PSK 4253 - Pipe Class E16H3A for Pressure Purposes. Austenitic-ferritic Stainless CrNiMo-steel
Proof of Fatigue Strength of Ferritic and Austenitic...
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Proof of Fatigue Strength of Ferritic and Austenitic
Nuclear Components
E. Roos, K.-H. Herter , X. Schuler , T. Weißenberg
MPA Universität Stuttgart
35th MPA-Seminar
October 9, 2009 in Stuttgart
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Abstract
For the construction, design and operation of nuclear components and systems the
appropriate technical codes and standards provide material data, detailed stress
analysis procedures and a design philosophy which guarantees a reliable behaviour
of the structural components throughout the specified lifetime. Especially for cyclic
stress evaluation the different codes and standards provide different fatigue analyses
procedures to be performed considering the various mechanical and thermal loading
histories and geometric complexities of the components. For the fatigue design
curves used as limiting criteria the influence of different factors like e.g., environment,
surface finish and temperature must be taken into consideration in an appropriate
way. Fatigue tests were performed with low alloy steels as well as with Nb- and Ti-
stabilized German austenitic stainless steels in air and simulated high temperature
boiling water reactor environment. The experimental results are compared and
valuated with the mean data curves in air as well as with mean data curves under
high temperature water environment published in the international literature.
1 Introduction
The basis for construction, design and operation of mechanical systems, structures
and components (SSC) are national technical codes and standards, like for nuclear
SSC the ASME-Code Section III [1], the German Nuclear Safety Standards KTA [2]
or the French RCC-M Code [3] and for non nuclear SSC e.g. the European standard
EN13445 [4]. The basic philosophy in the design of SSC is to demonstrate that the
integrity and the function is guaranteed throughout the lifetime. It is important that the
design concept accounts for most possible failure modes and provides rational
margins of safety against each type of failure mode. Some of the potential failure
modes which SSC designers should take into account are for example: Excessive
elastic deformation including elastic instability, excessive plastic deformation, brittle
fracture, fatigue and corrosion.
During design stage a complete picture of the stress state within the SSC obtained
by calculation or measurement of both mechanical and thermal stresses during
transient and steady state operation has to be created. It has to be demonstrated that
all stresses (primary, secondary) are within the allowable stress limits given by the
codes and standards, and that the usage factor developed by a fatigue analysis
(peak stresses) including the environmental effects are well below the limiting value
(cumulative fatigue life usage factor U<1).
It is possible to prevent failure modes caused by fatigue by imposing distinct limits on
the peak stresses at the highest loaded regions of the SSC or by reducing the load
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cycles since fatigue failure is related to and initiated by high local stresses/strains.
The design rules according to the technical codes and standards, [1, 2, 3, 4], provide
for explicit consideration of cyclic operation specific rules for assessing the
cumulative fatigue damage using design fatigue curves of allowable alternating loads
(allowable stress or strain amplitudes) vs. number of loading cycles (S-N curves),
caused by different specified or monitored load cycles. The influence of different
factors like welds, environment, surface finish, temperature, mean stress and size
must be taken into consideration in an appropriate way.
2 Fatigue approach for pressurized components
2.1 Fatigue S-N curves
Fatigue data are generally obtained from unwelded smooth cylindrical specimens
which were tested under strain control at room temperature and in air environment
with a fully reversed loading, i.e. strain ratio Rε=εU/εO=–1 and are plotted in the form
of nominal stress amplitude Sa vs. the number of cycles N to failure (mean data
curves). The total strain range Δεt obtained from the tests is converted to nominal
stress range 2Sa by multiplying the strain range by the modulus of elasticity E at test
temperature.
2ES2 t
a
εΔ⋅= (1)
Based on [5, 6] fatigue ε-N data can by expressed as
( ) ( )ClnBANln a −ε−= (2)
The current ASME as well as KTA best fit mean data curves [6, 7, 8, 11, 12] can be
written in terms of Eq. (2) as follows, categorized by the types of material. The fatigue
life for austenitic Cr-Ni steels is given by
( ) ( )1670029546 ,ln,,Nln a −ε−= (3)
for carbon steels by
( ) ( )0720027266 ,ln,,Nln a −ε−= (4)
and for low alloy ferritic steels by
( ) ( )1280023396 ,ln,,Nln a −ε−= (5)
The fatigue life to failure is defined as the number of cycles necessary for the tensile
stress to drop 25% from its peak or steady–state value during test, called N25. For a
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specimen size usually used in fatigue testing, e.g. 8-12 mm diameter cylindrical
specimens, this corresponds to a crack depth of about 3 mm [10].
The existing fatigue ε–N data to develop the S-N curves are categorized by the types
of material. Therefore most of the S-N curves given in the codes and standards are
to be applied for specific steels (e.g. distinguish between steels of different ultimate
tensile strength Rm).
