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    Low Temperature Austenite Decomposition

    in Carbon Steels

    Albin Stormvinter

    Doctoral Thesis

    KTH Royal Institute of Technology

    School of Industrial Engineering and Management

    Department of Materials Science and Engineering

    Division of Physical Metallurgy

    SE-10044 Stockholm, Sweden

    Stockholm 2012

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    Albin Stormvinter Low Temperature Austenite Decomposition in Carbon Steels

    KTH Royal Institute of Technology,

    School of Industrial Engineering and Management,

    Department of Materials Science and Engineering,

    Division of Physical Metallurgy,

    SE-10044 Stockholm,

    Sweden

    ISRN KTH/MSE--12/19--SE+METO/AVH

    ISBN 978-91-7501-449-4

    Akademisk avhandling som med tillstnd av Kungliga Tekniska hgskolan i Stockholm framlgges

    till offentlig granskning fr avlggande av teknologie doktorsexamen 27 september 2012, kl. 10.00 i

    sal F3, Lindstedsvgen 26, Kungliga Tekniska hgskolan, Stockholm.

    Albin Stormvinter, August 2012

    Printed by Universitetsservice US-AB, Stockholm, Sweden

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    To Emma

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    Abstract

    Martensitic steels have become very important engineering materials in modern society. Crucial parts

    of everyday products are made of martensitic steels, from surgical needles and razor blades to car

    components and large-scale excavators. Martensite, which results from a rapid diffusionless phase

    transformation, has a complex nature that is challenging to characterize and to classify. Moreover the

    possibilities for modeling of this phase transformation have been limited, since its thermodynamics

    and kinetics are only reasonably well understood. However, the recent development of

    characterization capabilities and computational techniques, such as CALPHAD, and its applicability

    to ferrous martensite has not been fully explored yet.

    In the present work, a thermodynamic method for predicting the martensite start temperature (Ms)

    of commercial steels is developed. It is based mainly on information on Ms from binary Fe-X

    systems obtained from experiments using very rapid cooling, and Ms values for lath and plate

    martensite are treated separately. Comparison with the experimental Ms of several sets of commercial

    steels indicates that the predictive ability is comparable to models based on experimental information

    of Ms from commercial steels.

    A major part of the present work is dedicated to the effect of carbon content on the morphological

    transition from lath- to plate martensite in steels. A range of metallographic techniques were

    employed: (1) Optical microscopy to study the apparent morphology; (2) Transmission electron

    microscopy to study high-carbon plate martensite; (3) Electron backscattered diffraction to study the

    variant pairing tendency of martensite. The results indicate that a good understanding of the

    martensitic microstructure can be achieved by combining qualitative metallography with quantitative

    analysis, such as variant pairing analysis. This type of characterization methodology could easily be

    extended to any alloying system and may thus facilitate martensite characterization in general.

    Finally, a minor part addresses inverse bainite, which may form in high-carbon alloys. Its coupling to

    regular bainite is discussed on the basis of symmetry in the Fe-C phase diagram.

    Keywords: Carbon steels, Electron backscattered diffraction, Martensite, Microscopy,

    Microstructure, Thermodynamic modeling.

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    Preface

    In the present doctoral thesis I would like to share my knowledge gained during the past four years,

    as a PhD student in physical metallurgy at the Department of Materials Science and Engineering at

    KTH Royal Institute of Technology, Sweden. The work has mainly been carried out within the

    martensite formation project (Supplements 1-5) which runs within the Hero-m research center,

    although Supplement 6 addresses inverse bainite and hence belongs to the bainite formation project

    within Hero-m. A large part of the characterization work on martensite is the result of two research

    stints conducted by the present author: at INP-Grenoble (France) in the spring of 2010 and at

    Tohoku University (Sendai, Japan) in the summer of 2011. In particular this doctoral thesis will

    discuss the following challenges, which are related to low temperature austenite decomposition in

    carbon steels:

    Thermodynamically based prediction of the martensite start temperature (Ms)

    Quantitative microstructure characterization: The effect of carbon content on the martensite

    morphology with emphasis on the transition from lath to plate martensite.

    Inverse bainite in hypereutectoid Fe-C alloys.

