/smash/get/diva2:605630/FULLTEXT01.pdf

/smash/get/diva2:605630/FULLTEXT01.pdf
Evaluation of current methods for creep analysis
and impression creep testing of power plant steels
Master thesis
Jonas Larsson
11/10-2012
Master of Science thesis
Supervisor: Jan Storesund
ITM: Material Science and engineering
KTH Royal Institute of Technology
Abstract
Destructive testing of creep exposed components is a powerful tool for evaluation of remaining
lifetime of high temperature pipe systems. The most common destructive evaluation method
used today is uniaxial creep testing. Uniaxial creep tests can produce accurate creep curves but
the test method has some drawbacks such as costliness and long testing times. It also demands
large sample material outtake which often involve weld repair.
Impression creep (IC) testing is a relatively new alternative test method for evaluating primary
and secondary creep rates. The scope of this work is to evaluate the benefits and drawbacks of IC
testing over uniaxial creep testing in order to determine its usefulness as a test method.
A literature survey was carried out over the area creep testing of high temperature pipe systems,
with particular focus on impression creep testing. The result of the literature survey clearly
showed several benefits with impression creep testing. An IC test series was performed in order
to determine the secondary creep rate of a service exposed 10CrMo9-10 high temperature pipe
steel. The IC tests were performed by VTT in Finland, using the same test parameter and sample
material as in previous projects where the creep properties of the test material were determined
by uniaxial creep testing.
The result of the predicted secondary creep rate obtained from the IC tests was compared with
the secondary creep rates measured during the uniaxial tests. The IC tests results did not align
satisfactory with the results from the uniaxial creep tests, which would have been expected. The
reason for this may be due to sources of error during impression creep testing, since very small
displacements due to creep have to be measured with high precision during the tests. Further
testing of the impression creep test method is recommended as a result of this work, in order to
evaluate the method.
Keywords: Impression creep test, 10CrMo9-10 steel, creep life assessment, creep analysis.
Sammanfattning
Förstörande provning av krypexponerade komponenter är ett kraftfullt redskap för utvärdering
av återstående livslängd hos rörsystem med höga drifttemperaturer. Den vanligaste formen av
förstörande provning i dessa fall är idag enaxlig krypprovning. Enaxliga krypprovningar
producerar fullständiga krypkurvor men provningsmetoden har vissa nackdelar såsom att den är
relativt dyr och tar förhållandevislång tid.
Impression creep eller (IC) –provning är en relativt ny, alternativ, testmetod för att utvärdera
primär och sekundärkryp. Det här arbetet ämnar utreda för- och nackdelar med IC-provning
gentemot enaxlig krypprovning, samt undersöka dugligheten av IC-provning som testmetod.
En litteraturstudie över området provning av krypegenskaper hos rörsystem med höga
drifttemperaturer, med extra fokus på IC-provning har genomförts. Resultatet av litteraturstudien
pekade tydligt på fördelarna med IC provning. En serie IC-tester utfördes också i syfte att
bestämma den sekundära kryphastigheten hos ett driftpåkänt 10CrMo9-10 låglegerat
tryckkärlsstål avsett för höga drifttemperaturer. IC-provningen gjordes av VTT Finland. Samma
testparametrar och samma provmaterial som hade använts i tidigare projekt där
krypegenskaperna hos provmaterialet har utvärderats bl.a. genom enaxlig krypprovning.
Resultaten från IC-provningen jämfördes med de sekundära krypningshastigheterna som hade
observerats vid den enaxliga krypprovningen. Resultaten från IC-provningen visade sig avvika
från resultateten från den enaxliga krypprovningen. Orsaken till det kunde inte förklaras.
Mätningar av mycket små förskjutningar samt små temperaturavvikelser föreslogs eventuellt
kunna leda till felkällor. Som ett resultat av det här arbetet förslås fortsatt utvärdering och
provning med IC-metoden behövs innan provningsmetoden kan tas i bruk.
Sökord: Impression creep test, 10CrMo-10 stål, återstående kryplivslängd, krypningsanalys.
Acknowledgements
I wish to express my thanks to my supervisors PhD Jan Storesund (Inspect) and PhD Anders
Eliasson (KTH) for their valuable guidance and support. Secondly I would like to thank
everybody at Inspect Technology who helped me during the project.
Contents
1 Introduction ............................................................................................................................................... 1
2 Background ................................................................................................................................................ 2
2.1 Fundamental about creep ................................................................................................................. 2
2.2 Designing for creep ........................................................................................................................... 3
2.3 Inspections of pressurized systems ................................................................................................. 7
2.4 Prediction of remaining life for a high temperature pipe systems .............................................. 8
2.5 Creep deformation mechanisms .................................................................................................... 11
2.6 Creep resistant steels ....................................................................................................................... 12
2.7 Different models for creep ............................................................................................................. 14
2.8 Non-destructive testing of creep exposed components............................................................. 26
2.9 Non-destructive tests methods ...................................................................................................... 28
2.10 Destructive evaluation methods .................................................................................................. 32
3. Investigation............................................................................................................................................ 42
3.1 About the test material .................................................................................................................... 42
3.1.1 Approximative estimation remaining life .............................................................................. 43
3.2 Metallurgical evaluation of test material ....................................................................................... 46
3.2.1 Microstructural evaluation of the weld.................................................................................. 46
3.2.2 Hardness measurements .......................................................................................................... 48
3.2.3 Carbide measurements ............................................................................................................. 52
3.2.4 Grain size measurements......................................................................................................... 53
3.3 Destructive evaluation of the test material................................................................................... 54
3.3.1 Impression creep test ............................................................................................................... 54
3.3.2 Uniaxial creep test .................................................................................................................... 58
3.3.3 Sensitivity analysis..................................................................................................................... 59
3.3.4 LSCP model prediction of secondary creep rate ................................................................. 60
4. Results and Discussion .......................................................................................................................... 61
5. Conclusion .............................................................................................................................................. 63
6. Bibliography ........................................................................................................................................... 64
Appendix. Microstructures. ...................................................................................................................... 68
1 Introduction
High temperature pipe systems in power plants are designed to operate in an environment where
they are subjected to high temperature- and stress loads as well as high pressures. These
unfriendly environments require usages of the material in its creep range.
Creep deformation of a material in its creep range is unavoidable after a certain time. A final
material failure due to creep deformation cannot in this case be avoided, but it can be controlled
by correct system design and regular monitoring of the microstructure by inspections, which can
prolong the final material failure by several years after the designed lifetime of the component.
Inspections of high temperature systems are performed both by i) safety reasons which is
mandatory in Sweden and Finland (1), ii) economic reasons where necessary component repairs
and replacements can be planned on forehand, and expensive unplanned maintenance stoppages
can be avoided.
High temperature systems are normally designed to last for 100 000-200 000 h. Thus, a majority
of all high temperature systems in Scandinavia exceeds their designed lifetime and can still be
productive many years after. The maintenance need for aged high temperature pipe systems
increases with time, thereby also the need for inspections.
The microstructure of a component in a high temperature system can be evaluated by nondestructive- and destructive evaluations in order to detect creep. In most cases are several
different non-destructive and destructive evaluations preformed since they are complimentary to
each other.
Impression creep testing is a destructive evaluation method and was invented in the 70:s (2). The
test method is not widely spread but is successfully used in some parts of the world, for example
Great Britain. Impression creep test has some advantages e.g. shorter test times, over
conventional creep strain tests when determining a materials minimum creep rate.
This work aims to i) produce an updated literature survey over the field of creep analysis of high
temperature systems and their components, with focus on a high temperature pipe system in
power plant made out of low alloyed steel. ii) Evaluate the benefits and possible disadvantages of
impression creep testing over conventional creep strain test. A sample of service exposed material
from a high temperature pipe system will be subject to a series of impression creep tests. The
results will be compared to earlier conventional creep tests performed on the same sample
material.
1
2 Background
2.1 Fundamental about creep
The creep curve
Creep occurs in materials that are exposed for static mechanical stress and elevated temperature
over time. The result of creep is a permanent deformation of the material. A general rule for
metals is that the temperature must be at least 0,4TM for creep to occur, where TM is the melting
temperature of the metal in Kelvin. Creep rate increase with increasing temperature and
increasing stress on the material, this is schematically illustrated in figure 1:
Figure 1. The effect of stress and temperature on the creep strain rate.
The creep process can be divided in to three phases; figure 2. The curve is the creep strain as a
function of time. The slope  is the creep strain rate. The creep rate varies over time and can be
divided in to three regions; primary, secondary and tertiary creep (3).
Figure 2. The typical creep curve where the creep strain is a function of the time.
Rupture occurs at the t ime t r .
2
Primary creep (also known as transient creep) shows a steady decrease in creep rate. Dislocations
are formed in the material during limited plastic deformation. Over time more and more
dislocations are formed, and dislocation hardening is taking place. The dislocation hardening is
the reason for the steady decrease in creep rate (3).
Secondary creep (also known as steady state creep or minimum creep) takes place at
approximately constant creep rate. The dislocation density is now sufficient and annihilation of
dislocations takes place. The annihilation effect can be described as a softening of the material,
which lowers the dislocation density and thereby also the internal energy of the material. The
ongoing formation of new dislocations and annihilations of old dislocations keeps the dislocation
density at a constant level, and therefor also the creep rate. The difference between creep
deformation and other deformation mechanisms which take place at lower temperatures, is the
effect of the increased temperature which creates thermal fluctuation in the material and allows
creep deformation to take place (3).
Finally, tertiary creep takes place which eventually will lead the material to rupture. Voids are now
present in the grain boundaries which weaken the material and leads to an increase in creep rate.
Formation and/or coarsing of carbides and nitrides, and an increased annihilation of dislocations
over formations of new ones can also contribute to increased creep rate. Voids will continue to
nucleate, grow, accumulate and finally form cracks in the grain boundaries until rupture occurs.
The final rupture is a result of the weakened microstructure caused by the void formation in the
grain boundaries, and the ageing process of the micro structure. If necking occur and the stress
intensity increase over the decreasing cross area, shortening the time to rupture (3).
The final rupture is caused by microstructural- and/or metallurgical changes. Examples of those
are grain boundary separation, formation of internal crack formation, weakening by cavities or
void formations, decreased cross area due to necking (3).
2.2 Designing for creep
A number of factors need to be considered when designing system and components to resist
creep in high temperature applications. Temperature and stress levels need to be kept low in
order to avoid creep deformation. Elevated temperature and stress levels lead to creep in many
engineering materials and limit the components creep life. The rule of thumb when designing for
creep is that: if the temperature is high, the stress levels must be relatively low, and vice versa.
The service temperature of the component is controlled by the process, where elevated
temperature in general leads to improved efficiency of the process. The maximum temperature is
often limited only by the creep resistance of the material that the component is made of,
including its welds. The service life of the component is controlled by the stress loads it is
subjected to at a certain service temperature. This is considered when designing the system.
Different parameters e.g. Larson-Miler index (equation 7) can be used to design a component to
withstand creep deformation for a certain time i.e. design life.
European standard for high temperature systems
High temperature systems have to meet the Swedish safety regulations which could be achieved
by applying a Swedish standard (Svensk standard). Swedish standard is the based on the
European Norm (EN). Its American analogy is the ASME regulation and certification norm. The
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European Norm sees that the system can withstand corrosion, fatigue, creep and other damage
mechanisms. This is by regulating the choice of material, geometry of the component and general
design of the system. The design of single components is controlled by the service temperature,
internal pressure, additional loads and the medium it is in contact with. The entire process of
designing a high temperature plant from generation of the blue print of the high temperature
plant, to startup is described in this thesis. The following example is for a high temperature
piping system in a power plant and is based in the norm SS-EN 13480 (4).
Material selection
The regulations require minimum values for ductility and impact toughness in the finished
product and the design need guarantied strength properties at operating temperatures. As
welding and forming decrease these properties, the content of carbon, sulphur and phosphor of
the material has to be limited. For materials that should be used in the creep range additional time
dependent properties as creep rupture strength is needed. Creep rupture strength is strength
values associated with certain plastic strain and values at a given design life time must fulfill the
requirements.
Design
The design procedure can be divided in to the two steps, i) design of single components and
ii) design of the entire system. Design of components geometry are made individually and based
on the design stress f .
i)
An example is the simple case when calculating the minimum thickness of a straight
pipe subjected to internal pressure, equation 1:
e
pc D0
2 f  pc
(1)
where e is the internal diameter, p c is the internal pressure and, D0 is the outer
diameter. Geometries of more complex part are calculated by the same principle but
with more substantial calculations. Other factor in addition to the internal pressure
and the outer diameter often has to be considered in these calculations. This includes,
corrosion, outer pressure, tolerances, safety factors, joints, elevated temperature
where creep need to be considered. Some factors that specifically have to be
considered if the material is subjected in the creep range are internal pressure, outer
pressure, cyclic loading and risk for fatigue, thermal transient etc. The design stress
must be established for both temperatures below and above creep range.
4
The design stress can be seen as the maximum stress allowed for the component, and
is given by the minimum value from eq. (1) and (2). Equation 2 can be used when
time independent properties are governing, i.e. below creep range.
R p 0 , 2 t Rm 
R
f  min  eH or
;