Based on experimental ε–N data fatigue life models for estimating the fatigue lives of
these steels in air have been developed at ANL [10] as best-fits mean data. The
fatigue life, N, of carbon steels is represented by
).(ln.T..)Nln( a 11309751001206146 −ε−−= (6)
and for low-alloy ferritic steels by
).(ln.T..)Nln( a 15108081001204806 −ε−−= (7)
where εa is the applied strain amplitude (%), and T is the test temperature (°C). For
austenitic stainless steels in air at temperatures up to 400°C the fatigue data are best
represented by the equation
).(ln..)Nln( a 112092018916 −ε−= (8)
The design curves included in the codes were derived by introducing factors of 2 on
stress and 20 on cycles of the mean data curve concerned, whichever gave the
lowest curve and is meant to account for real effects (“scatter of data and material
variability”, “size effects”, “surface finish and environment”, [7, 8, 9, 10]) occurring
during plant operation.
2.2 Effects influencing the fatigue life
The use of the fatigue design curves is restricted in the nuclear codes and standards
to a specific maximum temperature below the creep range. Using design fatigue
curves it is necessary to adjust the allowable stresses if the modulus of elasticity E at
operating temperature is different from that one used for the design curves. In air, the
fatigue life of ferritic steels decreases with increasing temperature, however the effect
is small. For austenitic stainless steels the temperature has no significant effect on
the fatigue life plotted as S-N curve. In the nuclear codes and standards it is
supposed that the design S-N curves may accounted for any temperature effects by
the subfactor of 2 for “data scatter and material variability” [10]. This results in a
comparable reduction in fatigue life like in the European standard EN13445 [4] using
a temperature correction factor ft*.
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In the codes and standards there are different and specific requirements concerning
the surface finish of components especially for welded regions, for different vessel
and piping products and different joints. A special regard to the influence of surface
finish depending upon peak-to-valley height Rz is obvious. Depending on Rz for
carbon steel or low–alloy steel and for austenitic stainless steels the surface finish
would decrease fatigue life depending on the number of cycles. Surface finish may
be considered for number of cycles >103. Below this number of load cycles, the
influence of surface finish may be negligible, Figure 1.
0,01
0,10
1,00
10,00
10 100 1000 10000
Load Cycles / N
Str
ain
Am
plit
ud
e ε
a /
%
ASME Mean DataANL Mean Data polishedsmooth finishedroughened
Strain controlledRoom temperatureMaterial 20MnMoNi5-5
0,01
0,10
1,00
10,00
10 100 1000 10000
Load Cycles / N
Str
ain
Am
plit
ud
e ε
a /
%
ASME Mean DataANL Mean Data polishedsmooth finishedroughened
Strain controlledRoom temperatureMaterial 20MnMoNi5-5
Figure 1: Influence of surface finish for ferritic low alloy steel in the very low cycle fatigue regime.
In nuclear codes and standards it is supposed that this effect is accounted for in the
subfactor for “surface finish and environment” of 4.
To account for this effect the procedure included in European standard EN13445 [4]
differentiate between unwelded SSC with correction factor fS and welded SCC fatigue
design curves for weld details.
During the last three decades great endeavours have been made to investigate the
influence of the coolant environment on fatigue life [13, 14, 15, 16]. Today it is
generally accepted that the Light Water Reactor (LWR) environment can have a
significant impact on the fatigue life of carbon and low alloy steels as well as on
austenitic stainless steels under given circumstances and has to be involved in
cumulative fatigue life considerations.
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The U.S. Nuclear Regulatory Commission (NRC) has recently issued the Regulatory
Guide 1.207 “Guidelines for Evaluating Fatigue Analyses Incorporating the Life
Reduction of Metal Components Due to the Effects of the Light-Water Reactor
Environment for New Reactors” [17]. This Regulatory Guide is based on the research
work and publications by ANL [13], which provide equations for mean fatigue life
curves in LWR environment of carbon steels, low alloy ferritic steels and austenitic
stainless steels, exemplarily shown in Figure 2.
10 100 1000 10000 100000 1000000
0,1
1
10
.
ASME III Design (KTA 3201.2)
Reg.Guide 1.207
New Air Design Curve
(Factor 2 / 12)
ANL mean air 2007
ANL mean water 2007
ASME mean air
austenitic SS ParameterT = 240 °Cε = 0.01 %/sFen = 3.55 (ANL)E240 °C = 183 000 MPa
Str
ain
Am
plit
ud
e ε
a / %
Load Cycles / N
Figure 2: Fatigue life curves for stainless steels
The ANL fatigue life model for LWR environments includes parameters for the effects
of temperature, strain rate, dissolved oxygen content in water and, in case of ferritic
steels, sulphur content of the steel.
The environmental effects are expressed in terms of an environmental correction
factor Fen as the ratio of fatigue life in air environment at room temperature to fatigue
life in LWR coolant at operating temperature.
waterair
en NNF = (9)
The Fen factor, depending on the above-mentioned parameters (temperature, strain
rate, oxygen, sulphur), is also used to estimate the environmental fatigue usage.
∑ ×= ienien FUU (10)
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This procedure is consistent with the methods proposed in Japan [18, 19, 20],
whereas the parameters to determine Fen are somewhat different in the Japanese
approach.
In Regulatory Guide 1.207 a new fatigue design curve in air is developed based on
the new ANL mean air curve [13], lowered by a factor of 2 on stress and a reduced
factor of 12 on cycles, respectively whichever is more conservative. Again this curve
does not include safety margins and the environmental effects have to be considered
on the basis of Fen as stated above.
