Castings by John Campbell

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Castings by John Campbell

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Contents

Preface vii
Dedication ix
Introduction xi
1.2 Transport of gases in melts 10
1.3 Surface film formation 12
Reactions of the melt with its
environment 2
2. Entrainment 17
2.1 Entrainment defects 20
2.2 Entrainment processes 3 1
2.3 Furling and unfurling 54
2.4 Deactivation of entrained films 61
2.5 Soluble, transient films 63
2.6 Detrainment 64
2.7 Evidence for bifilms 64
2.8 The significance of bifilms 67
3. Flow 70
3. I Effect of surface films on filling 70
3.2 Effect of entrained films on filling 73
3.3 Fluidity (maximum fluidity length) Lr 74
3.4 Continuous fluidity 95
3.5 Glossary of symbols 98
4. The mould 99
4.1 Inert moulds 99
4.2 Aggregate moulds 100
4.3 Mould atmosphere 105
4.4 Mould surface reactions 1 1 I
4.5 Metal surface reactions 114
5. Solidification structure 117
5.1 Heat transfer 117
5.2 Development of matrix structure 129
5.3 Segregation 139
5.4 Aluminium alloys 147
5.5 Cast irons 156
5.6 Steels 167
6. Gasporosity 178
6.1 Nucleation of gas porosity 178
6.2 Subsurface porosity 186
6.3 Growth of gas pores 195
6.4 Blowholes 200
7. Solidi$cation shrinkage 205
7.1 General shrinkage behaviour 205
7.2 Solidification shrinkage 206
7.3 Feeding criteria 210
7.4 Feeding – the five mechanisms 2 12
7.5 Initiation of shrinkage porosity 222
7.6 Growth of shrinkage pores 226
7.7 Final forms of shrinkage porosity 227
8. Linear contraction 232
8.1 Uniform contraction 232
8.2 Non-uniform contraction (distortion) 237
8.3 Hot tearing 242
8.4 Cold cracking 258
8.5 Residual stress 259
9. Structure, defects and properties qf the
finished casting 267
9.1 Grain size 267
9.2 Dendrite arm spacing 270
9.3 Compact defects 275
9.4 Planar defects 279
9.5 Effects of defects on properties of
castings 282
9.6 The statistics of failure 301
10. Processing 306
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 interactions 3 1 1
1 1.1 Internal oxidation 3 1 1
11.2 Corrosion 313
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 Al-
Si 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 author’s
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 volumes. The next
in line is Castings II – Practice listing my ten rules
for the manufacture of good castings with one
chapter per rule. It concentrates on an outline of
current knowledge of the theory and practice of
designing filling and feeding systems for castings.
It is intended as a more practical work. Finally, I
wish to write something on Castings III – Processes
because, having personal experience of many of
the casting processes, it has become clear to me
that a good comparative text is much needed. I
shall then take a rest.
Even so, as I mentioned in the Preface to
Castings, and bears repeat here, the rapidity of
casting developments makes it a privilege to live
in such exciting times. For this reason, however, it
will not be possible to keep this work up to date. It
is hoped that, as before, this new edition will serve
its purpose for a time, reaching out to an even
wider audience, and assisting foundry people to
overcome their everyday problems. Furthermore, I
hope it will inspire students and casting engineers
alike to continue to keep themselves updated. The
regular reading of new developments in the casting
journals, and attendance at technical meetings of
local societies, will encourage the professionalism
to achieve even higher standards of castings in the
future.