Fracture Plastic Flow and Structural Integrity - The Proceedings of the 7th Symposium Organised by the Technical Advisory Group on Structural Integrity in Nuclear Plant (TAGSI) Held at TWI, Abington, UK

210 Pages·2000·10.94 MB·English

by  Hirsch

FRACTURE,
PLASTIC FLOW
AND STRUCTURAL
INTEGRITY
The Proceedings of the 7th Symposium
organised by the
Technical Advisory Group on Structural
Integrity in Nuclear Plant (TAGSI)
held at TWI, Abington, UK
29 April 1999
Edited by
PETER HIRSCH
and
DAVID LIDBURY
Book 772
First published in 2000 by
10M Communications Ltd
1Carlton House Terrace
London SWI Y 5DB
© 10M Communications Ltd 2000
All rights reserved
10M Communications Ltd
is a wholly-owned subsidiary of
The Institute of Materials
ISBN 1-86125-095-99
Typeset in the UK by
Fakenham Photosetting, Norfolk
Printed and bound in the UK at
The University Press, Cambridge
Contents
Foreword vii
25 Years of TAGSI and LWRSG x
Materials Factors and Structural Integrity of Reactor Components 1
B. L. Eyre .
The Micro-Mechanisms of Fracture in Steels Usedfor High Integrity Structural
Components 21
John Knott
Welds, Their Quality and Inspection Ability for High Integrity Structures and
Components 45
R. E. Dolby, I. J. Munns, C. R. A. Schneider and R. H. Leggatt
Material Conditions that Lead to Crack Arrest 65
F. M. Burdekin
Fracture Toughness - a Measurable Materials Parameter 85
J. D. G. Sumpter
Microstructural Development in RPV Steels 103
Colin English, Jon Hyde and Sue Ortner
Local Approach Modelling of Constraint Contributions to The Ductile to
Brittle Transition 129
A. R. Dowling and D. P. G. Lidbury
Loading Rate Effects on Tensile Properties and Fracture Toughness of Steel 149
C. S.Wiesner and H. MacGillivray
Material Factors which Influence Remaining Life of Components Subject
to Reheat Cracking 175
P. Neumann, D. A. Miller and R. A. Ainsworth
A Historical Perspective of Materials Related Structural Integrity Issues
in the Nuclear Industry 185
Brian Tomkins
v
CHAPTER 1
Materials Factors and Structural Integrity of
Reactor Components
B. L. Eyre
Department ofMaterials, University of Oxford, Parks Rd, Oxford OXl 3PH
ABSTRACT
Materials factors have amajor influence onthereliable andsafeoperation ofengineering plant.
The consequences of component failure range from the relatively minor resulting in loss of
service to major with threats to safety and total loss of investment in the plant. Component
integrity is particularly vital for nuclear power plants because of the need to meet exception-
ally high safety standards.
A key issue for all nuclear power plants is the integrity of the primary coolant pressure
boundary. Thepaper focuses onthisissueboth because ofitsvital importance tosafeoperation
and because it illustrates very well the relationship between metallurgical factors in terms of
composition, structure and response to in-service conditions on the one hand and structural
integrity onthe other. Itwas established atanearly stage ofthe development ofboth PWR and
gas cooled reactor programmes that the reactor pressure vessel (RPV) ferritic steels are hard-
ened and embrittled by neutron irradiation. It isnow established that hardening and embrittle-
ment in steels isdue tothe combined effects of intrinsic irradiation damage and ifCu exceeds
athreshold level, finely dispersed Cuprecipitates. Itis also clear that other compositional fac-
tors including Ni, Mn and Pplay asignificant role.
The least understood aspect of hardening of RPV steels is that due to intrinsic irradiation
damage. The paper summarises the available evidence which isconsistent with atleast part of
the damage in steels being dislocation loops, most probably interstitial innature. Nevertheless,
more work is required particularly using direct observations on RPV steels to determine both
the nature and geometry of the damage clusters and the influence of composition and micro-
structure oncluster distributions.
