2003 campbell book - castings

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Castings

CastingsJohn Campbell OBE FREngProfessor of Casting Technology, University of Birmingham, UK

U T T E R W O R T H E I N E M A N NOXFORD AMSTERDAM BOSTON LONDON NEW YORK SYDNEY PARIS SAN DIEGO SAN FRANCISCO SINGAPORE TOKYO

Butterworth-Heinemann An imprint of Elsevier Science Linacre House, Jordan Hill, Oxford OX2 8DP 22 Wheeler Road, Burlington MA 01803 First published 1991 Paperback edition 1993 Reprinted 1993, 1995 (twice), 1997, 1998, 1999, 2000 Second edition 2003 Copyright 0 1991, 2003, Elsevier Science Ltd. All rights reserved

No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W I T 4LP. Applications for the copyright holders written permission to reproduce any part of this publication should be addressed to the publisherPermissions may be sought directly from Elseviers Science and Technology Rights Department in Oxford, UK: phone (+44)(0) 1865 843830; fax (+44)(0) 1865 853333; e-mail: permissions@elsevier.co.uk. You may also complete your request on-line via the Elsevier Science homepage (http://www.elsevier.com), by selecting Customer support and then Obtaining Permissions

British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of CongressISBN 0 7506 4790 6 For information on all Buttenvorth-Heinemann publications visit our website at www.bh.com

Typeset by Replika Press Pvt. Ltd, India Printed and bound in Great Britain by MPG Books Ltd, Bodmin, Cornwall

Contents

Preface vii Dedication ix Introduction xi1. The melt1. I 1.2 1.3

6.2 6.3 6.4

Subsurface porosity 186 Growth of gas pores 195 Blowholes 200

1

7. Solidi$cation shrinkage7.1 7.2 7.3 7.4 7.5 7.6 7.7

205

Reactions of the melt with its environment 2 Transport of gases in melts 10 Surface film formation 12

2. Entrainment2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8

17

General shrinkage behaviour 205 Solidification shrinkage 206 Feeding criteria 210 Feeding - the five mechanisms 2 12 Initiation of shrinkage porosity 222 Growth of shrinkage pores 226 Final forms of shrinkage porosity 227

Entrainment defects 20 Entrainment processes 3 1 Furling and unfurling 54 Deactivation of entrained films 61 Soluble, transient films 63 Detrainment 64 Evidence for bifilms 64 The significance of bifilms 67

8. Linear contraction8.1 8.2 8.3 8.4 8.5

232

Uniform contraction 232 Non-uniform contraction (distortion) 237 Hot tearing 242 Cold cracking 258 Residual stress 259

3. Flow3. I 3.2 3.3 3.4 3.5

70Effect of surface films on filling 70 Effect of entrained films on filling 73 Fluidity (maximum fluidity length) Lr 74 Continuous fluidity 95 Glossary of symbols 98

9. Structure, defects and properties qf the finished casting 2679.1 9.2 9.3 9.4 9.59.6 Grain size 267 Dendrite arm spacing 270 Compact defects 275 Planar defects 279 Effects of defects on properties of castings 282 The statistics of failure 301

4. The mould4.1 4.2 4.3 4.4 4.5

99

Inert moulds 99 Aggregate moulds 100 Mould atmosphere 105 Mould surface reactions 1 1 I Metal surface reactions 114

10. Processing

306

5 . Solidification structure5.1 5.2 5.3 5.4 5.5 5.6 6.1

117129

Heat transfer 117 Development of matrix structure Segregation 139 Aluminium alloys 147 Cast irons 156 Steels 167

10.1 Impregnation 306 10.2 Hot isostatic pressing 306 10.3 Working (forging, rolling and extrusion) 309 10.4 Machining 309 10.5 Painting 310