1,5 2,4 
 1,5
(2)
Where , ReH is the minimum specified value of upper yield strength at calculation
temperature when this temperature is greater than the room temperature R p 0, 2t is the
yield stress, Rm is the rupture stress.
The design factor f is calculated differently when creep is considered, equation 3:
f CR  SRTt / SFCR
(3)
Where the creep rupture strength at the temperature T and the predicted lifetime t
SRTt have been normalized on the safety factor SFCR which is dependent on the
predicted lifetime, see table 1.
Table 1. Safety factors for mean creep strength.
Time T in h
Safety factor
200 000
150 000
100 000
1.25
1.35
1.5
For pressure rated components the allowed pressure (PS) at a reference temperature
(RT) can read from a pressure-temperature (p/t) diagrams valid for temperatures up
to 50 0C. The allowed pressure for temperatures above 50 0C is calculated according
to equation 4:
PS  PN  min( f t : f CR ) / 140MPa
(4)
where f t is the nominal design stress at the temperature T, f CR is the nominal design
stress at the creep range.
ii)
The second step of the design procedure is to analyze the entire piping system. The
piping system s have enough stiffness and strength to withstand deadweight, pressure
and other loads but on the other hand the system needs to be flexible enough to
allow the thermal expansion and limit loads on connecting structures. This is
normally carried out as a flexibility analyses which in the majority of cases is an elastic
analyze. The elastic analyze treat the entire system and can reveal critical location due
to load or thermal expansion. This leads to a reevaluation of the design where
components might have to be replaced, and step i) and ii) of the design procedure
5
have to be iterated a couple of times before a satisfying result is obtained. When
designing for systems in the creep range, one additional stress limit due to creep have
to be considered.
Fabrication and installation
The fabrication and installation of the piping is controlled to ensure the quality of the
component. Some general considerations that should be taken include correct transportation,
handling, storage, fabrication, installation and joining of the piping system. The requirements for
formed pressure retaining parts are especially high and contain a variety of tests on the
component and its welds. Some of the nondestructive evaluations (NDE) which are required for
the component are:




test of wall-thickness
dimension checks e.g. angels at bends
hardness test, ultrasonic examination
magnetic particle penetrant examination
Reference replica testing is also required if creep is considered.
Some of the destructive examinations required are:



tensile test
notch impact test
examination of the microstructure from micrographs
Hot tensile test is required if creep is considered. Welds on the components must also undergo a
rigorous test procedure. Before and during welding the cleanness, selection of weld metal,
preheating, welding process and temperature must be controlled. Following inspections after
welding are:



visual inspection
compliance with drawings
tractability to the welder/operator
Inspection and testing
Some tests, inspections and certification of the welds are required before start-up. i) Various
nondestructive evaluations dependent on the material. This includes ultrasonic examination,
magnetic particle testing, penetrant testing, radiographic testing and replica testing. Proper
cleaning of the surface specific for each method is also required. ii) Additional sample testing
should be performed if not 100 % of the welds have been subject to NDE. The sample tests
should be chosen randomly and representative for a group of welds, where a grouped of welds
consists of welds performed to a specific welding procedure specification (WPS) and a certain
welder. If a tested weld reveals imperfections, two additional welds should be tested. If one or
both of the tested welds show imperfection, all weld in the same group must be tested and
necessary repair and replacement made, followed by a re-inspection. If no unacceptable
imperfection is found no action is taken. If creep or fatigue is believed to be the controlling
6
factors in the piping, all branch welds, socket/fillet welds, seal welds and circumferential weld
larger than 30mm require extended NDE. The extended NDE includes volumetric testing by
radiographic- and penetrant testing, and surface testing by replica- and radiographic testing.
Proof test of the piping is required before start-up. If creep is considered, the test pressure
should include the amount of any static heading (the pressure caused by a fluid due to difference
in height) acting in-service. For further reading about designing for creep, see ref (4).
2.3 Inspections of pressurized systems
Inspections of pressurized systems such as pressure vessels and their piping systems are in
Sweden regulated by the government (1). Three types of mandatory inspections are i) installation
inspection, ii) scheduled inspection and iii) audit inspections. i) Installation inspection has to be
carried out in a newly built steam pipe system before the start-up and is mainly due to safety
reasons. ii) Scheduled inspections are performed during the entire lifetime of the steam pipe
system. The intervals are set dependent on the pressure, volume, medium and sometimes the
temperature in the steam pipe system (1). This can be seen in figure 3 for a piping system
containing steam. The classes A, B and C are here dependent on the pressure and the nominal
diameter DN. A is the toughest class. The regulations also say that piping containing fluids with a
temperature higher than 350 oC are automatically set to class A. The times between inspections in
the case for steam containing pipes (figure 3) is for class A set to six years. More frequent
inspections are mandatory for corresponding pressure vessels.
Figure 3. The classifications A, B and C, dependence on the pressure P in bars
and the nominal diameter DN in mm (1).
7
iii) Audit inspections is required whenever the system have undergone any modification e.g. a
new joint. An audit inspection must be performed before the system can be again taken into
service again (1).
Non-mandatory inspections of the facility are often carried out more frequent and/or more
comprehensive than what the regulations require. The inspections can reveal information which
helps preventing premature material failure. This helps foreseeing necessary component
replacements and avoiding costly unplanned maintenance stoppages.
2.4 Prediction of remaining life for a high temperature pipe systems
The actual life of a high temperature system often differs from its designed life but is in general
longer than the design life. The lifetime is shortened if the system has to withstand temperatures
and stresses higher than designed for. Components which have to withstand the highest stress
and thermal loads are most vulnerable, where a temperature increase of 15-20 0C can shorten the
residual life time shortened by more than half. Repair or replacing of damaged components is
sometime necessary but affects the both component and its ambience. A new weld can leave
residual stress, embrittlement or cracking, which potentially lead to new maintenance issues (5).
Critical components
The lifetime of high temperature piping in service is often determined by the remaining creep life
in critical high temperature components. The remaining life of critical components needs
therefore to be considered. Creep defects can be categorized as volumetric and localized.
Volumetric creep is multiple micro cracks spread out relatively uniformly over a large are, and
originate due to nominal stress in the structure. Localized creep is concentrated to the weakest
and most exposed components in a structure, known as critical components. Critical components
often withstand the highest loads and temperatures in the system, they are therefore more
sensible to all sorts of deformation, like creep as well as wear, fatigue and hydrogen
embrittlement. Welds can be considered critical components, because they are associated with
lower creep strength and/or lower creep ductility. In addition, they are frequently placed in areas
where stress concentrations occur (6).
Some common critical components with respect to creep in power plant are:




superheater and reheater steam headers, valves, T- and Y- pieces
welds
bends
hot parts of steam and gas turbines
Common critical components in a chemical or petrochemical plant are:

high temperature reactor vessels, pipelines and heat exchangers (7).
8
Welds
A weld consist of the weld metal (WM), the surrounding heat affected zone (HAZ), and the
unaffected parent material (PM). The microstructure of the HAZ is dependent on the
composition of the steel and reached peak temperature during welding. Figure 4 show the change
in microstructure in a low alloy steel weld, when cooling from peak temperature in the center of
the weld.
Figure 4. The change in microstructure in a low alloy steel weld, when cooling from
peak temperature in the center of the weld.
Figure 4 show that if a peak temperature of 1100-1150 oC or higher is reached, the final
microstructure consists of coarse grains and is bainitic, sometimes also martensitite occurs
depending on the cooling rate. This type of HAZ is known as a coarse grained HAZ, and has the
characteristics a higher creep strength and lower creep ductility than the PM. A fine grained zone
in the area with peak temperatures between 1000-1050 oC and the AC3 –line in a Fe-C phase
diagram is where the phase austenite transform to the phase austenite + ferrite during cooling.
The boundary between the fine grained zone and the unaffected base metal is sensitive to type IV
cracks.
In the fine grained HAZ the peak temperature low enough, austenitic transformation will take
place without grain growth. During cooling, the austenite will transform to ferrite and bainit. Due
to many nucleation sites in the austenitic grain boundary and a relatively low growth rate because
of to the low temperature, the result is a fine grained HAZ.
Weld cracks
Cracks appearing in ferritic steel weld commonly classified in in to four classes. Their regular
locations and orientation can be seen in figure 5. Type I and II forms during the preheating or
the solidification of the weld, and are located in the weld metal. They are normally detected
during the inspection at the fabrication of the weld, and are therefore normally not considered as
9
a problem. Type III and IV cracks are located in the HAZ and figure more commonly in high
temperature components.
Figure 5. The regular locations and orientations of type I -IV
cracks in a weld (8).
Type III cracks are intergranular cracks
and can be found in the coarse grained
HAZ. Type III cracks or micro cracks
can be initiated during relaxation of
residual stresses by creep at service or
in a post weld heat treatment (PWHT).
Type III cracks can be only be
detected
by
conventional
nondestructive
testing
(NDT)
methods, whereas microcracks can be
detected both by NDT methods and
replica testing.
Type IV cracks or microcracks are the
most dangerous forms of cracks and is
often a life limiting factor for the
component. Type IV cracks are
located in the intercritical part of the
HAZ, in the boundary between the
fine grained zone and the unaffected
base metal, see figure 6. Type IV
cracks occurs more frequently in welds
subjected to bending forces, which
enhance the axial stress in the weld (9)
(10).
Figure 6. Type IV cracks between the HAZ and base metal in a
butt welded 0.5Cr-0.5Mo-0.25V steel pipe (8).
10
2.5 Creep deformation mechanisms
The underlying mechanism responsible for the creep deformation of the material varies with
temperature and level of stress. It is essential to know the underlying deformation mechanism
when designing for creep. The variation of deformation mechanisms according to stress and
temperature can be seen in a deformation mechanism map (figure 7), where normalized stress
(  /E-module) is plotted over normalized temperature ( T / Tm ). Creep deformation take place at
stresses lower than the yield stress. If the yield stress is exceeded, dislocation glide will be the
dominant plastic deformation mechanism instead of creep. The creep range of an application can
cover over more than one deformation mechanism zone, which have to be considered at design,
modeling and testing with respect to creep.
Figure 7. A deformation mechanism map for a 2.25 Cr-1Mo steel. Diffusional flow
based creep is d ominant at lower stress levels. Power -law creep is dominant at higher stress levels,
up to when dislocation glide is initiated . (11).
Grain boundary sliding takes place at high temperatures. With increased temperature follows
softening of the grain boundaries, causing the grains to slide which eventually leads to
intergranular cracking. Grain boundary sliding decrease with decreasing grain boundary area.
Large grains are therefore desired in a creep resistant material.
Diffusional flow based creep is caused by vacancies which diffuse from grain boundaries with
high tensile stress to grain boundaries of low tensile stress. At the same time, atoms diffuse in
opposite direction of the vacancies. As a result, the grains orient them self in the diffusional
direction, causing the material to elongate in the diffusional direction and contract in the
perpendicular direction. The diffusional flow can either follow the grain boundaries or go
through the grains (bulk flow), and are respectively called Cobble creep and Nabarro-Herring
creep. Cobble creep take place at lower temperatures where the activation energy needed is lower.
Nabarro-Herring creep take place at higher temperatures. Diffusional flow based creep has a
strong temperature dependence, a weak stress dependence and a moderate grain size dependence,
decreasing with increased grain sizes.
Dislocation creep or power-law creep takes place at lower temperatures and elevated stress levels.
Dislocation by vacancy diffused assisted creep is dominant at relatively high stress levels. The
assistance from the diffusion makes it possible for the dislocations to overcome obstacles. The
11
ruling mechanism at the highest levels of stress and elevated temperature is dislocation gliding
creep, where the dislocations glide through the crystal in the slip plane. The deformation
mechanism is essentially the same as for dislocation glide at room temperature (12) (3) (13).
2.6 Creep resistant steels
General
Materials used in high temperature applications have some common required features; being
resistant to creep deformation is the most obvious one. This is often accomplished by using
alloys causing finely dispersed precipitates; e.g. alloy carbides in ferritic steels. Increased heat
resistance can also be accomplished by heat treatment.
Most common creep resistant power plant steels used today can be divided in to low alloyed
ferritic steels, 9-12 % steels and austenitic steels. Low alloyed ferritic and 12 % Cr steels was the
standard steel used in power plant until the 90´s, where modified 9-12 % Cr steels were first
introduced in new power plants. Many of today’s running power plants are old and have creep
resistant components made of low alloyed ferritic steels. Because of their age they are in frequent
need of service, low alloyed ferritic steels are therefor still an important material out of a
maintenance perspective. Austenitic steels are frequently used in power plants in North America,
but are not commonly used in Europe.
Low alloyed ferritic steels
Mo- steels
Molybdenum steels have a ferritic-perlitic microstructure. Increased molybdenum content cause
an increase in the materials creep-rupture strength due solution hardening, but also a decrease of
the materials creep ductility. The molybdenum content are therefore relatively low, e.g. 0,3 % Mo
in Mo-steel. Graphitization and decomposition of the iron carbides takes place at temperatures
above 500 oC which limits the Mo- steels to lower temperature applications than that in power
plants. A common Mo-steel grade is 16Mo3, also called grade 204 according to ASTM standard.
CrMo- steels
The negative effects of Molybdenum in the Mo-steels can be avoided by alloying with
Chromium. The Molybdenum content can now be increased up to about 1 %, but further
alloying do not contribute to increase the materials creep resistance by the solution hardening
effect.
Chromium carbides are formed which stabilize the microstructure, and enables usage of CrMosteels at temperatures above 500 oC. The chromium content also increases the materials ability to
resist oxidation at elevated temperatures. To typical CrMo- steels are 10CrMo9-10 (grade 22) and
13 CrMo4-5/5-5 (grade 11).
Recent development has been done to improve the considered outdated CrMo- steels. Two new
important steels grades have been developed; grade 23 and grade 24. Both have a similar
microstructure as the conventional CrMo- steels but have been further alloyed to increase their
strength properties. Grade 23 has been alloyed with vanadium, boron and titanium, and grade 24
12
with vanadium, boron, tungsten, and niobium. Some of the most common creep resistant steels
can be seen in table 2 (13).
Table 2: Chemical compositions of creep resistant ferritic steels for power plants.
Chemical composition (mass %)
Grade
(European
and
American
standard)
16Mo3 (T*/P*204)
C%
Cr %
Mo
%
B%
Al %
0.060.10
max
0.20
0.400.50
0.0020.006
13CrMo4-5/5-5
(T/P11)
0.22.029
0.901.20
0.150.30
10CrMo9-10 (T/P22)
0.080.15
2.002.50
0.901.20
X10CrMoVNb9-1
(T/P91)
7CrMoVTiB10-10
(T/P24)
0.080.12
0.050.10
8.009.50
2.202.60
0.851.05
0.901.10
0.00150.007
HCM2S (T/P23)
0.040.10
1.902.60
0.050.30
0.00005
-0.006
max
0.06
0
max
0.04
0
max
0.04
0
max
0.40
max
0.02
0
max
0.03
0
X20CrMoV12-1
0.17 0.23
10 12.5
0.8 1.2
Nb%
Ni %
N
%
0.060.10
max
0.40
0.0300.070
max
0.010
0.180.25
0.200.30
max
0.030
0.200.30
0.020.08
0.3 0.8
V%
0.250.35
*T=tube steel, P= pipe steel
9-12 % Cr steels
X10CrMoVNb9-1 or grade 91 steel was developed in the 1970s and was introduced in the first
power plants in the early 1990s. Grade 92 and grade 122 are alternative steel creep resistant steel
grades developed out of grade 91. Together they are classified as high alloyed martensitic or 9-12
% Cr steels. They show similar creep rupture strength to austenitic steel, but have a higher
thermal conductivity and lower thermal expansion but are significantly cheaper as well.
Grade 91 is one of the most commonly used 9-12 % Cr high temperature pipe steels. A
component out of grade 91 steel is heat treated by normalization and tempering. The final
microstructures have a high dislocations density and consist of tempered martensitie with fine
carbonnitrides precipitations (MX) in the matrix, and extensive carbide precipitations (M23C6) in
the grain boundaries (14). Some benefits with grade 91 components are great rupture strength
which leads to increased safety margins, reduced wall thickness which lower the thermal storage
in the component and lower the thermal stress on the system, and therefore also reduce the risk
for thermal-fatigue cracking which normally occurs in thick walled components. The grade 91
steel have showed to be most vulnerable to creep in HAZ weld zone. During lab testing at
625 oC the creep and creep-fatigue rates were 10 times higher in the HAZ compared with the
base metal (15). The creep degradation in martensitic steels is today not fully understood. It is
believed to be strongly related to changes in the grain boundaries, e.g. sub grain size,
precipitations and dislocation density.
13
2.7 Different models for creep
Nortons law
Secondary creep can be the described by Norton´s law, equation 5:
s  A n
(5)
Where s is the strain rate during secondary creep,  is the stress and n is the stress exponent.
The stress exponent n varies with the deformation mechanism. A can be obtained from equation
6:
A  A´exp(Qc / RT )
(6)
where A´ includes microstructural parameters,  Qc is the activation energy for creep, R is the gas
constant and T is the temperature.
Because a creep test under actual condition would take several years and is not economically
feasible, accelerated creep test is normally performed to obtain creep data. An accelerated creep
test is normally performed with elevated stress and temperature, and can be aborted before the
time of rupture, if e.g. the only the secondary creep rate is wanted. Therefore there is a need to
approximate and extrapolate data obtained from accelerated creep tests.
Larson-Miller parameter
The Larson-Miller parameter is a commonly used time-temperature constant. It can be used to
extrapolate data, equation 7:
PLM  T (log t r  C )
(7)
Where PLM is the Larson-Miller parameter, t r is the time of rupture and C is a material
parameter. If the material parameter C is known, two or more tests lead to rupture at elevated
temperature can be used to determine the Larson-Miller parameter. This information can be used
to determine the time of rupture at lower temperatures at constant stress.
Monkman –Grant relation
The Monkman –Grant relation describe the relationship between secondary creep rate and the
time of rupture. It has over the years been showed to be valid for most metals and alloys used in
creep resistant application. The Monkman –Grant relation state that the strain accumulated
during secondary creep is constant at failure, and that the product of the secondary strain and the
time of rupture is constant, equation 8.
s  (t r ) m  CMG
(8)
Where C MG is a constant depending on the total elongation during creep, m is a constant close
to and often set equal to one. An engineering expression can then be written as equation 9:
s tr  CMG
(9)
14
Power law equation
The power law equations give useful relations between the stress and the time of rupture.
Equation (6) and (8) can be combined into the Power law equation (ref (12)), equation 10:
s  CMG / t r   n A´exp(Qc / RT )
(10)
Kachanov–Rabotnov
The Kachanov-Rabotnov equations can be used to produce a model for the damage
development in a material. The Kachanov-Rabotnov equation is based on the concept of
increasing damage caused by formations of voids due to creep. This can be represented by the
deimincing cross area of cylidrical spiecemen subjected to a constant load, see figure 8.
Figure 8. An illustration of the Kachanov-Rabotnov concept, which only should be viewed mathematically.
The damage factor concept was first introduced by Kachanov, and later modified by Rabotnov,
equation 11:
  1
A
A0
(11)
Where  is the damage factor, A is the effective area, A0 is the original area. The expression
should be viewed only as a mathematical formulation of the creep propagation and should not be
taken literally.
The Kachanov-Rabotnov model in one dimension can be written as equation 12 and 13:
 
B n
(1   ) v
(12)
 
B 
(1   )
(13)
Where  is the secondary creep rate,  is the damage process. n , v ,  ,  are material
constants.
The Kachanov-Rabotnov can be expressed for a multi-axial stresses, equation 14 and 15:
15
dD
A (( 1(1   ) e ) v
g
dt
 1
(1  D)
(14)
Dcrit  1  (1  g )1/( 1)
(15)
Where D and Dcrit are partial damage and partial critical damage.  e is the von Mises effective
strain,  1 is the max principal stress. v is the lateral contraction of the material.  is a material
constant, when   1 is the creep rate controlled by the greatest max principal stress, when
  0 the creep rate is controlled by von Mises effective tension. A and v are constants which
describes the relation between the secondary strain rate and level of damage. g and  are
constants describing the inhomogeneity in the partial damage. The Kachanov-Rabotnov model is
as can be seen, based on several constant that need to be determined, and may act as sources of
error. For a more extended derivation of the Kachanov-Rabotnov model, see ref. (16) (17).
The Wilshire equation
One recently discovered relationship is the Wilshire equation, which is a modification of the
power law equation (equation 10), equation 16:
( /  TS )  exp(k 1(t f exp(Qc* / RT )u )
(16)
The strain  has been normalized on the materials ultimate tensile stress  TS for the specific
temperature T . The parameters k 1 ,  Qc* , and u can be derived from creep rupture data.
Interesting is that  Qc* have showed to be very close to the lattice diffusion coefficient for many
metals and alloys. The Wilshire equation also accepts that t f  0 as    TS , while t   as
  0.
The secondary creep rate s and time to predefined strains t can also be obtained from the
Wilshire equation, equation 17 and 18:
( /  TS )  exp( k 2(s exp(Qc* / RT ) v )
(17)
( /  TS )  exp( k3 (t exp( Qc* / RT ) w )
(18)
Where k 2 , k 3 , v , w and Qc* can be obtained by deriving creep rupture data. The Wilshire
equation have showed be capable of predicting rupture times up to 100 000 h, from data based
on creep test of 1000 h for e.g. aluminum alloys. The Wilshire equation have showed very good
agreements for a variety of pure metals and alloys (bainitic, ferritic and several martensitic steels)
the author still recommend that the equation need further validation before being fully accepted
(18) (19).
Creep crack initiation and growth
Creep crack growth takes place in the grain boundaries where the voids have accumulated. The
crack growth is dependent on interlinking chain of cavities along the grain boundary and the
16
crack, see figure 9. The crack correlate with the grain boundary voids. Excising grain boundary
voids facilitates the propagation of the crack, new voids are at the same time formed in the area
close to the crack tip (20).
Figure 9. The growth of a grain boundary crack along a interlinking chain of
voids in the grain boundary.
Crack propagation data from laboratory specimens can be extrapolated in order to predict the
crack growth in in-service components. Crack tip parameter is often used as a transfer function
of the crack growth data. Crack tip parameters characterize the state of stress at the crack tip, and
are independent on the shape and size of the specimen/component. Two different approaches of
describing the crack growth is time dependent fracture mechanics (TDFM) and elastic plastic
fracture mechanism (EPFM), where TDFM is the most common one. One of the most
frequently used parameters in TDFM is the C* parameter. The C* parameter is a loading
parameter which show good correlations with the crack propagation rate. Two other common
loading parameters that should be mentioned are the C(t) and Ct parameters which can be used in
special cases (21).
Life assessment models based on creep crack growth data
Most component creep life time is characterized by continuum dame mechanism (CDF), where
the failure is controlled by either creep rupture or creep strain failure mode. But for a component
that have to withstand very high temperature and pressure the failure is rather controlled by creep
crack initiation (CCI) and/or creep crack growth (CCG). The size of the flaws in the material
manly determines if the failure is controlled by CCI or CCG. Small flaws leads to a long
incubation time of the crack, failure is thereby controlled by the CCI. Large flaws leads to a
relatively early initiation of the crack in the components service life, failure is thereby controlled
by CCG.
References stress methods
Creep strain is strongly stress dependent. Creep which tends to uniform the stress in a structure
can therefore be detected, by measuring the change in stress. The reference stress is a widely used
concept in fracture mechanics and can except for determine creep rates, be used for determine
displacement rate, times for stress restitutions and dissipation of energy in a structure. Reference
17
stress can also be used to estimate the time to failure for a specimen subjected to creep
deformation. Analyses of numerical simulations have showed that the stress field when n  
(where n is the exponent from the power law equation) can be used to describe the stress field at
lower n-values. The reference stress can be expressed as, equation 19:
 ref 
P
y
PL
(19)
where  ref is the reference stress field, P is the applied load, PL is the limit load value and  y is
the yield stress. Since the applied limit load PL is proportional to the yield stress  y , the reference
stress  ref is independent of the yield stress. Reference stress method (RSM) can be used to
design creep resistant components by proper dimensioning of the component. A benefit with the
reference stress method are that the parameters in equation 19 are in many cases relatively easy to
obtain (22) (23).
R5-procedure
The R5-procedures are used for defect assessment at high temperatures. The first R5-procedure
was developed in the United Kingdom and is related to the R6-structural assessment method.
Several R5-based methods or similar to R5 has been developed. The R5-procedures can be
divided into design procedures and assessment procedures, where the design procedures aims to
assess a test/components with an in-built conservatism, and the assessment procedures aims to
predict the test accuracy. Only the R5-Time dependent failure assessment diagram (TDFAD)
procedure along with the basic procedure will be described here, see ref (24) for further reading.
R5-rupture procedure
The R5-concepts built on the reference stress approach (eq. 19), and can be used to estimate the
risk for creep rupture for a structural feature. Is only valid for moderately sharp crack damages.
The rupture reference stress  ref for creep ductile materials is evaluated from equation 20:
R
R
 ref
 1  0.13  1 ref
(20)
 is a stress concentration factor for the adjustment of reference stress and is obtained from
equation 21:

 E ,max
 ref
(21)
The  E ,max is the maximum elastically calculated value of the equivalent stress and can be
obtained from an elastic analysis. The evaluation (20) is only valid for stress concentration factors
  4.
18
Elevated risk for creep rupture considering different operating conditions r can then be assessed
for each structural type of component, e.g. a pipe bend. This is done by calculating a total creep
usage factor U , which should be U  1 , and is obtained from equation:
k 

t
U  

R
r 1 t (
 f ref , Tref )  r
(22)
Where r is the cycle type, t is the duration of steady load operation during which creep is
significant totaled over all cycles of type r , k is the number of cycle types and t f is the allowable
time read from rupture curves at rupture reference stress  ref and reference temperature Tref , see
R
ref (24) for further reading.
R5-TDFAD approach
R5-Time dependent failure assessment diagrams (TDFAD) are based on Failure assessment
diagrams from failure assessment diagrams (FAD) as in the R6-method. The diagram can be used
to estimate the time to creep crack initiation, and is based on the two parameters K r for fracture
and Lr for limit load.
The fracture parameter K r is defined in equation 23:
c
K r  K / K mat
(23)
c
Where K is the stress intensity factor in a component and K mat
is the appropriate creep
toughness value for a material.
The limit load parameter Lr is defined in equation 24:
Lr   ref /  0c.2
(24)
Where the  0c.2 is the stress corresponding to 0,2 % inelastic (plastic plus creep) strain from an
average isochronous stress-strain curve for the temperature and assessment time of interest, an
schematic isochronous stress-strain curve can be seen in figure 10:
19
Figure 10. An isochronous stress-strain curve (25).
The time dependent failure assessment diagram is defined by equation 25 and 26:
 E ref
L3r 0c.2 
Kr  

c
c 
 Lr 0.2 2 E ref 
Kr  0
1 / 2
Lr  Lmax
r
(25)
Lr  Lmax
r
(26)
Where E is the Young´s modulus,  ref is the total strain from the average isochronous stressstrain curve at the reference stress  ref  Lr 0.2 at a given time and temperature. The fracture
c
parameter K r is then plotted against the limit load parameter Lr in order to obtain the time
dependent failure assessment diagram, figure 11:
20
Figure 11. A TDFAD for a 1CrMoV-steel at 550 o C (25).
max
The cut-off Lr is defined as, equation 27:
Lmax
  R /  0c.2
r
(27)
max
Where  R is the rupture stress. As in the R6-porcedure Lr should be less than; equation 28:
Lmax
  /  0.2
r
(28)
Where  can be obtained from equation 29 (according to the R6-procedure):
 
 0.2   u
(29)
2
Where  u is the ultimate tensile stress (24) (25).
Nikbin-Smith-Webster-Model
The Nikbin-Smith-Webster (NSW) -model was developed in the 1980s. The NSW-model
determines the creep crack growth from uniaxial creep data and the materials grain size. The
model assumes that that crack advantage take place when the creep crack ductility is exhausted at
the crack tip. One drawback with the model is that it predicts conservative results.
Prediction of the creep crack growth with NSW model
The stress intensity factor K is normally used to describe the crack propagation rate in normal
fracture mechanics, equation 30.
a  AK m
(30)
21
Where a is the crack propagation rate, A and m are material constants. K can be used to
describe creep deformation in very brittle materials where the creep deformation contribution to
the fracture is significantly smaller than for ductile materials.
When creep deformation predominates the stress distribution at the crack tip, the stress intensity
factor C * better describe the crack propagation rate a , equation 31:
a  D0C *
(31)
Where are D0 and  material parameters. The stress intensity factors C (t ) and C t can be used
when describing creep in a small scale creep region where stress redistribution is still taking place.
The zone ahead of crack propagation during creep is described in figure 12.
Figure 12. The zone ahead of a propagating crack (25).
Where r is the distance from the cracktip and rc is the creep process zone size. Creep damage in
the material takes place when r  rc .
An approximate expression of time of the time to rupture is; equation 32:
 fo   0  v
tr 
 
0   
(32)
Where t r is the time to rupture,  fo is the creep ductility at the stress  0 and v is a constant.
22
For a material that creep under the power law the steady crack tip stress field and strain rate
distribution at coordinates (r, θ) can be expresses as equation 33 and 34:
1 /( n 1)
 C* 

 ij   0 

I


r
 n 0 0 
 C* 



 ij   0 

I


r
 n 0 0 
~ij ( , n)
(33)
ij ( , n)
(34)
n /( n 1)
Where  0 is the equivalent stress, I n is a integration constant that depends on n , 0 is the
constant strain rate, ~ij is the non-dimensional stress tensor, n is the stress index in the power
law.
If assuming that creep crack propagation in the damage zone of the crack (in figure 12) when;
c
r  rc at the time t  0 and accumulates creep strain  ij by the time it reaches a distance r from
the crack tip, the conditions for crack growth is given by using the ductility exhaustion criterion,
equation 35:
t
 ijc   ijc dt
(35)
0
Where  ij is the creep strain tensor and ij is the creep strain rate tensor. If assuming that crack
c
c
propagation at the crack tip occurs when the materials creep ductility is exhausted,  reaches its
~
maximum value and  ij  1 , and integrating over constant grow rate and constant C * , the crack
c
propagation rate can then be described as equation 36:
 /( n 1)
a NSW
(n  1) 0  C * 



n  1   *f 0  I n 00 
rc( n1 ) /( n1)
(36)
Where  f 0 is the creep ductility at the stress rc , and  is a constant. Notice that equation 36 is on
*
the same form as equation 31. The model shows that the creep crack growth rate is inversely
proportional to the creep ductility stress at the crack tip. For a more detailed derivation of this
expression see ref (26).
Nikbin et. al. showed that C * varies between 0.7-1.0, and can be set to 0.85. Along with other
experimental results it has been shows that equation 36 can be rewritten to a more engineeric
expression, equation 37:
a 
3C *0.85
(37)
f
23
with a [mm/h],  f is the uniaxial creep ductility expressed as a fraction, and C * [MJ/m2h]. The
equation is called the NSW engineering creep crack growth law. Equation 37 have showed to be
valid for a variety of materials. Equation 37 only applies for plain stress condition which is the
case for components with a small thickness. It can be used to describe the stress state on the
surface of a component e.g. a pipe with shallow surface defects, or a relatively thin test specimen.
A similar expression which applies for plain strain conditions is; equation 38:
a 
150C *0.85
(38)
f
Equation 38 should only be used for specimens where plain strain condition applies, i.e. thicker
components, typical around 10-20 mm. It can be used to describe the stress state in three
dimensions which applies for of a component with relatively deep going defects, or a relatively
thick test specimen. A thick specimen for a creep test can be modified by creating notches in
order to obtain a plain stress state (12) (27) (26).
Modified Nikbin-Smith-Webster-Model
Recent attempts to improve the conservative overestimation of the NSW-model have been made.
The modified NSW (NSW-mod) equation is based on the assumption that fracture occurs at the
angel  (figure 12) where the equivalent stress ~e reaches a maximum value. For plane stress this
value has showed to be   0 at plane stress conditions and   90 at plane strain conditions.
Fracture then occurs when ~e and the multi axially strain factor reaches maximum value. The
modified NSW-equation; equation 39:
a NSWMOD
0  C * 
 (n  1) *
 f ( , n)  I n 00 
n /( n 1)
rc1 /( n1) ( , n)
(39)
For a more detailed derivation of this expression see (28). The result of the modified-NSWequation can be seen in figure 13. The result is from test of brittle/ductile alloys and creep ductile
engineering alloys. The prediction of the NSW- and the NSW-mod models can be seen in the
figure. The NSW-mod-model is less conservative than the NSW-model, and correlate well with
the data from the creep ductile engineering alloys (27).
24
Figure 13. Material independent creep crack growth engi neering assessment diagram (27).
Prediction of the creep crack initiation time
The creep crack initiation time (incubation time) can be estimated with the NSW model. The
crack propagation zone can be seen as a number of small elements, each with the width dr, figure
14:
Figure 14. The crack propagation zone divided in to small elements with width dr (24).
25
The stress for the first element can be expressed in equation 34 as; equation 40:
1 /( n 1)
 C* 

 ij   0 

I


dr
 n 0 0 
~ij ( , n)
(40)
The time to failure for the first element, with the maximum equivalent value of ~ij ( , n)  1 is
given from equation by equation 41:
 *fo  I n 00 dr  v /( n1)
dt 


0  C * 
(41)
The sensitivity of the crack monitoring equipment is proportional to the width of the elements
dr. If the monitoring equipment is sensitive enough, the C * can be approximated to a constant,
and the incubation time can be calculated (see ref (12)); equation 42;
 *fo  I n 00 ri  v /( n1)
ti


