Grain size control

197 Pages·2004·32.454 MB·English

by  Gladman, T

Grain Size Control
ALSO BY THE AUTHOR
The Physical Metallurgy of Microalloyed Steels
Grain Size Control
T.GLADMAN
Formerly Head of Physical Metallurgy, Swinden
Laboratories, British Steel Corporation, and British
Steel Professor of Metallurgy, University of Leeds,
UK.
@
MANEY
FOR THE INSTITUTE OF MATERIALS, MINERALS AND MINING
10M Communications Ltd is awholly-owned subsidiary of
The Institute of Materials, Minerals &Mining (10M3).
B0802
First published for 10M3 in 2004 by
Maney Publishing
I Carlton House Terrace
London SWI Y 5DB, UK
© 10MCommunications Ltd 2004
All rights reserved. No part of this publication may be reproduced, stored in aretrieval
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or otherwise, without the written consent ofthe copyright holder.
Requests for such permission should be addressed to Maney Publishing.
ISBN I 904350 22 4
Statements in the volume reflect the views of the author and
not those of the Institute or publisher.
Typeset by Old City Publishing, USA
Printed and bound in the UK by the Charlesworth Group
Contents
Preface ix
Acknowledgemnts xiii
Chapter 1 The Nature of Grain Boundaries 1
1.1 Historical Views of Metallic Structures 1
1.2 Modem Approaches to Po1ycrystalline Materials 4
1.3 Grain Boundary Energy 6
1.4 Grain Boundary Energy and Interfacial Tension 10
1.5 Grain Boundary Forces 11
1.6 Grain Boundary Segregation 15
1.7 Summary 17
References 18
Chapter 2 Granular Arrays - Topology, Grain Shape,
Stereology, and Grain Size 21
2.1 Topology 21
2.1.1 General Principles 21
2.1.2 Surface Energy and Space Filling Effects 23
2.1.3 Implications for Grain Shape 24
2.1A Experimental Assessment of Grain Shape 27
2.2 Grain Size 29
2.2.1 Stereological Principles 30
2.2.2 Stereo10gical Aspects ofVarious Polyhedra 35
(a) Mean Areal Intersection 35
(b) Mean Linear Intercept 37
2.3 Experimental Measurement of Grain Size 40
2.3.1 The Comparator Chart Method 40
2.3.2 The Heyn Intercept Method 41
2.3.3 Jeffries Planimetric Method 42
2.3.4 Relationships Between Grain Size Measures 43
References 44
Chapter 2Exercises 45
Chapter 3 Grain Growth 47
3.1 Establishing aGranular Structure 47
3.2 Normal Grain Growth 48
3.2.1 Analytical Treatments 49
3.2.2 Topological Models 56
3.2.3 Computer Based Models 58
3.2.4 Other Approaches 60
3.3 Abnormal Grain Growth 61
3.4 Growth Velocity and Impurity Effects 62
References 65
VI Grain Size Control
Chapter 4 Inhibition and Control of Grain Growth 67
4.1 Introduction 67
4.2 Second Phase Particles 68
4.2.1 The Zener Relationship 69
4.2.2 Granular Arrays with Small Volume Fractions of Uniformly
Sized Randomly Distributed, Second Phase Particles 71
(a) Growing Tetrakaidecahedra 71
(b) Shrinking Tetrahedra 74
4.2.3 Granular arrays with Small Volume Fractions ofUniformly
Sized Grain Boundary Particles 76
(a) Growing Tetrakaidecahedra 77
(b) Shrinking Tetrahedra 78
4.2.4 Comparison of Grain Boundary Particles and
Randomly Distributed Particles 79
4.2.5 The Effects of Mixed Particle Sizes 80
4.3 Free Surface Effects 84
4.3.1 Thermal Grooving 85
4.3.2 Variations in Free Surface Energy 88
4.3.3 Substrates and Capping of Foils 92
4.3.4 Secondary Recrystallisation in Foils or Thin Strip 94
4.4 Thermal and Mechanical Cycles 95
4.4.1 Thermal Cycling ofAllotropic Materials 96
44.2 Thermomechanical Treatments 100
44.3 Conventional Controlled Rolling of Steel 102
References 108
Chapter 4Exercises 109
Chapter 5 Second Phase Particle Contents and Particle Coarsening III
5.1 Introduction 111
5.2 Particle Solubility 112
5.2.1 The Solubility Product 112
5.2.2 Using Simple Solubility Product 115
5.2.3 Calculating the Particle Contents for Simple Compounds 118
5.2.4 Solubility Data for Other Grain Refining Additives in Steel 119
