in low alloyed steels can be roughly described by the classical
diffusional models [1, 13], however, the fine details, like the degree of partitioning of
substitutional alloying elements [7, 8, 10, 13] and the exact value of interface mobil-
ity [53], are still disputed and needs clarification for improving the diffusional models.
Much more effort is also required to discriminate the existing views on the mechanism
of bainitic transformation.
Up to now, the kinetics of ferrite and banite formation are studied in conventional
isothermal or continual cooling experiments. In such experiments nucleation of new
grains and their growth occur simultaneously, and the unknown parameters such as
the spatial density and distribution of the nucleation sites and the variation in the
rate of nucleation during the total transformation, have a very large effect on the de-
tails of the growth reconstructed from the overall transformation curve. Therefore,
new experimental approaches are indeed required to clarify the fine details of growth
mechanism.
1.2 Content of this thesis
In this thesis, two new experimental approaches, the cyclic partial phase transformation
experiments at high temperatures and interrupted cooling experiments at medium
temperatures, are designed to study the growth kinetics of the austenite to ferrite and
bainitic transformation more accurately. The new experimental results are used to
discriminate between existing phase transformation models.
In Chapter 2, the cyclic partial phase transformation concept to study the austenite
to ferrite and the ferrite to austenite growth, is described in detail. The mixed mode
model and classical diffusion controlled growth model have been reformulated to
the conditions of the cyclic phase transformations, and then the models are applied to
simulate the cyclic phase transformations in a Fe-C alloy. Finally, a comparison between
the mixed-mode model and diffusional model is made, and the effect of interface
mobility on the transformation kinetics is discussed. In order to discriminate between
Paraequilibrium [54,55] and Local Equilibrium [56,57] models, a series of cyclic partial
phase transformation experiments in Fe-C-Mn alloys have been designed in Chapter
3. Interesting new transformation stages are observed and reported. The modeling
results are compared with the experimental results in details, and the effect of Mn on
the transformation kinetics is discussed. In Chapter 4, the cyclic phase transformations
in a series of Fe-C, Fe-C-M(M=Mn, Ni, Cu, Si,Co) and Fe-C-Mn-M(M= Ni, Si,Co) alloys
are simulated by Local Equilibrium model to illuminate the effect of alloying element
on the length of the stagnant stage newly discovered in Chapter 3. The effect of
heating/cooling rate on the length of stagnant stage is also investigated for an Fe-Mn-C
alloy. A series of experiments are designed in Chapter 5 to prove the existence of the
residual Mn spike after the cyclic partial phase transformations in Fe-Mn-C alloys,
which is theoretically predicted in Chapter 3. The effect of residual Mn spikes on
the austenite to ferrite transformation kinetics during the final cooling of the cyclic
experiments is systematically investigated. Chapter 6 presents the in-situ observations
of interface migration during the cyclic phase transformations in a Fe-C-Mn alloy.
1.2. Content of this thesis 5
The directly measured interface velocities for the austenite to ferrite and vice versa
are compared with predictions by Paraequilibrium and Local Equilibrium models.
In Chapter 7 a series of interrupted cooling experiments at medium transformation
temperatures are designed to study the nature of the bainitic transformation in low
alloyed steels. A so called Gibbs energy balance approach is proposed to theoretically
explain the newly observed features in the interrupted cooling experiments. In Chapter
8, the Gibbs energy balance approach is applied to model the transformation stasis
phenomena in a series of Fe-C-X alloys. The Gibbs energy balance model predictions
are compared with those of the T0 concept, and the physical origin of the occurrence
of transformation stasis is discussed. The main finding as reported in this thesis are
reported in the Summary. In addition to the findings as reported in the thesis chapters,
some of the additional transformation research results are reported in two appendices.
6 Chapter 1. Introduction
concept
This chapter is based on
• H Chen, S van der Zwaag, Application of the cyclic phase transformation
concept for investigating growth kinetics of solid-state partitioning phase
transformations, Comp Mater Sci, 2010; 49:801-813.
2.1 Introduction
Both the austenite (γ) to ferrite (α) and the ferrite to austenite phase transforma-
tions in iron-based alloys are of great interest in the steel production, as the final
microstructure of products and their properties are determined by these solid-state
phase transformations. They have been widely investigated from both an experi-
mental and a theoretical perspectives. For more details about the austenite to fer-
rite [3–5, 8, 11, 42, 47, 50, 51, 58–69] and the ferrite to austenite phase transformations,
see Ref. [70–77]. Despite abundant efforts that has been paid in the past, the kinetics
of these two phase transformations is still not understood well [13].
Generally, integral models for both the austenite to ferrite and the ferrite to austenite
8 Chapter 2. The cyclic phase transformation concept
phase transformations involve two parts: nucleation and growth [78]. As for nucle-
ation,…