0  C * 
(42)
Prediction of the creep propagation rate with the damage parameter
The crack growth rate can in a service exposed material be determined by adopting the damage
parameter  e .  e describe the level of damage in the material, where e  0 for new material.
Equation 31 can then be written as equation 43:
a 
D0
C *
(1  e )
(43)
It should be mentioned that the equation do not consider the ageing effect of the material, which
can alter the uniaxial creep properties (12).
Service exposed material
Destructive evaluation can be used for obtaining actual materials data. The creep strain and
creep rupture rate can easily be calculated if actual material data is obtained. Determination of the
CCG rate in service exposed materials can only be done by performing proper creep crack
growth test, and cannot be determined only out of the material data (12).
Virgin material
The reference stress method can be used to estimate the CCG of virgin material. This can also be
done with linear fracture mechanics (12).
2.8 Non-destructive testing of creep exposed components
Non-destructive testing (NDT) can be used to characterize key microstructural features in a
material. Because there is no intrusion or damage made to the component during testing, NDT is
important tool to monitor out of-service components. Non-destructive testing is often followed
26
by a non-destructive evaluation (NDE), where the result from one or more NDT techniques is
evaluated in order to determine the status of the component, e.g. with respect to cracks, creep
damage and microstructural degradation.
The selection of test position for NDE
The result from localized NDT is highly dependent on the choice of test positioning on the
components. The number of non-destructive tests which may be performed on-site may be
limited by time. This is manly from period of time of the service stoppages where a number of
non-destructive tests have to be performed, often in a short time. A selection of the critical
positions which should be subjected to NDT for e.g. a steam pipe system, appropriate therefore
beforehand. The selection of test positions tend to become more important, since the trend today
is going towards shorter fewer service stoppages.
The selection of test position and testing of critical components can be divided in to
i)
ii)
iii)
identifying the critical components of the system by performing a system analysis
identifying the critical positions on the critical component, by component analyses
testing the critical components at the critical positions.
The choices of test positions in the system (critical components) are based on the result of a pipe
system analysis, which normally is an elastic analysis. The elastic analysis will reveal information
about the locations of the largest stresses in the structure. The elastic analysis is considered as the
standard choice of pipe stress analysis today. However, results from an recent study where a pipe
system analysis including material model with respect to creep in Abaqus software was conducted
see ref (29), show that a normal elastic analysis not always cover all critical locations which should
be tested for creep. The main differences was due to i) effects of creep relaxation, ii)
accumulations of creep strain due to repeated starts and stops, resulted in strain concentrations in
other positions than initially expected. The effects of creep relaxations after 1 year of service can
be seen in figure 15 (29).
Figure 15. The difference in stress distribution due to creep relaxation between initial start -up and
after 1 year of service. The effect of the creep relaxation is not considered in an elastic analyze.
Notice the difference manly in the bends (29).
The critical positions are given by the system analysis reveal the components which are selected
for NDE. The exact testing positions for each type of component are based on experience.
Appropriate recommendations for component test positions aim to cover all positions where the
27
most critical creep damage may occur as a result of component geometry, system stresses and
presence of welds. An example can be seen in figure 16, see (30) for further reading.
Figure 16. The recommended test positions for replica testing for a pipe bend, where (a) show the exact locations
and (b) show the positions from the cross section of the pipe (30) .
However, the study in ref (29) also showed that the predicted strain accumulations in a single
component may be altered when the system stresses are involved. It might therefore be more
favorable perform a stress/strain analysis on components with complex geometry instead of
using the experience based recommendations.
2.9 Non-destructive tests methods
Visual inspection
Visual inspections are normally carried out in the beginning of an inspection in search for regions
that are damaged, heavily wear or in any way deviate from normal. Visual inspection can detect
damaged regions which can easily be detected by the naked eye but are hard to measure with any
NDE. Such regions can be areas covered with general corrosion, or components with odd
geometrical features which are both hard to measure with NDE and often accumulated stress
(31).
Magnetic particle testing
Magnetic particle testing (MT) can be used to detect flaws on the surface or on the subsurface of
a component. First, a magnetic powder is placed on the pre-prepared sample surface. A magnetic
field is then introduced to the surface, normally created by an electric source such as a yoke. The
magnetic powder particles then orients themselves to the magnetic field, see figure 17. Flaws can
then be detected which can be seen as discontinuations in the oriented magnetic particles. The
change in the orientation of the magnetic particle tends to magnify the actual size of the flaw.
This effect facilitates the detection of small flaws. Flaws down to 3 mm can normally be detected,
sometimes even down to 1 mm under good conditions when proper surface preparation has been
carried out (31).
28
The magnetic powder used normally consists of pure iron particles and sometimes additions of
fluorescent color particles, which improve the contrast if lit with UV-light. The powder is often
mixed with a liquid in field.
The sample surface must be relatively smooth and should be carefully prepared since air gaps
between the yoke and the surface can lead to a decrease in the magnetic field.
The applied magnetic field can be customized to given conditions by varying the current, and by
varying current type between direct and AC current. Direct current tends to measure deeper in
to the material (31).
Figure 17. An illustration of magnetic particle testing (MT) of a weld (32).
Replication
Replica testing (RT) is used to investigate the surface of components in order to detect creep
damage. An illustration of the replica method can be seen in figure 18. The sample surface is first
prepared by polishing and etching. A plastic film of ethyl acetate is prepared by dissolving its
surface, and then placed on the sample surface. The film is removed once it has dried, and will
then contain a negative image (replica) of the microstructure of the sample surface. The replica is
then examined in a light optical microscope. The examination over a weld can reveal presence of
type III or type IV cracks (33) (34).
29
Figure 18. A schematic illustration of the replica method (35).
The replica is classified against the modified Neubauer damage class system, in order to
determine the status of the examined component. The modified Neubauer damage class system
for classification of creep damages is showed in table 3.
Tabell 3. The modified Neubauer damage class system for classification of creep damages (34).
Assessment class
Description of damage
0
New material, no thermal exposure
1
Creep/thermal exposure, no cavities
2a
Isolated cavities
2b
Numerous cavities without preferred orientation
3a
Numerous cavities without directional orientation
3b
Chains of cavities or grain boundary separations
4
Microcracks
5
Macroscopic cracks
30
The benefits of replica testing are:


Require relatively simple and cheap equipment
Small impact on examined component.
Some limitations are:



Require surface preparation.
Only examines the surface of the material, volumetric creep cannot be detected. The
surface is however representative for the entire component in most cases.
Only examine a small area and spots for examination must be chosen, which lead to a risk
That most damaged areas remain undiscovered (31).
Ultrasonics
Ultrasonic (UT) measurements can detect creep damages by detecting the difference in time over
path. The method of ultrasonic measurements is to send out an ultrasonic wave through the
material, and measure the wave’s time of flight. Because the velocity of the wave is known, the
path of the ultrasonic wave can then be determined from the result. Increased formation of creep
cavities is related to decreased material density. Because sound waves travel faster in a solid
medium, increased cavity formation leads to a decreased velocity of the wave, which thereby
indicates creep damage (36).
There is a variety of different ultrasonic measurement that can be applied for detection of creep
damage, but only i) bulk waves and ii) surface waves methods will be described here.
i) The bulk wave method measure the time of flight between when creating a back
wall echo. Ultrasonic bulk waves reveals information about the bulk of the object
and is thereby capable of determine volumetric creep.
ii) Ultrasonic surface waves are a more suitable technique to detect localized creep.
An ultrasonic wave is sent out to travel a fixed path which is set up and defined by
a so called pitch-and-catch arrangement. The time of flight is then companied with
the time of flight of a reference specimen, obtained by performing the same
measurement on a reference sample of virgin material. Rayleigh waves are used for
ultrasonic surface detection of creep. The penetration depth of Rayleigh waves can
be varied between 0.3-1.5 mm, but are normally one wavelength or 0.6 mm when
using a frequent of 5 MHz for steel (37).
A benefit with the ultrasonic testing is that the method is capable also can be used for thickness
measurements. Some limitations of the method is that undesired internal structure e.g. eroded or
corroded material can be a source of error and that it is difficult to use for welds.
Comparison of NDT
A recent review over todays and upcoming non-destructive tests was made in (8). A selected part
of the result can be seen in table 4, where five different non-destructive tests have been
reviewed (8).
31
Table 4. Some benefits and drawbacks for different NDE techniques (8).
Technique
Damage type
Sensitivity with
depth
Surface only
Bulk (average
over thickness)
Replication
Ultrasonic
velocity
Local/ Volumetric
Volumetric
Ultrasonic
backscatter
Hardness
Eddy current
Localized
Sub-surface
Localized
Volumetric
Surface only
Decaying rapidly
Selectivity
Good
Good only at late
stage; sensitive to
surface state
More research
needed
Poor at welds
Poor
In-situ
deployment
At maintenance
At maintenance
At maintenance
Large scatter
Not documented
2.10 Destructive evaluation methods
Destructive evaluation (DE) complements the NDE by revealing information that can only be
obtained by excavation of the material. Thus, an obvious disadvantage with destructive evaluation
is that a sample of the out of-service component has to be removed. The level of modification
due to the excavation affects the component. Small material quantities can often be taken out
with equipment that reduces the impact on the modification done to the component. One
example is the electric discharge sampling equipment used which can provide e.g. impression
creep test specimens without any repair welding. In some cases e.g. for creep testing specimens,
test methods using miniature test specimens have been developed in order to decrease the impact
of the modification to the component, the method is known as semi-destructive testing.
Electric discharge sampling equipment
The material sample used for the impression creep test can be removed from the pipe with an
electric discharge sampler. An electric discharge sampler (EDS) is a compact portable sampler
equipment which is capable of taking material samples from components on-site. The recently
developed patented method was invented by Japanese scientist Okamoto et. al. (38).
The sampling process is described in figure 19, where an electric discharge between the electrode
and sampling material is generated, and thereby melting the material.
Figure 19. a) Sampling process start, b) working process, c) end of process (38).
32
The equipment of the EDS can be seen in figure 20. It consists of a main body mounted on a
base plate which is fixated on an in-system component. The main body drives the rotation part
on where the electrode is mounted. The process is controlled by a control panel. A machining
cooling liquid enables cooling of the electrical discharge part.
Figure 20. a) The main body and base plate fixated to a pipe, b) the control panel and power supply,
c) machining liquid used for cooling of the electrical discharged parts, d) the electrode which cut out the material sample.
A sample cut out by the EDS can be seen in figure 21:
Figure 21. A typical material sample cut out by the EDS .
33
Some benefits with the EDS, according to the inventors and authors (38):




Simpler and faster sample removal compared to traditional alternatives. A material sample
with the dimensions 40*23*2,3 mm can be removed in 3-4 hours depending on steel type.
Enables local sampling.
Small modification to the sampled component.
The thermal effect caused by the electric discharge is small and can be neglected.
Creep testing
Creep testing or uniaxial creep testing is frequently used destructive evaluation method to
determine the creep properties of a material, see figure 22.
Figure 22. An illustration of a creep test and a uni axial creep test specimen lead to rupture.
Almost all creep tests are in some way accelerated due to time and economic reasons. This is
normally performed by either increased the temperature or stress level compared to service
temperatures and stresses. Thus creep tests can be divided in to i) iso-stress tests and ii) isothermal tests.
i)
Iso-stress creep tests take place under constant stress levels but with varying
temperatures. This method is often used when testing out of-service in-situ material,
where the applied stress correspond the stress in-service. The normal procedure is to
produce a test series where 4-6 of specimens are crept to rupture. The result can be
plotted as log time vs. temperature. The rupture times can then be extrapolated in to a
straight line where temperature and corresponding rupture time can be seen, see
figure 23.
34
Figure 23. The result of an iso-stress creep test of a 2.25Cr1Mo steel at 80 MPa (31).
ii)
Iso-thermal test is performed under constant temperature and varying load. The
normal procedure is a test series where a number of specimens are lead to rupture.
The resulting fracture times cannot normally be approximated in to a straight line
since different mechanisms of deformation often take place over the tested range of
stresses. The result from iso-thermals tests can be used to obtain parameters for
constitutional equations which describe the creep progress process, e.g. the n- and Aparameter in the Norton equation (equation 5).
Creep crack propagation test are in difference from creep test, not lead to rupture. The increase
in displacement as a function of time is measured by either an extensometer or removing the
specimen and measure the elongation at room temperature. An extensometer is to measure the
elongation during uninterrupted tests. The elongation can also be measured by removing the
specimen and measure it, the test is then called an interrupted test. An extensometer can however
still be used for additional measuring (39).
Creep test are frequently performed across welds by using cross-weld specimens. The different
microstructures over the weld can be seen as metallurgical notches which creates multi-axial
stress conditions (31).
A total of 4-5 creep tests are normally performed during a creep test procedure. Different test
specimen sizes is classified and showed in table 5 (40):
Table 5. Creep test specimen size classes.
Specimen size
Full size
Sub-size
Miniature
Diameter d0
d0> 5mm
3< d0 <5mm
d0< 3mm
Reference length
3 ≥ d0
3 ≥ d0
≥ 10 mm
Even larger creep test specimen can be used e.g. d0=10 mm and gauge length of 100 mm. A
problem with such full sized specimens occurs when testing in-service components. The large
take out of material require more comprehensive repair welding and risk the structural integrity
of the component. Creep testing with miniature specimens has therefore been developed and
35
was tested in e.g. ref (41), where miniature specimens was plated with nickel to counter the
problems with oxidation which is a common problem for miniature specimens for low alloy steel.
The result however showed that the nickel plating hindered the oxidation of the surface but lead
to decreased creep rupture times than expected.
Stress Rupture Test
Stress rupture tests are performed in order to obtain the time to rupture or the rupture
elongation. Notched specimens which also can be used for creep tests, is used to obtain multiaxial stresses (39).
Notched stress rupture test
A uniaxial notched rapture stress test is carried out the same way as the uniaxial stress rupture
test. The difference is the specimen, which contains one or more circumferential notches, located
in the parallel section, perpendicular to the applied load direction. Single notch test is the most
common test, and the notch is most often placed in the middle of the specimen. Different types
of notches can be used. The most common types are V-notch and Brideman (semicircular)
notches. V-notch types are traditionally used in order to determine the notch strengthening or
notch weakening of materials used in high temperature environments. Brideman notch type is
more suitable for notch root measurements (39) (42).
Hardness measurement
The hardness tends to decrease with time and elevated temperatures, due to thermal degradation
of the material. Hardness measurement can therefore be used in order to indicate the level of
degradation of the microstructure. The hardness of a virgin material may vary from one batch to
another. Hardness measurements may therefore be a relatively blunt tool for absolute
comparisons. Hardness measurement in combination with examination of the microstructure is
therefore to be recommended when determining the status of the microstructure.
Increased hardness in the HAZ close to a weld can indicate that there is a problem with the weld,
e.g. improper heat treatment after welding (10).
Impression creep
The idea of the impression creep test is to estimate the hot hardness of a material in order to
predict its usefulness at elevated temperatures. An illustration of the impression creep (IC) test
can be seen in figure 24. The IC test measures the displacement over time of an intender set to
push in to a specimen. The method was first presented in 1977, the method have since then
through extensive analyses showed to be a reliable test method (39) (43).
36
Figure 24. An illustration of the impression creep test. The punch with the area
impression with the height
h
A loaded with the load L , which creates an
in the material.
The material during impression creep test exhibits the same stress and temperature dependence
as the stress as a conventional tensile creep test (2). IC tests can therefore be used to produce
reliable creep strain rates, equivalent to those obtained from a normal creep test. Impression
creep can also be used for localized in-situ testing measurements, where samples from various
components in a system can be tested in order to determine the status each component. This
information can be used when selecting/priorities further component testing. Some benefits with
impression creep test over conventional creep test are (43):