2.3 Fatigue analysis
Introduced by ASME in the early 1960s [7, 8, 6] for many years all pressure vessel
fatigue design rules followed the ASME approach. They are based on the concept
that the fatigue life of any SSC can be estimated from the S-N curve (design curve)
for the material concerned, obtained from uni-axial fatigue tests on small polished
specimens (mean data curve), by applying an appropriate fatigue strength reduction
factor (FSRF) Kf to account for discontinuities or weld joints. Instead of using the
FSRF applicable to pressure vessels, the piping analysis employ stress indices (Ci
and Ki) or stress concentration factors Kt for local areas based on linear elastic
calculations. Initially the S-N curves covered lives up to only 106 cycles. The overall
design approach was directed mainly at high-strain low-cycle fatigue (LCF)
conditions and little attention was paid to weld details as sources of fatigue. The
design curves were derived by introducing factors of 2 on stress and 20 on cycles of
the mean data curve concerned, whichever gave the lowest curve and is meant to
account for real effects (“scatter of data and material variability”, “size effects”,
“surface finish and environment”, [7, 8, 9, 10]) occurring during plant operation. All
pressure vessel and piping fatigue design rules are based essentially on the same
approach based on data from primarily low-cycle fatigue (LCF) tests.
As demonstrated in Figure 3 fatigue analysis can be performed by different concepts
which must not necessarily yield in the same result. The approach used in nuclear
codes and standards [1, 2, 3] can be assigned to local concepts (local notch strain or
stress concepts) in principal based on linear elastic stress calculations using stress
indices resp. stress concentration factors to take into account the structural
discontinuities in the SSC including welds [9].
Conservatism in the fatigue analysis may arise from the overall fatigue evaluation
procedure in conjunction with the fatigue design curves. This includes:
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• Stress analysis rules based on linear elastic calculations using stress indices,
stress concentration factors and plastic correction factor Ke.
• Use of S-N curves instead of service life curves.
• Reduced notch effect at cyclic conditions (fatigue stress concentration factor
smaller compared to stress concentration factor under static loads).
• Use of specified design transients with high number of cycles.
Fatigue Strength AssessmentFatigue Strength Assessment
ExperimentalAssessment
ExperimentalAssessment Analytical / Numerical AssessmentAnalytical / Numerical Assessment
Global ConceptBased on external forces or based on
nominal stresses within the component sections in question
Global ConceptBased on external forces or based on
nominal stresses within the component sections in question
Local ConceptBased on local stresses and related to criticalvalues of local phenomena like crack initiation
or crack propagation
Local ConceptBased on local stresses and related to criticalvalues of local phenomena like crack initiation
or crack propagation
Nominal StressConcept
Nominal StressConcept
Structural StressConcept
Structural StressConcept
Local Notch stressConcept
Local Notch stressConcept
Crack PropagationConcept
Crack PropagationConcept
Full ScaleComponent Test
Tests up to failure of thecomponent under typical
loading conditionsNF=f(loading),
eg. acc. to ASME-VIII, Div.2 or up to the tech-
nical crack initiation Ni=f(loading)
Full ScaleComponent Test
Tests up to failure of thecomponent under typical
loading conditionsNF=f(loading),
eg. acc. to ASME-VIII, Div.2 or up to the tech-
nical crack initiation Ni=f(loading)
Nominal Stress-Woehler-Line
- Material- Geometry- Surface finish /
Weld qualityNominal Stress Collektive
- Loading sequence- Geometry- Load amplitude
Accumulation of damage
NF=f(σNom)
Structural Stress resp.Strain Woehler-Line
- Material- Geometry- Surface finish /
Weld qualityStructural Stress resp.Strain Collektive
- Loading sequence - Geometry- Load amplitude
Akkumulation of damage
NF=f(σStruc)
Strain Woehler-line /cyclic Stress-StrainCurve
- Material- Weld quality
Local Notch Stress- Geometry- Load amplitude
Stress-Strain History- Load-time function
Accumulation ofdamage
NI=f(σNotch)
Crack Growth Curve
(cyclic)- Material- Load amplitude
Load-Time Function ΔKeff- Geometry- Crack geometry
Crack Growth / LifetimeWoehler-Line / Service-Life Curves (crack init. Up tofailure, ΔσNom)
Fatigue Strength AssessmentFatigue Strength Assessment
ExperimentalAssessment
ExperimentalAssessment Analytical / Numerical AssessmentAnalytical / Numerical Assessment
Global ConceptBased on external forces or based on
nominal stresses within the component sections in question
Global ConceptBased on external forces or based on
nominal stresses within the component sections in question
Local ConceptBased on local stresses and related to criticalvalues of local phenomena like crack initiation
or crack propagation
Local ConceptBased on local stresses and related to criticalvalues of local phenomena like crack initiation
or crack propagation
Nominal StressConcept
Nominal StressConcept
Structural StressConcept
Structural StressConcept
Local Notch stressConcept
Local Notch stressConcept
Crack PropagationConcept
Crack PropagationConcept
Full ScaleComponent Test
Tests up to failure of thecomponent under typical
loading conditionsNF=f(loading),
eg. acc. to ASME-VIII, Div.2 or up to the tech-
nical crack initiation Ni=f(loading)
Full ScaleComponent Test
Tests up to failure of thecomponent under typical
loading conditionsNF=f(loading),
eg. acc. to ASME-VIII, Div.2 or up to the tech-
nical crack initiation Ni=f(loading)
Nominal Stress-Woehler-Line
- Material- Geometry- Surface finish /
Weld qualityNominal Stress Collektive
- Loading sequence- Geometry- Load amplitude
Accumulation of damage
NF=f(σNom)
Structural Stress resp.Strain Woehler-Line
- Material- Geometry- Surface finish /
Weld qualityStructural Stress resp.Strain Collektive
- Loading sequence - Geometry- Load amplitude
Akkumulation of damage
NF=f(σStruc)
Strain Woehler-line /cyclic Stress-StrainCurve
- Material- Weld quality
Local Notch Stress- Geometry- Load amplitude
Stress-Strain History- Load-time function
Accumulation ofdamage
NI=f(σNotch)
Crack Growth Curve
(cyclic)- Material- Load amplitude
Load-Time Function ΔKeff- Geometry- Crack geometry
Crack Growth / LifetimeWoehler-Line / Service-Life Curves (crack init. Up tofailure, ΔσNom)
Figure 3: Different Methods to perform fatigue analysis
However the codes and standards permit the use of new and improved approaches
to fatigue analysis like e.g.