    The outline of this thesis is as follows; In the first chapter, introduction, the topic low temperature

    austenite decomposition and the aims and purposes related to the three challenges listed above are

    introduced. In Chapters 2-4 my intention has been to condense selected works on three important

    subjects related to this work: Phase transformations, Characterization techniques, and

    Thermodynamics. I make no claim that these chapters would advance the understanding of low

    temperature austenite decomposition further; instead they should be viewed as either an introduction

    to the different topics for the novice or as an interpretation of the dense literature on the subjects

    done by the present author. The readers are referred to the cited works for more details. The expert

    reader may in fact skip Chapters 2-4 and move on to Chapters 5-7 where I summarize the results

    from the supplements and discuss them in relation to existing literature.

    Albin Stormvinter

    Stockholm, 2012-08-01

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    Appended papers

    I. Investigation of Lath and Plate Martensite in a Carbon Steel

    A. Stormvinter, P. Hedstrm and A. Borgenstam

    Solid State Phenomena, 172-174, (2011), p. 61-66.

    II. Thermodynamically Based Prediction of the Martensite Start Temperature for Commercial

    Steels

    A. Stormvinter, A. Borgenstam and J. gren

    Metallurgical and Materials Transactions A, Online first, 2012,

    DOI: 10.1007/s11661-012-1171-z

    III. A transmission electron microscopy study of plate martensite formation in high-carbon low

    alloy steels

    A. Stormvinter, P. Hedstrm and A. Borgenstam

    Submitted manuscript

    IV. Effect of Carbon Content on the Orientation Relationship between Austenite and bct-

    Martensite in Fe-C Alloys resolved by Electron Backscattered Diffraction

    A. Stormvinter, G. Miyamoto, T. Furuhara, and A. Borgenstam

    Submitted manuscript

    V. Effect of Carbon Content on the Variant Pairing of Martensite in Fe-C alloys

    A. Stormvinter, G. Miyamoto, T. Furuhara, P. Hedstrm and A. Borgenstam.

    Submitted manuscript

    VI. On the Symmetry Among the Diffusional Transformation Products of Austenite

    A. Borgenstam, P. Hedstrm, M. Hillert, P. Kolmskog, A. Stormvinter and J. gren

    Metallurgical and Materials Transactions A, 42(6), (2010), p. 1558-1574.

    * The appended papers will be referred to as [16] in the present thesis

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    My contribution to the appended papers

    I. Major part of: literature survey, experimental work, and writing.

    II. Major part of: literature survey, modeling work, and writing.

    III. Major part of: literature survey, experimental work, analysis, and writing.

    IV. Major part of: literature survey, experimental work, analysis, and writing.

    V. Major part of: literature survey, experimental work, analysis, and writing.

    VI. Part of experimental work, contributed to discussions and took part in the writing.

    Other related reports not included in the thesis

    A. On the Three-Dimensional Microstructure of Martensite in Carbon Steels

    Peter Hedstrm, Albin Stormvinter, Annika Borgenstam, Ali Gholinia, Bartlomiej Winiarski,

    Philip J. Withers, Oskar Karlsson and Joacim Hagstrm

    Proceedings of the International Conference on 3D Materials Science 2012.

    Parts of this work have been presented at the following international conferences

    1. Investigation of the Transition from Lath to Plate Martensite in Fe-C,

    Albin Stormvinter, Peter Hedstrm and Annika Borgenstam,

    Solid-Solid Phase Transformations in Inorganic Materials (PTM2010),

    6 - 11 June 2010, Avignon, France.

    2. A Phenomenological Model for the Prediction of Martensite Start Temperature in Steels,

    Albin Stormvinter and Annika Borgenstam,

    The TMS Annual Meeting & Exhibition 2011 (TMS2011),

    27 February - 3 March 2011, San Diego, USA.

    3. Thermodynamically Based Prediction of the Ms Temperature for Commercial Steels,

    Albin Stormvinter and Annika Borgenstam,

    International Conference on Martensitic Transformations 2011 (ICOMAT2011),

    4 - 9 September 2011, Osaka, Japan.

    4. Martensitic Meso- and Nanostructures in High-Carbon Low-Alloyed Steels,

    Albin Stormvinter, Peter Hedstrm, and Annika Borgenstam,

    The TMS Annual Meeting & Exhibition 2012 (TMS2012),

    11 - 15 March 2012, Orlando, USA.

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    Table of Contents

    Abstract ................................................................................................................................................................ v