The position on Cu precipitation is better understood and the hardening has been modelled
based onelastic modulus difference between theprecipitates andmatrix. Nevertheless, import-
ant questions remain concerning the factors influencing precipitate distributions and structure.
Lastly, the influence of other elements, and particularly interstitial solutes and tramp impu-
rities on the damage structures and hardening and embrittlement need further clarification.
1 INTRODUCTION
In his 1959 Thomas Hawkesly Lecture,' Sir Alan Cottrell made the analogy between
nuclear engineering to agame of snakes and ladders: 'in which theprizes go to those teams
whose metallurgists andchemists aremost adept atputting upladders, removing snakes and
anticipating the patterns of hazards through the game, and whose engineers and physicists
show most ingenuity in adapting their plans tothatpattern.' Clearly, the experience over the
last four or fivedecades has demonstrated how apposite this analogy is. Since the introduc-
tion of nuclear power generating reactors in the 1950s we have encountered many unex-
pected snakes associated with materials behaviour from the Windscale firetriggered by the
1
Fracture, Plastic Flow and Structural Integrity
Wigner energy release from the graphite, tothewideranging stress corrosion cracking prob-
lems on light water reactors (LWRs).
It is also true that the associated challenges stimulated large research and development
programmes in all of the countries involved in nuclear power development. This has
resulted in major advances, not only in resolving the particular problems, but also in our
wider understanding of the basic mechanisms governing materials behaviour.
This paper isconcerned with the links between metallurgical factors and structural integ-
rity of reactor components. It will focus on the case of reactor pressure vessel integrity. As
well as being of central importance to the safety and reliability of reactors it also provides
aparticularly agood example ofthe complex inter-relationships between metallurgical fac-
tors, deformation and failure mechanisms and structural integrity.
2 BACKGROUND
Because of the high safety requirement and the economic repercussions of materials fail-
ures, particularly stringent demands are placed on ensuring structural integrity in nuclear
power plants. The service conditions are onerous in terms ofrequired component lifetimes
and operating conditions and for reactor components there is the added factor of neutron
irradiation.
It was anticipated early on that neutron irradiation effects on materials properties would
be important. But many unexpected integrity related problems have also been encountered
with components outside ofthe nuclear island andthese have often been associated with the
special operating conditions ofnuclear plants interms for example ofstress conditions, tem-
peratures and environment.
3 REACTOR PRESSURE VESSEL INTEGRITY - EARLY WORK
The pressure vessel forms the primary coolant and pressure boundary and its integrity is a
key issue for allreactors. Loss ofcoolant and de-pressurisation resulting from unstable fail-
ure would constitute a severe accident, as well as total loss of the investment in the plant.
More important would be the catastrophic human and social consequences, not to mention
atotal loss of confidence in nuclear technology.
Professor Cottrell has had amajor influence on this area. This extends from the work he
led atHarwell in the 1950sto the questions he has raised about the integrity of PWR reac-
tor pressure vessels (RPVs) and his continuing vigilance to ensure that such questions are
rigorously answered.
3.1 IRRADIATION EFFECTS ON MECHANICAL PROPERTIES OF STEELS
Itwasrecognised from theearly 1950s,2-4that iron and steels are subject toirradiation hard-
ening andembrittlement. Figure 1shows anexample ofthechanges intensile properties and
ductile to brittle transition temperature (DBTT) resulting from neutron irradiation of a
carbon-manganese steel such as are used for the Magnox RPVs.
Major research and surveillance programmes were mounted from the outset of the civil
2
Materials Factors and Structural Integrity ofReactor Components
IRRADIATED
.>(9.5 x 1018n/cm2)
100
80 UNIRRADIATED
I
60
(a) .e-n
ci.
C")
0 40
x
CIJ
CIJ
wa.-:
20
CIJ
10 20 30
--------t~.
STRAIN
%
40 UNIRRADIATED
i
30
CIJ
en
....J
(b) J..!..
u..