• elastic plastic finite element calculations, which may result in lower stress/strain
amplitudes,
• improved (lower) plastic correction factors Ke and
• use of load cycles and temperature transients derived from on-line fatigue moni-
toring.
Furthermore the stress analysis procedure is based on strength hypotheses which
are fully verified for static loads and for fatigue strength (endurance limit). In the
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range of fatigue strength for finite life appropriate strength hypotheses are still under
verification, especially when the principal stresses or strains change directions or
there is a phase shift in stress or strain components.
3 Experimental Investigations
3.1 Air environment
Tests in air environment at room
temperature, 288°C and 350°C were
performed under strain controlled
conditions and few under stress
controlled conditions, Figure 4. Test
material for the smooth cylindrical
specimens and smooth hollow
cylindrical specimens was low alloy
ferritic steel 20MnMoNi5-5 (similar to
ASTM A533, Gr. B, Cl. 2) and
stabilized austenitic stainless steel
X 10 CrNiNb 18 9 S (similar to ASTM
TP347) and X 6 CrNiTi 18 10 S
(similar to ASTM TP321).
The major results are as follows.
• For the low alloy ferritic steel
20MnMoNi5-5 the fatigue life data
at room temperature are shown in
Figure 5. Included are the ASME
mean data curve Eq. 5 and the
ANL mean data curve Eq. 7.
• For the low alloy ferritic steel 20MnMoNi5-5 the fatigue life data at temperatures
between 240 and 350oC are shown in Figure 6 and compared with the ASME
mean data curve Eq. 5 and the ANL mean data curve Eq. 7 (T=300oC).
Figure 4: Equipment for fatigue testing in air environment
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0,01
0,10
1,00
10,00
10 100 1000 10000 100000 1000000 10000000
Load Cycles / N
Str
ain
Am
plit
ud
e ε
a /
%
Strain controlledRoom temperatureMaterial 20MnMoNi5-5
ASME Mean Data Curve
ANL Mean Data CurveT = 20°C
Stress controlled
0,01
0,10
1,00
10,00
10 100 1000 10000 100000 1000000 10000000
Load Cycles / N
Str
ain
Am
plit
ud
e ε
a /
%
Strain controlledRoom temperatureMaterial 20MnMoNi5-5
ASME Mean Data Curve
ANL Mean Data CurveT = 20°C
Stress controlled
Figure 5: Fatigue strain vs. life data for the low alloy ferritic steel 20MnMoNi5-5 in air
at room temperature
For more relevant ε–N data from MPA Universtät Stuttgart (MPA) tests for low alloy
ferritic steels (20MnMoNi5-5, 22NiMoCr3-7, 15MnNi6-3, 15NiCuMoNb5) at tempera-
tures up to 350oC are shown in Figure 7. Included are the ASME mean data curve
Eq. 5, the ANL mean data curve Eq. 7and a mean data curve representing the MPA
ε–N data by
).(ln..)Nln( a 1023024929945 −ε−= (11)
Between 103 and 105 load cycles there is a good conformity. In the very LCF and
HCF regime for a detailed evaluation more data are needed.
