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Martensitic Transformation PDF

472 Pages·1978·10.502 MB·English
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MATERIAL S SCIENC E A N D TECHNOLOG Y EDITORS ALLEN M. ALPER A. S. NOWICK GTE Sylvania Inc. Henry Krumb School of Mines Precision Materials Group Columbia University Chemical & Metallurgical Division New York, New York Towanda, Pennsylvania A. S. Nowick and B. S. Berry, ANELASTIC RELAXATION IN CRYSTALLINE SOLIDS, 1972 E. A. Nesbitt and J. H. Wernick, RARE EARTH PERMANENT MAGNETS, 1973 W. E. Wallace, RARE EARTH INTERMETALLICS, 1973 J. C. Phillips, BONDS AND BANDS IN SEMICONDUCTORS, 1973 /. H. Richardson and R. V. Peterson (editors), SYSTEMATIC MATERIALS ANALYSIS, VOLUMES I, II, AND III, 1974; IV, 1978 A.J. Freeman and J. B. Darby, Jr. (editors), THE ACTINIDES: ELECTRONIC STRUC­ TURE AND RELATED PROPERTIES, VOLUMES I AND II, 1974 A. S. Nowick and J. J. Burton (editors), DIFFUSION IN SOLIDS: RECENT DEVELOP­ MENTS, 1975 J. W. Matthews (editor), EPITAXIAL GROWTH, PARTS A AND B, 1975 J. M. Blakely (editor), SURFACE PHYSICS OF MATERIALS, VOLUMES I AND II, 1975 G. A. Chadwick and D. A. Smith (editors), GRAIN BOUNDARY STRUCTURE AND PROPERTIES, 1975 John W. Hastie, HIGH TEMPERATURE VAPORS: SCIENCE AND TECHNOLOGY, 1975 John K. Tien and George S. Ansell (editors), ALLOY AND MICROSTRUCTURAL DESIGN, 1976 Μ. T. Sprackling, THE PLASTIC DEFORMATION OF SIMPLE IONIC CRYSTALS, 1976 James J. Burton and Robert L. Garten (editors), ADVANCED MATERIALS IN CATALYSIS, 1977 Gerald Burns, INTRODUCTION TO GROUP THEORY WITH APPLICATIONS, 1977 L. H. Schwartz and J. B. Cohen, DIFFRACTION FROM MATERIALS, 1977 Paul Hagenmuller and W. van Gool, SOLID ELECTROLYTES: GENERAL PRINCIPLES, CHARACTERIZATION, MATERIALS, APPLICATIONS, 1978 Zenji Nishiyama, MARTENSITIC TRANSFORMATION, 1978 In preparation G. G. Libowitz and M. S. Whittingham, MATERIALS SCIENCE IN ENERGY TECH­ NOLOGY Martensiti c Transformatio n Zenji Nishiyama Fundamental Research Laboratories Nippon Steel Corporation Kawasaki, Japan Edited by Morris E. Fine Departmento f Material s Scienc e an d Engineerin g Northwester n Universit y Evanston, Illinoi s M. Meshii Departmento f Material s Scienc e an d Engineerin g Northwester n Universit y Evanston, Illinoi s C. M. Wayman Departmento f Metallurg y an d Minin g Engineerin g Universityo f Illinoi sa t Urbana-Champaig n Urbana, Illinoi s ACADEMIC PRES S New York San Francisco London 1978 A Subsidiar yo f Harcour t Brac e Jovanovich , Publisher s COPYRIGHT © 1978, BY ACADEMIC PRESS, INC. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER. ACADEMIC PRESS, INC. HI Fifth Avenue, New York, New York 10003 United Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road, London NW1 Library of Congress Cataloging in Publication Data Main entry under title: Martensitic transformation. (Materials science and technology series) Includes bibliographical references. 1. Martensitic transformations. 2. Crystallography. I. Nishiyama, Zenji, Date TN690.M2662 669'.94 77-24960 ISBN 0-12-519850-7 PRINTED IN THE UNITED STATES OF AMERICA First original Japanese language edition published by Maruzen Co., Ltd. Tokyo, 1971. Preface to English Edition The text of this edition has been revised somewhat to include new in­ formation which became available after the publication of the original book. When appropriate, some material has been deleted. The translation was prepared by: Dr. S. Sato, Hokkaido University; Dr. I. Tamura, Kyoto University; Dr. S. Nenno, Dr. H. Fujita, Dr. K. Shimizu, Dr. K. Otsuka, Dr. H. Kubo, and Mr. T. Tadaki, Osaka Univer­ sity; Dr. M. Oka, Tottori University; Dr. S. Kajiwara, National Research Institute for Metals; Dr. T. Inoue, Dr. M. Matsuo, and Dr. I. Yoshida, Fundamental Research Institute, Nippon Steel Corporation. The English translation was edited by Dr. Morris E. Fine and Dr. M. Meshii, Northwestern University and Dr. C. M. Wayman, University of Illinois. The author would like to express his sincere appreciation to the translators and the technical editors. ix Preface to Japanese Edition The martensitic transformation is an important phenomenon which con­ trols the mechanical properties of metallic materials and has been studied extensively in the past. At first, the studies were made mainly by optical microscopy, and the high degree of hardness of the martensite in steels was interpreted as being due to its fine microstructure. Without inquiry into its fundamental nature, the martensitic transformation was explained chiefly from the thermodynamical point of view, and it seemed in those days that the theory was reasonably well established. Subsequently, with advances in research techniques, e.g., x-ray diffraction and electron microscopy, the structures of various martensites were determined and the presence of sub­ structures such a^ arrays of lattice defects was established. New views of martensitic transformation have been developed that consider the new ex­ perimental facts. The author considered it timely to summarize the more