5.2.5 Multiple Additions of Grain Refining Additives-
Mutually Exclusive Compounds 123
5.2.6 Mutually Soluble Components 124
5.3 Conversions to Volume Fractions 129
5.4 Particle Size and Particle Coarsening 131
5.4.1 Precipitation 131
54.2 Ostwald Ripening 133
54.3 Implications ofthe Wagner Equation for Particle Selection 137
5.4.4 Calculation of Particle Growth Rates 140
References 140
Chapter 5Exercises 141
CONTENTS va
Chapter 6 Applications in Grain Size Control 143
6.1 Steel Normalising Treatments 143
6.1.1 Fine Grained Normalised Steels 145
6.1.2 The Grain Coarsening Temperature 147
6.1.3 Secondary Recrystallisation 148
6.1.4 The Grain Size atVery High Temperatures inthe
Austenite Range 150
6.2 The Effects ofAluminium Content on Coarsening 151
6.2.1 The Solute Content 152
6.2.2 Effects of Prior Treatment 154
6.2.3 General Observations onAluminium Treatment 157
6.3 Titanium Nitride Technology 158
6.4 Hot-rolled Products 164
6.4.1 Control of the Starting Grain Size 164
6.4.2 Inhibition of Grain Growth inHot Working 166
6.4.3 Retarded Recrystallisation 166
6.5 Secondary Recrystallisation ofCold Worked andAnnealed Strip 168
6.6 The Use of Oxide Particles 169
References 174
Answers to Excersies 177
Index 181

Preface
IMPORTANCE OF GRAIN STRUCTURES IN METALS
There are very few mechanical properties that are unaffected by either the grain size or the
grain characteristics in polycrystalline metals. Two that readily spring to mind are the elas-
tic modulus and the specific density. These properties depend on inter-atomic forces and
the atomic structure of the particular metals. Most other properties show varying levels of
sensitivity to grain size or grain orientations.
Probably the best known effect of grain size is that expressed by the Hall-Petch rela-
tionship, which states that the yield strength of a polycrystalline metal is related to the
inverse of the square root of the grain size, and very significant increases in strength can be
attained by maintaining a very small grain size. Similar effects are seen in other strength
properties such as the ultimate tensile strength, and the fatigue strength.
The effect of grain size on the ductile cleavage transition temperature for b.c.c. metals
is very pronounced. The transition temperature is negatively related to the inverse of the
square root of the grain size, so that in order to avoid brittle cleavage fracture, the requisite
low transition temperatures are attained by refining the grain size. The beneficial effect of
fine grain sizes on the impact temperature makes grain refinement the only strengthening
mechanism that will simultaneously improve toughness.
The effects of grain size on creep properties aremore complex, though no less significant,
and depend quite fundamentally on the creep conditions and the predominant creep mecha-
nism involved. At relatively high stresses (or low temperatures) diffusion controlled disloca-
tion creep predominates, and the creep rate isvirtually independent of grain size.At relative-
ly low stresses (or high temperatures) creep occurs by a bulk diffusion process involving
transport ofatoms from transverse boundaries to lengthways boundaries, and the creep rate is
inversely related to the square of the grain size. Thus, smaller grains allow faster creep rates
and reduce the creep strength. At intermediate stresses (or temperatures), creep occurs pre-
dominantly by aprocess ofgrain boundary diffusion known asCoble creep, andthe creep rate
is inversely related to the cube of the grain size. In order to give the extended lives required
in creep service, stresses and temperatures are commonly set at levels in or near to the bulk
diffusion regime and therefore fine grain sizes arenot sought for creep resisting applications.