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0,01
0,10
1,00
10,00
10 100 1000 10000 100000 1000000 10000000
Load Cycles / N
Str
ain
Am
plit
ud
e ε
a /
%
Strain controlledTemperatures 240-350oCMaterial 20MnMoNi5-5
ASME Mean Data Curve
ANL Mean Data CurveT = 300°C
0,01
0,10
1,00
10,00
10 100 1000 10000 100000 1000000 10000000
Load Cycles / N
Str
ain
Am
plit
ud
e ε
a /
%
Strain controlledTemperatures 240-350oCMaterial 20MnMoNi5-5
ASME Mean Data Curve
ANL Mean Data CurveT = 300°C
Figure 6: Fatigue strain vs. life data for the low alloy ferritic steel 20MnMoNi5-5 in air
at temperatures 240-350oC
0,01
0,10
1,00
10,00
10 100 1000 10000 100000 1000000 10000000
Load Cycles / N
Str
ain
Am
pli
tud
e ε
a
/ %
Strain controlledTemperatures 20-350oCMaterial: 20MnMoNi5-5
22NiMoCr3-715MnNi6-315NiCuMoNb5
MPA Data best-fit
Stress controlled
ASME Mean Data Curve
ANL Mean Data CurveT = 20°C
0,01
0,10
1,00
10,00
10 100 1000 10000 100000 1000000 10000000
Load Cycles / N
Str
ain
Am
pli
tud
e ε
a
/ %
Strain controlledTemperatures 20-350oCMaterial: 20MnMoNi5-5
22NiMoCr3-715MnNi6-315NiCuMoNb5
MPA Data best-fit
Stress controlled
ASME Mean Data Curve
ANL Mean Data CurveT = 20°C
Figure 7: MPA fatigue strain vs. life data for the low alloy ferritic steels in air at
temperatures up to 350oC
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For the stabilized austenitic stainless steels X6CrNiNb18-10 and X6CrNiTi18-10 the
fatigue life data at temperatures between room temperature and 350oC are plotted in
Figure 8 together with the ASME mean data curve Eq. 3, the ANL mean data curve
Eq. 8 and a mean data curve representing the MPA ε–N data by
).(ln.,)Nln( a 1025401891264246 −ε−= (12)
The MPA mean data curve is in very good agreement with the ANL mean data curve
for load cycles >103.
0,01
0,10
1,00
10,00
10 100 1000 10000 100000 1000000
Load Cycles / N
Str
ain
Am
plit
ud
e ε
a /
%
Strain controlled Temperatures 20-350oCMaterials: X10CrNiNb18-9
X10CrNiTi18-9
MPA Data best-fit
ANL Mean Data Curve
ASME Mean Data Curve
0,01
0,10
1,00
10,00
10 100 1000 10000 100000 1000000
Load Cycles / N
Str
ain
Am
plit
ud
e ε
a /
%
Strain controlled Temperatures 20-350oCMaterials: X10CrNiNb18-9
X10CrNiTi18-9
MPA Data best-fit
ANL Mean Data Curve
ASME Mean Data Curve
Figure 8: Fatigue strain vs. life data for the stabilized austenitic stainless steels
X 10 CrNiNb 18 9 S and X 10 CrNiTi 18 9 S in air at temperatures 20°C-350oC
• Cyclic tests with multi-axial loading (torsion or torsion with superimposed longitu-
dinal loading), Figure 9, fit into the mean data curves of the uniaxial cyclic tests,
Figure 10. The calculation of the equivalent stress amplitude according to ASME-
BPVC Section III, Subsection NH (based on the equivalent strain range, Eq. (13))
shows good results in comparison to the tests performed.
( ) ( ) ( ) ( ) ( )22222223
122
zxiyzixyixiziziyiyixiVi γΔ+γΔ+γΔ+εΔ−εΔ+εΔ−εΔ+εΔ−εΔν−
=εΔ∗
(13)
with ν*=0.5 for plastic und ν*=0.3 for elastic material behaviour.
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Cooledspecimen grip
Heating device
Cooledspecimen grip
Specimen withthermo couple
Figure 9: Equipment for fatigue testing of hollow cylindrical specimens under tension-torsion loading in air environment
10
100
1000
10000
100000
10 100 1000 10000 100000 1000000
Load Cycles / N
Str
ess
Am
plit
ud
e
σV
a /
MP
a
Strain controlledRoom temperatureMaterial 20MnMoNi5-5
Torsion with superimposed Axial Loading
90o Phase Shift
ANL Mean Data Curve
10
100
1000
10000
100000
10 100 1000 10000 100000 1000000
Load Cycles / N
Str
ess
Am
plit
ud
e
σV
a /
MP
a
Strain controlledRoom temperatureMaterial 20MnMoNi5-5
Torsion with superimposed Axial Loading
90o Phase Shift
ANL Mean Data Curve
Figure 10: Fatigue stress vs. life data from hollow cylindrical specimen testing with
low alloy ferritic steel 20MnMoNi5-5 in air at room temperatures
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3.2 Simulated High temperature boiling water reactor environment
The investigations of environmental effects on fatigue life of stainless steels per-
formed in the USA and in Japan are predominantly based on unstabilized austenitic
stainless steels. There is a need to investigate whether the fatigue behaviour of the
niobium or titanium stabilized austenitic stainless steels used in German nuclear
power plants in contact with oxygenated high temperature (HT) water (boiling water
reactor BWR coolant) can be predicted by the curves and methods developed in the
USA and in Japan. In a second research program tests were performed with the low
alloy ferritic steel 22NiMoCr3-7.
Experiments at MPA [21], [22] are performed in a miniature autoclave, Figure 11,
which is fixed on the specimen shoulders and encloses the gauge length of the
specimen, similar to the test equipment at ANL [13]. The electrochemical potential is
measured in the same loop in a separate cell with an Ag/AgCl Electrochemical
Potential (ECP) electrode, Figure 11.