recent research results on martensite and undertook the writing of this book. Because of the emphasis on phenomena, the presentation is based on the known crystallographical data and, accordingly, some readers may not be familiar with this approach. Therefore, an elementary description of the martensite transformation that may also be regarded as a summary is given in Chapter 1. This chapter is written in terms as elementary as possible and, though it lacks strictness, even the beginner or nonprofessional will be able to appreciate the organization of this book. The main thrust of the book begins with Chapters 2 and 3, in which crystallographic data are given in detail. Chapter 4 deals with thermodynamical problems and kinetics and Chapter 5 with conditions for the nucleation of martensite and problems concerning stabilization of austenite. The last chapter discusses the theory of the mechanism of the martensitic transformation. xi xii Preface t o Japanes e editio n The text is arranged according to phenomena; thus, data for a certain material are scattered throughout and may be difficult to locate. To over­ come this inconvenience, the alloys are given in terms of element-element in the index. The frank opinions of the author may, in some instances, be dogmatic or prejudiced. For the reader who may doubt the author's opinions or other descriptions and for the reader who may want to study the subject in more detail, all references known to the author are included. Nevertheless, some important papers may have been unintentionally omitted. The author would very much like to be informed of such papers. The author is planning to write a second book concerning other problems associated with martensite, e.g., the massive transformation, the bainitic transformation, the tempering of martensite, and the hardening mechanism in martensite. The author is indebted to the support given him by the Fundamental Research Laboratories, Nippon Steel Corporation, and especially for the encouragement of Academician S. Mizushima, Honorable Director, and Dr. T. Otake, Director of the Laboratories. In preparing the manuscript many valuable data were offered by foreign and domestic researchers. The author wishes to acknowledge them: The author wishes to express his thanks to his friends and colleagues for their kindness in reading and correcting the manuscript: Professor S. Sato, Hokkaido University; Professors I. Tamura and N. Nakanishi, Kyoto University; Professor Y. Shimomura, University of Osaka Prefecture, Professors F. E. Fujita, S. Nenno, H. Fujita, and K. Shimizu, Osaka Uni­ versity; Dr. S. Kajiwara, National Research Institute for Metals; and Mr. K. Sugino and Mr. H. Morikawa, Fundamental Research Laboratories, Nippon Steel Corporation. Further, the author expresses his gratitude to Professor J. Takamura, Kyoto University, for his valuable advice. This book contains the experimental data obtained by the author and his colleagues at the Institute of Iron and Other Metals, Tohoku University, and at the Institute of Scientific and Industrial Research, Osaka University. The author expresses his appreciation for the research opportunities in these institutions. 1 Introduction to Martensite and Martensitic Transformation Compared with that obtained by slow cooling, iron-carbon steel quenched from a high temperature has a very fine and sharp microstructure and is much harder. The mechanical properties and structure of quenched steels have long been studied because of their technological importance. The struc­ ture of quenched steel is called martensite in honor of Professor A. Martens, the famous pioneer German metallographer who greatly extended Sorby's initial work. Initially, the term was ambiguously adopted to denote the microstructure of hardened but untempered steels. As the essential proper­ ties of quenched steel have become better known, the meaning of the word has been gradually clarified as well as extended to nonferrous alloys in which similar characteristics occur. Although the term martensite has oc­ casionally been used somewhat ambiguously, there exists a critical restric­ tion on the use of the word. A substance's structure must have certain definite properties in order to be called martensitic structure; similarly, a phase transformation must have certain properties in order to be called a martensitic transformation. It is the object of this chapter to define marten­ site and martensitic transformations. We shall take up first the basic properties of martensite in steels, par­ ticularly in carbon steels, and then discuss what martensite is in a wider sense. 1 2 1 Introduction (a) (b) FIG. 1.1 (a) Body-centered cubic lattice (a iron), (b) Face-centered cubic lattice (γ iron). 