The grain sizecanalso influence ductility atthehigh strainrates associated with metal form-
ingifthedeformation temperature isinacriticaltemperature range, duetotheformation ofcav-
itiesontransverse boundaries. Athigher temperatures, rapid diffusion prevents cavitation, whilst
atlowertemperatures dislocation deformation predominates. Thiseffectisalsoseenatlowertem-
peratures when creepdeformation ratesareimposed. Intergranular cohesion canbereduced quite
dramatically by the segregation of impurities to grain boundaries, and can result inbrittle inter-
granular fracture. Inmaterial embrittled inthisway,aductile/intergranular transition temperature
may be observed, thetransition temperature increasing with increasing impurity concentration.
IX
x Grain Size Control
Other properties may depend onother grain characteristics such asgrain orientation rather
than grain size. The occurrence of a preponderance of grains of a certain orientation is
described as atexture, and certain properties are known to depend onthe type oftexture pro-
duced. Aclassic example of this isthe dependence of magnetic characteristics on the texture
of thin strip for the high-silicon magnetic transformer steels. Development of a strong Goss
texture can result in dramatic improvements of the efficiency of transformers. These textures
have been developed traditionally by using a secondary recrystallisation treatment which
encourages the abnormal growth of specific grains of favourable orientation. Another exam-
ple of texture effects is to be found in sheet metal-forming of both ferrous and non-ferrous
metals. Suitably textured material canbeproduced having superior sheet metal pressing prop-
erties, and also minimising the in-plane variability of strain that gives rise to earing inpressed
sheet, which intum reduces the waste from edge trimming.
There are literally thousands of publications illustrating the effects of grain size and
grain structure on the properties of metals, but these are outside the scope of the present
book. It should be clear from the above remarks, however, that the grain size can produce
dramatic effects on the properties of metals, and indeed many alloy specifications call for
specific levels of grain size control, or for property levels that can only be attained by such
acontrol. The same can be said for texture control.
DEVELOPMENTS IN THE PAST FIFTY YEARS
It is almost fifty years since McLean (1957) published his book on 'Grain Boundaries in
Metals'. This book provided an excellent account of the understanding of many matters
concerning grains, grain boundaries, grain size, and associated phenomena existing at that
time. Indeed, the understanding of grain structure atthat time was very well advanced, inthat
many of the phenomenological features, associated with grains and grain/growth had been
derived by the many physicists, chemists, and mathematicians, who had been attracted by the
very catholic field of metallurgy, and which were soundly based on thermodynamic or other
scientific argument. Aparticularly interesting case relates tothe effects of small second phase
particles in inhibiting grain growth. The effect of spherical particles had been derived by
Zener, using classical surface energy physics for the interaction force and a simple collapsing
bubble asthe driving force forboundary motion, giving theclassical Zener equation,
R = 4r/3f
where the grain radius, R, isrelated to the particle radius, r, and the volume fraction ofpar-
ticles, f.McLean rightly concluded that large inclusions inmetals areprobably unimportant
inrestricting grain growth, "but inclusions too small to be seen may limit the grain size, as,
perhaps, in fine grained steels". This was indeed aprophetic statement, because atthis time,
the only solid fact was that additions of aluminium to steel would produce grain refinement
and there was little indication ofthe nature of the growth inhibiting features, particularly as
growth eventually occurred despite the continued presence of aluminium and it's stable
compounds. Itisimportant torealise that electron microscopy, invented inthe early 1930's,
was just starting to be used in studies of steel in the mid 1950's, thereby opening up the
study of grain refining particles, especially when combined with electron diffraction and, at
a later stage, energy dispersive and wavelength dispersive analytical techniques for these
small particles. Also, theories of Ostwald ripening were not well developed. The original