Al ignment Fix ture MT S6 09
Load Cel l M TS 661. 21B-02
Col le t Gr ip M TS 646. 10
PrüfmaschineLCFImit ECP Prüfbehälter(Ansicht vonhinten)
660
22
2
1
2
2
8
1 2
8
8 88
8
4 4
4 4
1
4
2
5 5
1 x3
6
Vorlauf Rücklauf
9 x2
7
miniature autoclavefor LCF-experimentsin BWR environment
autoclave forECP-measurement
Figure 11: Autoclave for Low Cycle Fatigue (LCF)-Experiment
3.2.1 Test with austenitic stainless steel
The specimens are machined from 2 nuclear grade pipe materials (wall thickness
35 mm and 45 mm, resp.) of the austenitic stainless steels X10CrNiNb18-9 and
X10CrNiTi18-9, stabilized with Ni and Ti, respectively, which are comparable to the
US steels A347 and A321, [21], Table 1. The specimens of steel X10CrNiTi18-9 are
additionally tested in a sensitized material state after an annealing treatment at 620
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- 13.15 -
°C for 8 hours in order to simulate the depletion of solute Chromium as a result of a
welding procedure and post-weld heat treatment, respectively.
Low cycle fatigue (LCF) tests are conducted in air and simulated BWR HT-water
environment at 240°C, the oxygen content of the water is adjusted to 400 ppb. In
addition to high purity water, few tests are performed at an increased conductivity of
0.8 µS/cm by dosing sulphate (90 ppb SO4 by adding highly diluted sulfuric acid
H2SO4). The flow velocity is quasi stagnant (0.004 m/s).
All experiments in water were preceded by a soaking period of 100 h at test
conditions. The LCF tests in water were conducted with strain amplitudes of 0.6, 0.9
and 1.2 % and an uniform strain rate of 0.01 %/s, Table 2 (strain rate in air 0.1 %/s).
Table 1: Specification of the tested materials
Test materials
Stainless steel 1.4550 (X 10 CrNiNb 18 9 S) 1), Nb – stabilized (C = 0,06 %)
from pipe with 35 mm wall thickness
Stainless steel 1.4541 (X 10 CrNiTi 18 9 S) 2), Ti – stabilized (C = 0,06 %)
from pipe with 45 mm wall thickness
● as delivered (solution annealed), EPR 3) - value = 0,3%·
● sensitised 620°C/8h, EPR - value = 1.3 – 1.4 % 1) acc. to current standard KTA 3201.1: X 6 CrNiNb 18 10 S 2) acc. to current standard KTA 3201.1: X 6 CrNiTi 18 10 S 3) EPR: Electrochemical Potentiokinetic Reactivation
Table 2: Test matrix of the LCF experiments with austenitic stainless steels
material material state εa [%] environment
1.4541 as delivered
0.6 0.9 1.2 air 240 °C
1.4550 as delivered 0.6 0.9 1.2 air 240 °C
1.4541 as delivered 0.6 0.9 1.2 water 240 °C
1.4541 sensitized 0.6 0.9 water 240 °C
1.4541 as delivered 0.6 0.9 sulphate 240 °C
1.4550 as delivered 0.9 1.2 water 240 °C
Strain rate in air: 0.1 %/s
Strain rate in water: 0.01 %/s
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The fatigue life in simulated BWR environment is conservatively covered by the ANL
mean water curve for both austenitic stainless steels, Figure 12 and Figure 13. The
influence of the simulated BWR environment, characterized by the Fen factor, tends
to decrease with increasing strain amplitude. The fatigue life of the sensitized
material seems to be marginally reduced compared with the original solution
annealed material state of steel X10CrNiTi18-9, Figure 14, (reduction in average 4
and 12 % at strain amplitudes of 0.6 and 0.9 %, respectively), but with regard to the
low number of specimens there is no statistical evidence of this fact. For the
experiments in sulphate containing environment the decrease in fatigue life is quite
moderate in the range of 20 % compared to the results in high purity water, Figure
15, again all data points are conservatively covered by the ANL prediction curve.
100 1000 100000,2
0,3
0,4
0,5
0,6
0,7
0,80,91,0
2,0
.ε = 0.01 %/s
E 240 °C = 183 000 MPa
ANL water 2007 X10CrNiNb18-9 HT-water 240 °C
Str
ain
Am
plit
ud
e ε
a /
%
Load Cycles / N
ANL air 2007
ASME air
Figure 12: Fatigue life of Nb-stabilized austenitic stainless steel in HT-water compared
with ANL mean water curve
The results so far suggest that the fatigue life of the stabilized austenitic stainless
steels used in German nuclear power plants is similar to that of the unstabilized aus-
tenites of United States and Japan and can be reliably predicted by the existing me-
thods.