1.1 Martensite in carbon steels 1.1.1 Allotropic transformations in iron In order to discuss martensitic transformations in steel, we must consider first the allotropic transformation of elemental iron. Iron changes phase in the sequence a->/J->y -»<5on heating. Alpha iron, which is the stable phase at room temperature, has the atomic arrangement shown in Fig. 1.1a, which depicts a unit cell of the body-centered cubic (b.c.c.) lattice, in which the atoms lie at the corners and body center of a cube. On heating to 790°C iron changes to the β phase, which has the same b.c.c. structure as α iron. The sole distinction is that α iron is ferromagnetic whereas β iron is para­ magnetic. Since the magnetic change is not a change in crystal structure, we now use the term α iron to include β iron. The next transformation, which gives γ iron, takes place at 910°C (the A point). Gamma iron has the face- 3 centered cubic (f.c.c.) atomic arrangement, in which the unit cell contains atoms at the corners and face-centers of a cube, as shown in Fig. 1.1b. The last solid-state transformation on heating, y -><5, takes place at 1400°C; δ iron has the same b.c.c. structure as α iron. The y -> α transformation on cooling is closely related to the martensitic transformation which we will discuss later. 1.1.2 Phase diagram of carbon steels and the martensite start temperature, M s The outline of the phase diagram for a binary Fe-C alloy is given in Fig. 1.2. The ferrite α solid solution in this diagram has the b.c.c. arrange­ ment of iron atoms, like pure α iron, the carbon atoms occupying randomly 1.1 Martensite in carbon steels 3 α + cementite FIG. 1.2 Phase diagram of Fe-C system. 5 400h \ 300 - 200- 100 ol I I I I 1 I "0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 C (%) a small fraction of the sites marked χ, Δ, • in Fig. 1.3a. Since these sites are interstitial sites lying between the iron atoms, the α phase is an inter­ stitial solid solution of iron and carbon. The austenite, or γ phase is also an interstitial solid solution of iron and carbon, in which the iron atoms are arranged in an f.c.c. lattice like that of pure γ iron, the carbon atoms occupying randomly a fraction of the interstitial sites marked χ in Fig. 1.3b. In addi­ tion to the difference in structure, the α phase and γ phase have different (b) Ύ FIG. 1.3 Atomic arrangements in (a) ferrite (a) and (b) austenite (γ): Ο, Fe atom; χ, Δ, •, positions available for C atom. 4 1 Introduction carbon solubilities. As is shown in the phase diagram, the solubility of carbon in the α phase is small and is at most 0.03% at the eutectoid tem­ perature, 720°C, whereas the maximum solubility of carbon in the y phase amounts to 1.7%,* corresponding to 8 at. %. M temperature. Quenching of steel generally means that the steel is s rapidly cooled to a low temperature from a temperature above the A 3 temperature or the eutectoid temperature (A ). Any α phase or cementite x that may be present in the heated condition is little changed on quenching. What is important is the y phase. As the phase diagram shows, on slow cooling the y phase is decomposed into α phase and cementite. This is not the case on quenching, for then the martensitic transformation, a main subject of this book, takes place. This can be detected by observed rapid changes of the physical properties, such as dilatation. The martensitic trans­ formation starts at a temperature designated as the M temperature. Here s Μ signifies martensite and the subscript s designates start. The M tem­ s perature depends upon the carbon content, as is indicated by the dotted line in Fig. 1.2. Note that this curve has a slope similar to that for the A tem­ 3 perature but lies far below the A temperature line. The M temperature of 3 s pure iron is only about 700°C, which is much lower than the A point, 3 910°C. The reason for this difference will be presented later. 1.1.3 Crystal structure of martensite (a) in carbon steels The crystal structure of martensite obtained by quenching the y phase in carbon steels has a body-centered tetragonal (b.c.t.) lattice which may be regarded as an α lattice with one of the cubic axes elongated, as illustrated in Fig. 1.4b, where the vertical axis is elongated. This is the structure of martensite observed metallographically and the symbol α' is often used to denote it, since the martensite structure may be thought to be derived from the structure of the α phase. The prime is sometimes used as an indication of the tetragonality due to carbon atoms in ordered solid solution, but in this book a' will indicate the structure having characteristics of martensite, even including the b.c.c. phase without carbon atoms when this phase is produced by a martensitic transformation. The symbol (') will be used generally to signify a martensite phase. The lattice parameters of a' in steels vary with carbon content in a nearly linear fashion (see Figs. 2.1, 2.2). The tetragonality c/a and the volume of the unit cell increase with the carbon content. From this fact alone it can be deduced that a' is a solid solution of iron and carbon. The position for carbon atoms in the lattice as determined by various measurements is that marked χ in Fig. 1.4b. Therefore, a' is also an interstitial solid solution, but f Recently 2.0% was reported.

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