Shorter testing times, typically 300 hours for IC and about 7000 hours for conventional
creep test.
The negative impact on the component can be reduced because i) less test material is
needed, ii) more lenient sampling equipment can be used, e.g. electric discharge sampling
equipment.
The area reduction which occurs during conventional creep test does not have to be
considered during IC testing.
Localized sampling over individual zones in the material, e.g. a weld or a HAZ can be
made.
Can be used as a creep strength ranking test of components parent materials, in order to
aid the selection of damaged components in need of testing in a system. This was
successfully accomplished for an aged 1/2CrMoV pipework system in ref (44).
Set-up
Some recommendations on the standardization on IC-testing were published in ref (45). An
illustration of an impression test rig can be seen in figure 25. The fundamental equipment of an
impression creep test rig can be consists of:





Loading system
Data recovery system
Heating and temperature system
Deformation measurement system
Inert gas environment, if necessary.
37
Figure 25. An illustration of the impression creep test rig (45).
The intender used should consist of a material significantly harder than the test material. Suitable
intender materials are Ni-based super alloys such as Waspaloy and NIMONIC 105 for the
purpose of testing standard heat resistant pipe steels such as and 10CrMo9-10 and P91.
Since a materials hardness decrease over time at elevated temperatures, flat bottomed intenders
are used during IC in order to obtain steady stress state. The steady stress state is a result of the
constant load in difference from hardness measurements with e.g. pyramidal indentation, where
the stress intensity varies with the size of the contact area of the intender. The shape of the IC
testing intender should be absolute parallel to the surface of the test material. The shape of the
intender should be checked in-between every IC test. Grinding of the intenders surface is
necessary after a number of tests.
The shape of the intender can be rectangular or cylindrical. Rectangular intenders can favorable
be used to measure localized zones in the material, e.g. a weld or a HAZ. The width of a
rectangular intender is 1 mm or 0,8 mm, the length of the intender should always exceed the
width of the test specimen.
38
The recommended dimensions of the test specimen is w x b x h = 10 x 10 x 2,5 mm,
accompanied with the 1 mm wide intender, see figure 26. Smaller specimen dimensions can be
used if not enough test material is available In this case, a second recommendations for the
dimensions have been recommended; w x b x h = 8 x 8 x 2 and a intender with if 0,8 mm . The
specimen surface should be carefully grinded before testing in order to remove any surface
defects and residual stresses caused by the machining. The intender should be located in the
middle of the specimen.
Figure 26. An impression creep specimen (45).
The displacement of the punch is continually monitored and recorded by an extensometer. The
loading system used should be as accurate as + 1 % stress accuracy. The normal load required is
between 1-3 kN. A standard impression creep test for grade 91 take place at 600 0C and 90 MPa
(45).
Conversation of impression creep data
The obtained data result from an impression creep test will have the unit deformation in the
specimens thickness direction vs time, see figure 27.
Figure 27. A typical result from an IC test. Th is is for a grade 91 weld at 650 o C.
The plot show total deformation vs time.
The data need to be converted to in order to obtain the desired unit minimum creep strain rate
(MCR) vs stress, figure 28.
39
Figure 28. Typical converted result data from an IC test of a 316 stainless steel
and a CrMoV steel at 640 o C. The plot show minimum creep strain rate vs stress.
Conversion parameters are used in order to transform the IC test data and obtain the minimum
creep strain rates. The conversion equations are; equation 44 and 45:
  p
ssc 
(44)
 ss
d
(45)
where p is the uniaxial stress equivalent to the stress  ,  ss is the steady-state impression depth
rate  ss equivalent to the uniaxial steady-steady state strain rate ss , d
is the diameter of the
intender and  and  are the conversion parameters.
For a polycrystalline material in the range of n=2-15 in the Norton equation (5) where only a
single deformation mechanism is dominating, the conversation parameters can be set to; equation
46 and 47:
  0,296
(46)
  0,755
(47)
The conversation parameters are affected by the dimension of the specimen and the diameter of
the intender. Results from FE-analyses have showed that for best alignment it is recommended
to use the dimensions 10 x 10 x 2,5 mm for the specimen and a diameter of 1 mm for the
intender. If there is insufficient specimen material available their the dimension 8 x 8 x 2 mm and
a intender diameter of 0,8 mm is recommended as previously mentioned. Over all is a squareshaped specimen recommended over any other shape (46).
The derivation of the conversion parameters  and  can be seen in ref (47).
40
Small punch test
A small punch test measure the displacement a loaded punch pushed through a small disc of
specimen material at an elevated temperature. The material specimen disc is either simply
supported or clamed between two dies. The test set up can be seen in figure 29. Normal
dimensions of the disc are 8-10 mm in diameter, and a 0,2-0,5 mm in thickness. The diameter of
the punch is normally between 2-2,5 mm. The punch pushes with a constant load. A protective
atmosphere of argon gas can be used to diminish oxidation on the specimen caused by the
elevated temperature (39).
Figure 29. Schematic set up for small punch creep testing (48).
41
3. Investigation
3.1 About the test material
The low alloyed steel 10CrMo9-10 (also known as 2.25Cr-1Mo or T/P22) is a standard material
used for high temperature pipe systems. 10CrMo9-10 is still used a as high temperature pipe
material when building new plants today, but figures much more frequently in plants built before
the nineties. Many of these now aged plants are however still in-service, which makes 10CrMo910- steel highly interesting from a service perspective.
The 10CrMo9-10 test material used in this work have been in-service for over 200 000 h and
comes from a thick-walled pipe used in a high pressure pipe system the power plant
Asnaesvaerket, Denmark.
Some data for the pipe (49) can be seen in table 6:
Table 6. Data for the investigated component.
Service time
Internal pressure
Service temperature
Inner diameter
Outer diameter
Pipe wall thickness
212.000 hours
138 bar
540 °C
185 mm
260 mm
37 mm
The investigated pipe includes a butt weld pipe, consisting of two pipes welded together, see
figure 30.
Figure 30. A schematic illustration of a butt welded pipe.
During the manufacturing of 10CrMo9-10 steel, the standard heat treatment consist of
normalization and tempering, typically at 900 °C for 15 minutes and 700 °C for at least 30
minutes respectably depending on component thickness) before air cooling.
The material properties of 10CrMo9-10 are well documented, e.g. reference 49-54. The material
in the referred literature is either fabric-new, has been in service or is heat treated in order to
simulate the effects from the in-service life. Differences between microstructures can be caused
by:
42
i)
Initial variations in the microstructure. Small variations in the
microstructure of the same type of material are common and are mainly
dependent on the heat treatment parameters during manufacturing.
Material from the same batch can be used to obtain a similar initial
microstructure when preforming e.g. in-situ testing of multiple specimens.
ii)
Microstructural variations due to environmental impact. Varying stress
and thermal loads during the service life of the components may cause
unique microstructures. The microstructure can therefore differ somewhat
even between similar components in the same pipe system. The unique
microstructure cannot be completely reproduced by in-situ heat treatment
either. This because the details of the components service-life including
stress loads rarely can be obtained, and because increased temperatures
often is used in order to shorten the test time of in-situ experiments.
The Larson-Miller parameter describes the thermal-degradation of the microstructure and can be
used when comparing different creep studies of the same steel type. When evaluating material for
creep one should be careful when choosing reference data to compare with. As mentioned in i)
ii), both the initial microstructure and specific environmental parameters affects the final
microstructure. The initial microstructure of a service exposed material can be hard to obtain,
and stated service data of a component, especially the stress load affection, can differ somewhat
from the real case. Comparison between studies using e.g. Larson-Miller parameter is in many
cases necessary, but i) and ii) can serve as a major source of error and should be considered
when comparing different studies.
The very same material in this work was also used in (49) (50), and will therefore be used for
accurate comparison.
3.1.1 Approximative estimation remaining life
The remaining lifetime of the tested component can be roughly estimated by calculating the
internal stress in the pipe. The internal stress  was calculated by using values from table 6 in
equation 48:

pd
2t
(48)
where t is the inner diameter. The internal stress was then compared with reference data (ref 68).
The result can be seen in figure 31.
43
150
Log stress
(540 oC)
15
8000
Log time
80000
800000
Figure 31. The time to rupture of 10CrMo9-10- steel, at varying stress loads at 540 oC. The remaining life of the tested component
was roughly estimated to 500000 h.
The remaining lifetime of the test component was estimated from the reference values in figure
31. Since the reference values only goes from 10000 h to 250000 h and the test component
clearly has a much longer remaining life, a rough estimation of 500000 h was made. Stress-time
creep curves are difficult to predict, and should always be verified by actual tests. The estimation
should therefore be considered rough but conservative, and show that 42 % of the components
life is consumed.
Thermal degradation of the microstructure
Thermal degradation of the microstructure occurs in most steel types when subjected to
sufficient temperature exposure over time, which is the case for most high temperature steels
during their life time. This can lead to reduction of dislocation, carburizing and oxidation, and in
some cases also grain growth and recrystallization. The type and extent of each thermal
degradation mechanism is dependent on service parameters such as temperature, environment,
service time and the initial microstructure.
The initial microstructure is mainly determined by the heat treatment of the steel during its
manufacturing, where the most important parameter for ferritic steels is the cooling rate after the
heat treatment. The thickness of the component will affect the homogeneity of the
microstructure, where the inhomogeneity increases with increased component thickness.
Carburizing is a mechanism that can be as used to measure the thermal degradation of the
microstructure in low alloyed ferritic pertlitic steel. The spherodizsing of carbides in low alloyed
ferritic pertlitic steel is when the cementite leaves the perlite structure and spherodizse. The
spherodizse cementite tends to coagulate in the grain boundaries. The final microstructure will
consist of spherodizsed cementite in a matrix of ferrite. The initial carbides is cementite (Fe3C)
and M2C- carbide, the final carbides consist of a mixture of M7C3-, M22C6- and M6C carbides.
Thermal degradation of the carbides in 9-12 % Cr steels can be seen as coagulation of M23C6
carbides.
44
If only one thermal degradation mechanism is considerable, or if the degradation takes place in a
very short temperature interval, the rate of the degradation can be described as the general
Arrhenius kinetic reaction rate equation; equation 49:
dV
 Ae Q / kT
dt
(49)
Where A is a material dependent constant, Q is the activation energy for the degradation and k is
the Boltzmann constant. The reaction rate (degradation rate) can be expressed only as a function
of time and temperature. The most common expression is the Larson-Miler parameter
PLM=T(log t + C) (eq. 7). (51). Equation 49 can also be used to measure the creep rate in the
material (except for the thermal degradation).
The material parameter C in eq. 7 is determined by plotting the time and temperature results
from several heat treatment tests where varying times and temperatures have been used. Such a
plot can be seen in figure 32, where logarithms time t is plotted against the inverse temperature
1 / T . The microstructural degradation for a sample can then be graded based its temperatureand time history. The result of the microstructural degradation can then be used to divide the
time-Temperature diagram in to classes. A straight line can then be applied between each class,
where the slope of each line corresponds to 1/PLM. Each line crosses the y-axis at certain point,
which is the C-value of the material. Typical C-values for ferritic steels are C=14 when evaluating
thermal degradation, and C=20 when evaluating creep rates (52).
Figure 32. The Larson-Miller relationship PLM=T(log t + C). Each bo rder between the line corresponds to
the border between different levels of thermal degradation (A -F) (52).
Calculation of the Larson-Miller parameter
The Larson-Miller parameter for the test material was calculated from the data for the pipe in
table 6 and the material parameter C=14 in equation 7:
PLM=15712
Microstructural degradation classes
The evaluation of the thermal degradation was made to the according to following scale (51):
45





A
B
C
D
E

F
As-new microstructure of untempered steel
As-new microstructure of normal tempered steel
First signs (LOM) of degradation in bainite or martensite
First discernible (LOM) separate carbides
Sizeable carbides particular on grain boundaries, original structure (bainite
regions/lath structure) still partially discernible
Ferrite and carbides, original structure no longer discernible
3.2 Metallurgical evaluation of test material
Material samples were taken from the matrix of both sides A and B of the circular pipe and from
the circular weld. Microstructural examination, hardness- and average grain size measurements
were made for the weld and the matrix material.
Degradation class
E
D
C
B
F
Classification of level of microstructural degradation of matrix A and B
The thermal degradation of the microstructure of matrix A is classified to level E, and matrix B is
classified to level D-E. The result was compared to reference (51), see figure 33:
A
13000
Ref
Material A
Material B
14000
15000
16000
17000
18000
19000
PLM = T( log t + C)
Figure 33. The microstructural degradation of material A and B , compared with reference (51).
The result show good correlation the reference result. This indicates that the service data of the
pipe is correct and the transformation using the Larson-Miller parameter was successful.
Microstructural evaluation after electrolytic polishing
Electrolytic polishing was performed over the two weld samples A and B in aim to facilitate the
appearance of creep cavities. No creep cavities could be detected which indicates that creep has
not yet taken place in the weld.
3.2.1 Microstructural evaluation of the weld
A sample of the weld was taken out and divided in to two separate samples from the middle of
the weld, see figure 34.
46
Figure 34. Weld sample A and B. The red line shows the positions of the hardness measurements.
The width of A is 22 mm and B 19 mm.
The different zones of the weld can be divided in to:



unaffected parent material (PM)
heat affected zone (HAZ)
weld metal (WM)
The heat affected zone consists of several separate microstructures. They are (from PM to WM):





Tempered zone (TZ)
Intercritical zone (ICZ)
Fine grained zone (FGZ)
Coarse grained zone (CGZ)
Solid-liquid transition zone (SLT)
The different microstructures originate from the difference in peak temperature during the
welding which earlier is illustrated in figure 4.
The different zones of the weld in figure 34 can clearly be seen with the naked eye or in the light
optical microscope (LOM), but are hard to capture with a camera because of the reflection from
the polished metal. A more illustrative figure of the different microstructures can be seen in
figure 35.
47
Figure 35. An illustrative figure of the different microstructures in weld sample A and B.
The weld is of type multilayer bead weld. When applying a bead, surrounding material is affected
by the increased temperature which causes a heat treatment. This effect can result in grain
growth, recrystallization, reaustenitization, less distinct zones in the HAZ, and needs to be
considered when evaluating the weld.
From weld sample B, the microstructure of PM, FGZ, CGZ and WM were identified and can be
seen in figure Appendix 1. The microstructures were identified in a light optical microscope at 20
and 40 times magnification.
3.2.2 Hardness measurements
Hardness measurements were performed over the matrix material, and weld sample A and B.
Vickers hardness involves a pyramidal diamond indenter. The weight used during the hardness
measurements was 0,5kg, which generate the unit Vickers hardness 0,5 [HV0,5].
Matrix
The hardness in the matrix from the matrix materials A and B was measured. The obtained result
was virtually identical on both sides. The hardness of both sides A and B was measured to 136
HV 0,5. The result compared to (51) can be seen in figure 36:
48
170
160
150
Hardness HV
Material
A and
B,
HV0,5
Ref
HV1
140
130
120
110
100
12000
13000
14000
15000
16000
17000
18000
PLM= T(logt +14)
Figure 36. The hardness of material A and B, compared with reference (51).
The result show good correlation the reference result. This indicates that the service data of the
pipe is correct and the transformation using the Larson-Miller parameter was successful.
Weld sample
The locations of the hardness sampling can be seen in figure W1 and W2. The exact path of the
hardness measurements over the HAZ of sample A can be seen in detail in figure 37. The
distance between each measurement is 0,5 mm.
49
Figure 37. The exact path of the hardness measurements over the HAZ of weld sample A. The zones; parent
material, inter critical zone, fine grained zone, coarse grained zone, solid -liquid transformation zone and w eld
material is marked.
50
The result of the hardness measurement in weld A, B and C can be seen in figure 38, 39 and 40
respectively.
Hardness Weld A
220
210
200
190
HV 0,5
180
170
160
150
140
130
PM
120
1
2
3
ICZ
4
5
FGZ
6
7
8
9
CGZ
SLT
WM
10 11 12 13 14 15 16 17 18 19 20
Position
Figure 38. The result of the hardness measurement for weld sample A. The direction of the hardness
measurement was from the parent material to the weld material.
Hardness Weld B
220
210
200
190
HV0,5
180
170
160
150
140
130
PM
120
1
2
ICZ
3
4
5
FGZ
6
7
8
9
CGZ
SLT
WM
10 11 12 13 14 15 16 17 18 19 20
Position
Figure 39. The result of the hardness measurement for weld sample B. The direction of the hardness
measurement was from the parent material to the weld material.
51
Weld sample A
220
210
200
190
HV 0,5
180
170
160
150
140
130
120
1
3
5
7
9
11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41
Hardness measurement
Figure 40. A more extensive hardness measurem ents of weld A than in figure 37 .
Figure 37 and 38 show an increased hardness in the HAZ and weld material compared with
parent material. The hardness decreases with distance from the fusion line, where the coarse
grain zone show higher hardness than the fine grained zone. Similar results have been reported in
(53) (54) (49) .
Figure 40 do not indicate a greater hardness in the HAZ compared to the weld. This can be an
effect the materials long in-service lifetime, where the hardness difference in the weld has evened
out over time. It can also be caused by the induced heat treatment when applying the multibeads, or just a statistical deviation due to the relatively few hardness measurements preformed
over the WM.
3.2.3 Carbide measurements
The closeness of the carbides was measured and compared to (51), figure 41. The method used:
i)
ii)
iii)
taking a picture of the microstructure
drawing four lines over the picture, each one crossing the center
counting the total number of carbides on the lines and dividing the number of
carbides with the total length in order to obtain the result in carbides/mm.
52
Carbides/mm
450
400
350
Carbides/mm
300
Ref
250
Material
A
Material B
200
150
100
50
0
14000
14500
15000
15500
16000
16500
17000
17500
PLM= T (log t + 14)
Figure 41. The closeness of the carbides in th e material A and B, compared with reference (51).
The result correlate with the classification of the degradation of the microstructure of the test
material. The results in figure 33 and figure 41 show a slight deviation from the reference values.
This can be due to:
i)
ii)
The effect of carbide formation due to stress exposure. In-service material A and B
have been subjected to stresses during their lifetime, whereas the reference material
has not. Stress exposure tends to increase the growth rate of carbides which results in
larger but fewer carbides. (10) (69)
Differences in microstructure of the virgin material, such as grain size, fraction
bainite/ferrite and initial carbides.
3.2.4 Grain size measurements
Grain size measurement was performed for the welds and the matrix material.
Matrix
The result of the grain size measurement of the matrix can be seen in table 7.
Table 7. The average grain size in matrix A and B.
Matrix
A
Grain size [µm]
25
B
36
53
Matrix material B show slightly higher average grain size than material A. This can be due to
initial larger average grain size, but the difference is quite small and is in the range of margin of
error.
Weld
The result of the grain size measurement of the weld can be seen in table 8.
Table 8: The average grain size in weld A and B.
Weld
A
B
Location
PM
FGZ
CGZ
PM
FGZ
CGZ
Grain size [µm]
40
18
60
45
20
55
The grain size measurements of matrix material were conducted with more properly than the
measurements of the heat affected zones in the welds. The grains in heat affected zones were
much harder to spot, in difference to the grains in the matrixes. This is probably due the effects
of the multilayer beads which have been discussed earlier. The results from the grain sizes in the
matrix is therefore believed be more accurate than the results from the weld, which only should
be viewed very briefly.
The results from table 8 show that the average grain size in material B is larger than in material A.
The initial grain size affects the final grain size in the HAZ.
The result from table shows very a similar average grain size in weld A and B. No obvious
conclusions can be drawn from the result. The difference might however indicate that material A
and B are from different batches.
3.3 Destructive evaluation of the test material
3.3.1 Impression creep test
The impression creep test was performed by VTT in Finland. The specimen material is material B
of the tested 10CrMo9-10 pipe.
Test rig set-up
The VTT test-rig uses a somewhat different set-up than the one developed at Nottingham
university, described in detail in ref (45). The main differences can be summarized as:
i)
Different extensometer design. Since IC testing has very little in common with
uniaxial creep testing a more suitable extensometer design is used, that differ from
other test-rigs, e.g. (45) where an extensometer design similar to uniaxial creep are
used.
54
ii)
iii)
The heating is accomplished by two heat resistant cables, placed on the upper and
lower tool, see figure 42.
The load cells are placed below the furnace, water cooling of the system can thereby
be excluded.
The set-up of the test rig can be seen in figure 42-44. The ceramic extensometer works
frictionless and is placed close to the intender, which minimize the effect of the thermal
fluctuation. The rod-part of the ceramic extensometer is pushed directly by the upper tool, which
further reduce effect of the thermal fluctuation, since it is directly driven by the tool. The
effective gauge length is <10 mm, compared to ~50 mm in ref (45). This modification results in a
confirmed more stable displacement signal than compared to ref (45).
The heating of the system is provided by two Hotset WRP (Ø3,3mm) 750 W resistance cables,
see ref (55) for details. The heating cables are controlled separately, which minimize the
temperature difference between upper and lower tool. Two thermo couples made of noble metal
were used for temperature control and data logging. (56).
Figure 42. The lower tool with mounted ceramic extensometers and heating resistant cable.
Figure 43. The upper and lower tool. The heating cable has been covered with a metal sheet.
55
Figure 44. The complete test rig set-up. The middle part has been covered by an insulation box.
Details for the test
The test specimen hade the size 10*10*2,5 mm and the intender width was 1 mm. The
temperature during the test was 540 °C.
The test series was performed by three uninterrupted steps where the load was increased inbetween each step: 95 MPa for 600 h, 110 MPa for 500 h and 140 MPa for 400 h.
Equation 45: ( ssc 
 ss
) was used to calculate the secondary creep rate.
d
The coefficient  was calculated to   2.1464 , based on the measured specimen thickness =
2.416 mm(the nominal value for a 2,5 mm thick specimen is 2,18 (45).
The width of the intender d was measured to d  0.9692 (nominally 1 mm).
 is the measured displacement rate.