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- 13.17 -
100 1000 100000,2
0,3
0,4
0,5
0,6
0,7
0,80,91,0
2,0
.ε = 0.01 %/sE 240 °C = 183 000 MPa
ANL water 2007 X10CrNiTi18-9
HT-water 240 °C
Str
ain
Am
plit
ud
e ε
a /
%
Load Cycles / N
ANL air 2007
ASME air
Figure 13: Fatigue life of Ti-stabilized austenitic stainless steel in HT-water compared with ANL mean water curve
100 1000 10000,2
0,3
0,4
0,5
0,6
0,7
0,80,91,0
2,0
.ε = 0.01 %/sE 240 °C = 183 000 MPa
ANL water 2007
X10CrNiTi18-9 sensitized HT-water 240 °C
Str
ain
Am
plit
ud
e ε
a /
%
Load Cycles / N
ANL air 2007
ASME air
Figure 14: Fatigue life of sensitized Ti-stabilized austenitic stainless steel in HT-water compared with ANL mean water curve
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- 13.18 -
100 1000 100000,2
0,3
0,4
0,5
0,6
0,7
0,80,91,0
2,0
.ε = 0.01 %/sE 240 °C = 183 000 MPa
ANL water 2007
X10CrNiTi18-9 HT-Sulphate 240 °C
Str
ain
Am
plit
ud
e ε
a /
%
Load Cycles / N
ANL air 2007
ASME air
Figure 15: Fatigue life of Ti-stabilized austenitic stainless steel in HT sulphate water compared with ANL mean water curve
3.2.2 Tests with low alloy ferritic steel
Corresponding LCF experiments were performed with the ferritic steel 22NiMoCr3-7
(comparable to US steel A508, class 2) representative for reactor pressure vessel
(RPV) steels of European production [22]. The specimens were taken from a nuclear
grade vessel shell (wall thickness 143 mm without cladding) which was primarily
produced for a German nuclear power plant (BWR). In these LCF tests strain
amplitudes of 0.3, 0.5 and 0.9 % were applied at strain rates of 0.1 and 0.01 %/s.
The water condition (high purity water, sulphate 90 ppb SO4) and temperature of
240°C was identical with the tests of austenitic material.
According to the procedure proposed by ANL and Regulatory Guide 1.207, the mean
data curve for the fatigue life in water environment depends on the strain rate, i.e. 2
different curves are calculated for the “fast” and the “slow” strain rate at test
conditions. The corresponding environmental correction factors according to ANL are
Fen = 2.57 for the high and Fen = 3.27 for the slow strain rate.
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- 13.19 -
10 100 1000 10000 100000 1000000
0,1
1
10
Fen
= 3.27 (0.01 %/s)
HT-water 240 °C 0.1 %/s HT-water 240 °C 0.01 %/s
Fen
= 2.57 (0.1 %/s)
ANL water 20070.01 %/s
ANL water 20070.1 %/s
ASME mean air ferritic Steel 22NiMoCr3-7
Str
ain
Am
plit
ud
e /
%
Load Cycles / N
Figure 16: Results of the LCF experiments in high purity water at different strain rates.
Figure 16 shows the results of the LCF experiments in high purity water. At the fast
strain rate of 0.1 %/s the fatigue life of the specimens is in good accordance with the
corresponding mean data curve (denoted by ANL water 2007, 0.1 %/s), whereas at
the slow strain rate of 0.01 %/s, all experimental data are below the corresponding
mean data curve, i.e. in this case the effect of environment is underestimated for this
material. Investigations reveal that the cyclic stress-strain behaviour of this steel is
influenced by the strain rate, resulting in a higher hardening of the material at slow
strain rates, which might affect the sensitivity to corrosion effects under cyclic
loading. In air the strain rate has no influence on fatigue life.
In Figure 17 the fatigue behaviour at the fast strain rate of 0.1 %/s in sulphate is
compared with the experiments in high purity water. At strain amplitudes of 0.5 and
0.9 % hardly any influence of sulphate on fatigue life can be detected. Only at the
lowest strain amplitude of 0.3 %, associated with the longest exposure time in
environment, the fatigue life in sulphate tends to lower values.
At the slow strain rate of 0.01 %, only 2 experiments were performed at each strain
amplitude, Figure 18. As in high purity water all fatigue data in sulphate fall below the
ANL mean data curve. There is no significant effect of sulphate but a tendency to a
little lower average values compared with high purity water.
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- 13.20 -
10 100 1000 10000 100000 1000000
0,1
1
10
ANL water 20070.1 %/s
ASME mean air
HT-Wasser 240 °C 0.1 %/s
Sulphate 240 °C 0.1 %/s
Fen
= 2.57 Sulphate Content 70-100 ppb
ferritic Steel 22NiMoCr3-7
Str
ain
Am
plit
ud
e /
%
Load Cycles / N Figure 17: Fatigue behaviour at the fast strain rate of 0.1 %/s in sulphate compared
with high purity water
10 100 1000 10000 100000 1000000
0,1
1
10
ANL water 20070.01 %/s
ASME mean air
HT-water 240 °C 0.01 %/s
Sulfat 240 °C 0.01 %/s
Fen
= 3.27 Sulphate Content 70-100 ppb
ferritic Steel 22NiMoCr3-7
Str
ain
Am
plit
ud
e
/ %
Load Cycles / N Figure 18: Fatigue behaviour at the slow strain rate of 0.01 %/s in sulphate compared
with high purity water
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- 13.21 -
4 Conclusions
The results of fatigue tests performed at MPA in air environment to develop ε–N data
for the German low-alloy steels (e.g. 20MnMoNi5-5) are well represented by the new
ANL mean data curve for room temperature as well as for elevated temperatures
(slight decrease in fatigue life with increasing temperature).
For the German niobium or titanium stabilized austenitic stainless steels in air
environment the temperature has no significant effect on the fatigue life. This is in
conformity with the new ANL mean data curve to be applied up to 400oC.
The results of LCF experiments in simulated BWR water environment at 240°C show
a good correlation between fatigue life of German nuclear power plant materials and
international fatigue mean curves (ANL mean data curve). Only at low strain rates,
the environmental effects on fatigue life of the tested low alloy steel (22NimoCr3-7)
tend to be overestimated by the ANL mean data curve.