20
Cl
w
en IRRADIATED
a: (11.9x 1019n/cm2)
0
CIJ
en
«
>- 10
~
a:
w
z
w
-80 -60 -40 -20 0 +20 +40 +60 +80 +100
TEMPERATUREoC ~
Fig. 1 Effects ofneutron irradiation onthe stress-strain and ductile tobrittle Charpy transition curves.
3
Fracture, Plastic Flow and Structural Integrity
power programmes to both monitor and determine the hardening and embrittlement mech-
anisms. The accumulated data have demonstrated the central role played by metallurgical
factors in determining the extent of hardening and embrittlement and the importance of
understanding theunderlying mechanisms. Progress has been made onboth these fronts and
trend curves giving hardening and transition temperature shifts as a function of damage
level for given temperature ranges have been derived principally from the surveillance data
for both Magnox and LWR RPVs.But important questions remain regarding details of the
mechanisms and the trend curves are empirical to the extent that they involve statistical
analysis ofthedata. We arenotyetinaposition topredict reliably property changes forcon-
ditions that lie significantly outside the existing data base.
3.2 MECHANISMS
Early Harwell work on the mechanisms of irradiation hardening in ferritic steels used the
Hall-Petch5,6 analysis relating the applied yield and fracture shear stresses to grain size:
0' = 0'. + k . d -112 (1)
y l Y
O'F = 0';+ k*.d-1/2 (2)
where 0';is generally assumed to be the friction stress opposing the movement of glide dis-
locations on their glide planes. It consists of a thermally activated component, which
together with the dislocation source activation term, ky (= O'd. 1112,O'd is the unpinning stress
and 1isthedistance from thepile-up tothenearest source), accounts forthe temperature and
strain rate dependence of the yield stress. The second component of represents athermal
O'i
obstacles such as solute atoms, precipitates, forest dislocations and clusters and istherefore
relevant to irradiation hardening. The grain diameter is 2d.
Cottrell? used a modified Stroh" mechanism for crack nucleation at the head of a dislo-
cation pile-up involving theformation ofacracked dislocation attheintersection oftwo slip
bands. Forbee metals thecracked dislocation formed by such areaction lies ona{1OO}plane
with a <100> type Burgers vector. Combining the energy to form a crack with the Petch
analysis for the grain-size dependence of the yield stress he defined acriterion for unstable
propagation of abrittle crack as follows:
0' • k . d1l2 = B· u. 'Y (3)
y y
where Jl isthe shear modulus, 'Yisthe effective fracture surface energy and Bis ageometri-
cal term ranging from -1 for uniaxial tension to -1/3for triaxial tension.
The Cottrell equation provides a valuable insight into the key factors (materials through
0' and k ,physical through Jl and 'Yand mechanical through B), controlling brittle fracture.
y y
It defines the transition temperature through the temperature dependence of and and a
ky O'y
transition grain-size for a given temperature and strength. Importantly, in terms of under-
standing irradiation embrittlement, it gives a critical strength above which brittle fracture
will occur at a given temperature, through the O't term in O'y. Inter-granular embrittlement is
represented through changes in the fracture energy 'Y.
Subsequent metallographic observations have shown thatin structural steelsbrittle cracks
4
Materials Factors and Structural Integrity ofReactor Components
En 2 Steel
Tensile
1.5Xl019 n.cm.-2
Stress
o.y.at
-1':J6oC
}
OY-- 0i+KY1-1/2
2 4 10
Grainsize mm+?
1-112
e
Fig. 2 Effects of neutron irradiation on the grain size dependence of the yield stress at 200
and - 196°e for acarbon-manganese steel.
frequently nucleate atgrainboundary carbides andRitchie, Knott andRice9 have shown that
the critical stepforunstable propagation isforthe crack toovercome the firstbarrier usually
a grain boundary. Thus, the fracture stress must be sustained over a distance that is related
to the micro-structure. Ritchie etal. concluded that this istwo grain diameters for the plain
carbon steels they considered.