ss
56
Results
The impression creep test results were plotted as displacement vs time for the three different
loads. The curve obtained using 95, 110 and 140 MPa load can be seen in figure 45-47 (57).
Figure 45. The result of the impression creep test showed as displacement vs time for 95 MPa (57).
Figure 46. The result of the impression creep test showed as displacement vs time for 110 MPa (57).
57
Figure 47. The result of the impression creep test showed as displacement vs time for 140 MPa (57).
The temperature and load fluctuation during the IC test was measured to ± 0.6 °C 1.5 N
respectively (57).
The results of the impression creep test can be seen in table 9.
Table 9: The result of the impression creep test .
Stress
95 MPa
Indenter displacement rate from
IC test
[mm/h]
1,04E-05
Secondary creep rate IC
test
[1/h]
5,01E-06
110 MPa
2,04E-05
9,79E-06
140 MPa
1,22E-04
5,88E-05
3.3.2 Uniaxial creep test
Uniaxial creep tests of parent metal (PM) B and the weld metal (WM) of the IC-tested 10CrMo9-10
pipe has been carried out in ref (58) and (49). The results can be seen in table 10. The data will
be used for comparison with the results from the IC test as well as for calculations of secondary
creep rates by use of the LSCP model (59).
58
Table 10. The creep properties of the parent material PM, heat affected zone HAZ, and weld metal WM of
material B.The values is obtained from an uniaxial creep test, see ref (49).
Stress
95
110
120
140
Strain
rate
PM [1/h]
2,13E-5
7,38E-5
9,56E-5
3,04E-4
PM Rupture
time [h]
Strain rate
HAZ [1/h]
5363
1615
1182
313
3,04E-05
1,74E-04
4,62E-04
HAZ
Rupture
time [h]
4187
1169
475
Strain rate WM
[1/h]
WM Rupture
time [h]
4,45E-07
1,34E-06
1,24E-05
interrupted
24238
2645
Table 10 show a lower strain rate and longer rupture times for the weld material WM than for the
parent material PM and the heat affected zone HAZ, which is normal.
3.3.3 Sensitivity analysis
The secondary creep rate obtained from an IC test is derived from equation 45. Since  and d
are constants, the secondary creep rate  ssc is directly dependent on the slope of the steady state
displacement rate in the IC test curve, which can be seen in table 9 for 95 MPa stress load. The
effective displacement in the table 9 which the displacement rate is derived from is only 3,3*10^3 mm. Since the effective displacement is very small, a minor measurement error could
potentially lead to a large error in of the steady state displacement rate, and thereby also the
secondary creep rate  ssc . In table 11 a measure error of 0.01 mm has been simulated by
increasing the distance of the effective displacement at the end of the test with 0.01 mm. A new
secondary creep rate has then been calculated, and divided over the secondary creep obtained
from the IC test to get an error factor. The result show that a measure error of 0.01 mm could
lead to an error factor of 2.8-4.
Table 11. The simulated effect of an measure error of 0,01 mm.
Stress [MPa]
95
110
140
Effective
displacement
[mm]
3,3*10^-3
5,1*10^-3
0,024
Sec. creep rate IC
test with 0.01 mm
measure error
1,99*10^-5
2,90*10^-5
1,66*10^-4
Sec. creep
rate IC test
5,01E-06
9,79E-06
5,88E-05
Error factor creep
rate 0.01 mm
measure error
4
3
2,8
The temperature during the impression creep test was 540 °C. Thermal expansion caused by the
high temperature could have affected the accuracy of the displacement measure during the ICtest. Variations in temperature as well as small differences in measured and true temperature
values could also have affected the result. The temperature variations during the IC test was
± 0.6 °C.
The effect of temperature fluctuations has been investigated by comparing the secondary creep
during uniaxial creep testing at 530 °C and 540 °C, see table 12. The error factor has been
obtained by deriving the secondary creep rate at 530 °C with the secondary creep rate at 540 °C,
in order to simulate a measurement temperature measure error of 10 °C. The secondary creep
rates was derived from the Norton equation (eq 5), using stress values for 10 000 h and 100 000
59
at 1 % strain, ref (60). The result shows an error factor between 1,7-2,5. The result cannot be
directly compared with the IC-test results since these are based on uniaxial creep testing of fabric
new 10CrMo9-10 steel. However, it shows that the temperature difference between 530 °C and
540 °C has large impact on the secondary creep rate during uniaxial creep testing. Temperature
differences could probably affect IC-testing in a similar way.
Table 12. The difference in secondary creep during uniaxial creep testing at 530 °C and 540 °C of fabric new
10CrMo9-10 steel. Initial creep data for 10CrMo9-10 steel was obtained from (60).
Stress [MPa]
90
110
140
Sec. creep rate at 530 °C
4,16*10^-7
1,15*10^-6
3,92*10^-6
Sec. creep rate at 540 °C
1,03*10^-6
2,08*10^-6
6,56*10^-6
Error factor 10 °C
2,5
1,8
1,7
3.3.4 LSCP model prediction of secondary creep rate
The Logistic Creep Strain Prediction (LCSP) model is a robust creep strain model, developed to
predict full creep curves from minimal data. The model is relatively new and is developed by
VTT Finland. The model can predict the secondary creep rates from rupture times. The model
can be used to predict secondary creep rates from minimal data. For details of the LSCP model,
see ref (59).
The secondary creep rate for aged 10CrMo9-10 steel have been calculated by VTT Finland using
the Logistic Creep Strain Prediction model. The result can be seen in table 13. The predictions
are based on
i)
ii)
true rupture data obtained from uniaxial creep test of PM (table 13)
mean rupture times from creep tested fabric new 10CrMo9-10 steel. The rupture time
data is obtained from EN 10216-2.
Table 13. The creep properties
Predicted strain rate from LCSP model with
true rupture time [1/h]
1,81E-5
7,02E-5
1,0E-4
3,99E-4
Predicted strain rate from LCSP model with mean
rupture time [1/h] (EN 10216-2)
1,52E-6
3,7E-6
6,76E-6
2,3E-5
60
4. Results and Discussion
The result from in table 10, 11 and 13 has been plotted in figure 48.
Figure 48. A comparison of the secondary creep rate (strain rate) from i) impression creep test, ii) uniaxial creep
test of PM, HAZ and WM, iii) LSCP model prediction based on true rupture times obtained from uniaxial
creep test and mean rupture times
The uniaxial creep test result in figure 48 will here be viewed as the most correct prediction of the
test materials secondary creep rate, and will be used to measure accuracy of the results from the
IC-test and the LCSP model prediction.
Figure 48 show secondary creep rate from the impression creep test is somewhat lower than the
creep rate measured during the uniaxial creep test of the test material. The deviation in secondary
creep rate between the IC test and the uniaxial creep test for the different stress loads seems to
be fairly constant. The error factor for the for the values predicted by the impression creep test
can be obtained by dividing the secondary creep rate from the IC test with the secondary creep
rate values from the uniaxial creep test of PM. The error factor for the IC test secondary creep
rate values are 4,5 for 95 MPa, 7,6 for 110 MPa and 5,2 for 140 MPa.
The LCSP model prediction of the secondary creep rate based on true rupture data obtained
during the uniaxial creep test show very good agreement with the secondary creep rate obtained
from the uniaxial creep test. This shows that the true rupture time data can be used to predict an
accurate result of the secondary creep rate using the LCSP model. The predicted result of the
secondary creep rate based on the tabled mean rupture data show poor agreement with the
uniaxial creep test results of the service exposed material. The difference in actuary of the LCSP
models prediction based on tabled mean- and true rupture times was expected.
61
The difference in result of the LCSP prediction of the secondary creep rate based on true rupture
times and tabled mean values is a result of the difference between the true rupture times and the
tabled mean rupture time values (obtained from EN 10216-2). Since the mean rupture times are
based on fabric new material and the actual rupture times was obtained from aged material, the
difference is likely caused by the thermal degradation of the aged material. The thermal
degradation of the microstructure of the aged material was evaluated in the present work, and
classified to level D-E on the A-F scale, where A is fabric new material.
The large difference in end result using the LCSP model indicate that it can be beneficial to
collect actual material data by testing of excavated service exposed material instead of using
tabled mean values, for prediction secondary creep rates.
The literature survey show that impression creep testing is beneficial over traditional uniaxial
creep testing in some aspects, mainly shorter test time, smaller amount of specimen material
needed and lower cost. Quick and low-impact sampling of in-service components can probably
be performed with the Electric Discharge Sampler (EDS). Creep testing of components using IC
testing combined with the EDS-sampler could potentially be used to test a larger number of
components, but also enables testing of components which cannot be tested with conventional
creep testing, due to the negative impact from the large outtake of sample material.
The result from the IC test did however not show a satisfactory alignment with the result from
uniaxial test. The deviation of the IC test results could not been explained. It is suggested that
sensitivity in displacement measurement and possibly also temperature to could have affected the
end result.
Further testing of the impression creep test method is recommended as a result of this work, in
order to determine its reliability as a test method before it can be implemented in real projects.
This is necessary since the source of error of the predicted secondary creep rates could not be
determined.
62
5. Conclusion
A literature survey was carried out over the area creep testing of high temperature pipe systems,
with particular focus on impression creep testing. The IC test method show benefits such as
shorter testing times and smaller material outtake needed for test material.
An impression creep test was performed in order to determine the secondary creep rate of a
service exposed 10CrMo9-10 high temperature pipe steel.
The result of the predicted secondary creep rate obtained from the IC test was compared with
the secondary creep rates measured during the uniaxial test. The IC test result did not align
satisfactory with the results from the uniaxial creep test, which would have been expected. The
reason for this may be due to sources of error during impression creep testing but could not be
fully explained.
During the IC test, very small displacements have to be measured. It was shown that small error
in measurement can lead to large deviations in predicted secondary creep rate, which can be seen
as a weakness of the test method.
Further testing of the impression creep test method is recommended as a result of this work, in
order to evaluate the method.
63
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Appendix. Microstructures.
The appendix contain microstructures of the weld and parent material A and B.
Matrix
The result of the microstructural evaluation of matrix A and B be seen below in figure 49-54:
Figure 49. Matrix A in 20x zoom.
Figure 50. Matrix B in 20x zoom.
68
Figure 51. Matrix A in 40x zoom.
Figure 52. Matrix B in 40x zoom.
69
Figure 53. Matrix A in 100x zoom.
Figure 54. Matrix B in 100x zoom.
Weld
From weld sample B, the microstructure of PM, FGZ, CGZ and WM were identified and can be
seen in figure 55-61. The microstructures were identified in a light optical microscope at 20 and
40 times magnification.
70
Figure 55. The PM in 20x zoom. The micro structure is bainitic and ferritic. The dark lines are MnS-inclusion from the slag.
Figure 56. The FGZ in 20x zoom.
71
Figure 57. The CGZ in 20x zoom.
Figure 58. The WM in 20x zoom.
72
Figure 59. The PM in 40x zoom.
Figure 60. The FGZ in 40x zoom.
73
Figure 61. The CGZ in 40x zoom.
Figure 62. The WM in 40x zoom.
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The exact areas where figure is taken can be seen in figure 63. All pictures of the microstructures
(figure 55-62) except the coarse grain microstructural pictures (figure 57 and 61) were taken
inside the large circle in weld B because the different zones of the HAZ were quite distinct. The
CGZ- microstructural pictures were taken in the area marked with the smaller circle. Because of
the multibead weld had caused an increased temperature after solidification, which caused a
substantial austenitic transformation in the grain boundaries. This was not the case in the FGZ
closer to the surface of the weld.
Figure 63. The exact locations of where the pictures of the microstructure (Figure W3-W10) were taken.
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