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 i 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 tillstånd av Kungliga Tekniska högskolan i Stockholm framlägges till offentlig granskning för avläggande av teknologie doktorsexamen 27 september 2012, kl. 10.00 i sal F3, Lindstedsvägen 26, Kungliga Tekniska högskolan, Stockholm. © Albin Stormvinter, August 2012 Printed by Universitetsservice US-AB, Stockholm, Sweden ii To Emma iii iv 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. v vi 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 vii Appended papers I. Investigation of Lath and Plate Martensite in a Carbon Steel A. Stormvinter, P. Hedström 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. Ågren 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. Hedström 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. Hedström and A. Borgenstam. Submitted manuscript VI. On the Symmetry Among the Diffusional Transformation Products of Austenite A. Borgenstam, P. Hedström, M. Hillert, P. Kolmskog, A. Stormvinter and J. Ågren Metallurgical and Materials Transactions A, 42(6), (2010), p. 1558-1574. * The appended papers will be referred to as [1–6] in the present thesis viii 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 Hedström, Albin Stormvinter, Annika Borgenstam, Ali Gholinia, Bartlomiej Winiarski, Philip J. Withers, Oskar Karlsson and Joacim Hagström 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 Hedström 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 Hedström, and Annika Borgenstam, The TMS Annual Meeting & Exhibition 2012 (TMS2012), 11 - 15 March 2012, Orlando, USA. ix x Table of Contents Abstract ................................................................................................................................................................ v Preface ................................................................................................................................................................ vii 1 Introduction ..................................................................................................................................................... 1 1.1 Aim of the present work ........................................................................................................................ 2 2 Low temperature austenite decomposition ................................................................................................. 5 2.1 Martensite ................................................................................................................................................