1 1. Environmental interactionsInternal oxidation 11.2 Corrosion 3131 1.1

3 11

311

6. Gasporosity

178178

Nucleation of gas porosity

References 3 18 Index 329

Preface

Metal castings are fundamental building blocks, the three-dimensional integral shapes indispensable to practically all other manufacturing industries. Although the manufacturing path from the liquid to the finished shape is the most direct, this directness involves the greatest difficulty. This is because so much needs to be controlled simultaneously, including melting, alloying, moulding, pouring, solidification, finishing, etc. Every one of these aspects has to be correct since failure of only one will probably cause the casting to fail. Other processes such as forging or machining are merely single parts of multi-step processes. It is clearly easier to control each separate process in turn. It is no wonder therefore that the manufacture of castings is one of the most challenging of technologies. It has defied proper understanding and control for an impressive five thousand years at least. However, there are signs that we might now be starting to make progress. Naturally, this claim appears to have been made by all writers of textbooks on castings for the last hundred years or so. Doubtless, it will continue to be made in future generations. In a way, it is hoped that it will always be true. This is what makes casting so fascinating. The complexity of the subject invites a continuous stream of new concepts and new solutions. The author trained as a physicist and physical metallurgist, and is aware of the admirable and powerful developments in science and technology that have facilitated the progress enjoyed by these branches of science. These successes have, quite naturally, persuaded the Higher Educational Institutes throughout the world to adopt physical metallurgy as the natural materials discipline required to be taught. Process metallurgy has been increasingly regarded as a less rigorous subject, not requiring the attentions of a university curriculum. Perhaps, worse still, we now have

materials science, where breadth of knowledge has to take precedence over depth of understanding. This work makes the case for process metallurgy as being a key complementary discipline. It can explain the properties of metals, in some respects outweighing the effects of alloying, working and heat treatment that are the established province of physical metallurgy. In particular, the study of casting technology is a topic of daunting complexity, far more encompassing than the separate studies, for instance, of fluid flow or solidification (as necessary, important and fascinating as such focused studies clearly are). It is hoped therefore that in time, casting technology will be rightly recognized as a complex engineering discipline, worthy of individual attention. The author has always admired those who have only published what was certain knowledge. However, as this work was well under way, it became clear to me that this was not my purpose. Knowledge is hard to achieve, and often illusive, fragmentary and ultimately uncertain. This book is offered as an exercise in education, more to do with thinking and understanding than learning. It is an exercise in grappling with new concepts and making personal evaluations of their worth, their cogency, and their place amid the scattering of facts, some reliable, others less so. It is about research, and about the excitement of finding out for oneself. Thus the opportunity has been taken in this revised edition of Castings to bring the work up to date particularly in the new and exciting areas of surface turbulence and the recently discovered compaction and unfurling of folded film defects (the bifilms). Additional new concepts of alloy theory relating to the common alloy eutectics AlSi and Fe-C will be outlined. At the time of writing these new paradigms are not quite out of the realm of speculation, but most areas are now well grounded in about 200 person years of effort in the authors

viii

Preface

laboratory over the last 12 years. Furthermore, many have been rigorously tested and proved in foundries. This aspect of quoting confirmation of scientific concepts from industrial experience is a departure that will be viewed with concern by those academics who are accustomed to the apparent rigour of laboratory experiments. However, for those who persevere and grow to understand this work it will become clear that laboratory experiments cannot at this time achieve the control over liquid metal quality that can now be routinely provided in some industrial operations. Thus the evidence from industry is vital at this time. Suitable laboratory experiments can catch up later. The author has allowed himself the luxury of hypothesis, that a sceptic might brand speculation. Broadly, it has been carried out in the spirit of the words of John Maynard Keynes, I would rather be vaguely right than precisely wrong. This book is the first attempt to codify and present the New Metallurgy. It cannot therefore claim to be authoritative on all aspects at this time. It is an introduction to the new thinking of the metallurgy of cast alloys, and, by virtue of the survival of many of the defects during plastic working, wrought alloys too. The primary aim remains to challenge the reader to think through the concepts that will lead to a better understanding of this most complex of forming operations, the casting process. It is hoped thereby to improve the professionalism and status of casting technology, and with it the products, so that both the industry and its customers will benefit.

It is intended to follow up this volume Castings I - Principles with two further