A sulphate content of the simulated BWR environment (90 ppb SO4) has no
significant influence on the fatigue behaviour.
Literatur
[1] Rules for Construction of Nuclear Power Plant Components. ASME Boiler and Pressure Vessel Code, Section III, Subsection NB, The American Society of Me-chanical Engineers (2007)
[2] Safety Standards of the Nuclear Safety Standards Commission (KTA). KTA Rules 3201 and 3211, Carl Heymanns Verlag KG, Cologne, latest edition
[3] French Design and Construction Rules for Mechanical Components of PWR Nu-clear Islands (RCC-M). AFCEN - Association Française pour la Construction des Ensembles Nucléaires, Paris
[4] European Standard EN 13445-3:2002 Unfired pressure vessels, Part 3: Design
[5] Coffin, L.F., A Study of the Effect of Cyclic Thermal Stresses on a Ductile Metal, Trans. ASME 76, 1954, pp. 931-950
[6] B. F. Langer., "Design of Pressure Vessels for Low-Cycle Fatigue", Journal of Basic Engineering, Vol. 84, 3 (1962)
[7] Criteria of Section III of the ASME Boiler and Pressure Vessel Code for Nuclear Vessels, the American Society of Mechanical Engineers, Library of Congress Catalog Card Number: 56-3934 (1963)
[8] Criteria of the ASME Boiler and Pressure Vessel Code for design by analysis in Section III and VIII, Division 2, the American Society of Mechanical Engineers, Library of Congress Catalog Card Number: 56-3934 (1969)
[9] Fatigue strength reduction and stress concentration factors for welds in pressure vessels and piping, Welding Research Council Bulletin, WRC 432, June 1998
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- 13.22 -
[10] O. K. Chopra, W. J. Shack, Effect of LWR Coolant Environments on the Fatigue Life of Reactor Materials, NUREG/CR-6909, ANL-06/08, February 2007
[11] C. E. Jaske, W. J. O'Donnell, "Fatigue Design Criteria for Pressure Vessel Al-loys", Journal of Pressure Vessel Technology (1977)
[12] D. R. Diercks, "Development of Fatigue Design Curves for Pressure Vessel Al-loys using a Modified Langer Equation", Journal of Pressure Vessel Technology, Vol. 101 (1979)
[13] O. K. Chopra,, Effects of LWR Coolant Environments on Fatigue Design Curves of Carbon and Low Alloy Steels, NUREG/CR-6583, ANL-97/18, March 1998
[14] S. Rosinski, Materials Reliability Program (MRP), Evaluation of Fatigue Data Including Reactor Water Environmental Effects (MPR-49), EPRI Technical report 1003079, Dec. 2001
[15] H. D. Solomon, C. R. E. Amzallag, R. E. Delair, A. J. Vallee, Strain Controlled Fatigue of Type 304L SS in Air and PWR Water, 3rd Intern. Conference on Fa-tigue of Reactor Components, Seville, Spain, October 3 – 6, 2004
[16] M. Higuchi, Japanese Program Overview, 3rd Intern. Conference on Fatigue of Reactor Components, Seville, Spain, October 3 – 6, 2004
[17] Guidelines for Evaluating Fatigue Analyses Incorporating the Life Reduction of Metal Components due to the Effects of the Light-Water Reactor Environment for New Reactors, Regulatory Guide 1.207, U.S. Nuclear Regulatory Commis-sion, March 2007
[18] M. Nishimura, T. Nakamura, Y. Asada, TENPES Guidelines for Environmental Fatigue Evaluation in LWR Nuclear Power Plant in Japan, 2nd Intern. Confer-ence on Fatigue of Reactor Components, Snowbird, USA, July 29 – 31, 2002
[19] M. Higuchi, K. Sakaguchi, H. Akihiko, Y. Nomura, Revised and New Proposal of Environmental Fatigue Life Correction Factor (Fen) for Carbon and Low-Alloy Steels and Nickel Base Alloys in LWR Water Environments, Proceedings of PVP2006-ICPVT-11-93194, July 2006
[20] T. Nakamura, M. Higuchi, K. Takehiro, Y. Sugie, JSME Codes on Environmental Fatigue Evaluation, Proceedings of PVP2006-ICPVT-11-93305, July 2006
[21] T. Weissenberg, Zentrale Untersuchung und Auswertung von Herstellungsfeh-lern und Betriebsschäden im Hinblick auf druckführende Anlagenteile von Kern-kraftwerken, Arbeitspaket 3.1, Einfluss des Reaktorkühlmediums auf das Ermü-dungsverhalten austenitischer CrNi-Stähle, BMU-Vorhaben SR 2501, MPA Uni-versität Stuttgart, November 2007
[22] T. Weissenberg, Ermüdungsverhalten ferritischer Druckbehälter- und Rohrlei-tungsstähle in sauerstoffhaltigem Hochtemperaturwasser (Fatigue Behaviour of Ferritic Steels for Pressure Vessels and Piping in Oxygenated High Temperture Water), Final Report, Reactor Safety Research – Project No.: 1501309, MPA Universität Stuttgart, September 2009 (German Language)