Figure 2shows the effects ofneutron irradiation onthegrain-size dependence oftheyield
stress at 20° and -196°C for a C-Mn steel similar to those used for the Magnox RPVS.10
Based onequations 1-4, anumber ofconclusions were drawn regarding irradiation harden-
ing and embrittlement processes.
5
Fracture, Plastic Flow and Structural Integrity
Sheer Stress
"tj (p.s.i.Xl()3) En 2 Steel
100·
-1960C
50
d11.l = "tj ksy
trans ~~y
2 4 6 8 10
Transition Grain Size ~1/2 pm-112
Fig. 3 Plots ofthe transition grain size asafunction ofthe yield stress in acarbon-manganese steel
irradiated at 20°C, -77°C and - 196°C.
• From the constancy of the slopes of the ay versus d1l2 plots it was concluded that the
irradiation hardening is due to point defect clusters increasing the friction term, ai'
with no increase in dislocation pinning.
• The temperature dependence ofthe yield strength isnot changed by irradiation, which
is consistent with irradiation not effecting the Peirls-N abarro stress or the stress to
unpin dislocations.
• With regard to the fracture strength, it was observed that af (either measured directly
orderived from Cottrell's analysis) is sensitive to grain-size and irradiation dose, but
only through their effects in increasing the yield strength through the friction term.
• As expected, the transition grain size calculated from (3) was observed to decrease
with irradiation dose as shown for example in Fig. 3.
• Hull and Mogford observed that irradiation results in a decrease in strain ageing on
10
heating to temperatures in the 150°Cto 200°C range, indicating the trapping ofpoint
defects by interstitial solutes, aresult later confirmed byLittle and Harries. This has
11
abearing on the nucleation ofpoint defect clusters and second phase particles.
As illustrated in Fig. I, the irradiation induced increase in yield stress and decrease in duc-
tility is accompanied by amarked decrease in work-hardening capacity. We shall return to
6
Materials Factors and Structural Integrity ofReactor Components
• SILICON KILLED MILD STEEL,
cr:
w ANNEALED CONDITION
~E
....I.€ •• SILICON KILLED MILD STEEL,
AGED 500 HOURS AT650°C
~ ~ 10 100
U5~
ills •• ALUMINIUM GRAIN SIZE
t-en CONTROLLED MILD STEEL C\EI
~w~cr: ANNEALED CONDITION Z~
ent-
«en
o~ mCl 5 - 50
z->--
1/2
~ay=A(0t)
o 5 10 15
(NEUTRON DOSE)1/2 x 108
Fig. 4 Plots showing theincrease inyield stress asafunction of(neutron dose)112forthree mild steels.
consider the implications of this in terms of the effect of irradiation on deformation mech-
anisms and upper shelf toughness.
It was also established.F as shown for example by Fig. 4, that irradiation hardening in
C-Mn and low alloy steels follows a (dose) relationship at neutron doses below the level
112
for damage saturation:
(6)
The constant A is dependent on the material and irradiation temperature. The interdepend-
ence of yield strength, hardness and transition temperature shift is reflected by similar
(dose relationships also applying to these latter two parameters.
)112
4 RPV INTEGRITY: CURRENT PICTURE
The initial results from the surveillance programmes indicated that the extent of irradiation
embrittlement, asmeasured for example by DBTT shifts, was modest and within the allow-
able limits. But at the end of the 60's a further snake was encountered in the form of
enhanced embrittlement inPWR RPV welds containing higher copper levels than theparent
plate.P This highlighted the importance of obtaining a more comprehensive understanding
of compositional factors and the influence of long-term ageing onirradiation induced prop-
erty changes and structural integrity.
Since thePotapov andHawthorne':' observations, extensive work both tocharacterise and
understand the complex interactions between composition and irradiation damage in deter-
mining the mechanical property changes has been carried out. While this has focussed on
Cu it has broadened to include other alloy and impurity elements, notably nickel and phos-
7