Long term creep deformation and crack growth predictions for grade 91 steels and risk-based methods in their component life assessment
Author(s)
Maleki Azarbaijani, Shervin
Type
Thesis
Abstract
Grade 91 steel has been extensively used for high temperature since it was introduced in the
1980s. However, as the 9%Cr material is relatively new, there is limited experimental data
and understanding with respect to quantifying the effect of variables, for example service
exposure, on life prediction and long-term behaviour, in particularly weldment. Many
incidents reported about premature weld failures of Grade 91 steel suggest that the design
standards and guidelines may be non-conservative for the pressure vessels and piping
made of Grade 91 steel. Furthermore, Grade 91 welds are also known to be prone to Type
IV cracking, leading to premature failure controlled by creep crack growth (CCG).
A comprehensive literature review was carried out and extensive data is collected. This
includes data for a different range of steel materials at different creep temperatures and test
stress which in turn provided a range of test durations from a few hundred hours to several
thousand hours (ie >60,000 hours) but with the main focus of the analyses remaining on
Grade 91. The NSW based creep crack growth model is found to be capable of predicting
upper/lower bounds of creep crack growth well for Grade 91 when data is obtained from
relatively short to medium-term laboratory experiments (< 10,000 hours). It is also found that
the extrapolation of the model fails to predict the long-term creep crack propagation rates
unless the actual long-term uniaxial creep data is available.
Two types of creep testing are performed in this study to investigate the creep behaviour of
Grade 91 steel over a range of stresses on the basis of continuum damage mechanics and
fracture mechanics: Uniaxial creep and CCG. For each type of testing, creep deformation
behaviour and CCG characteristics in different microstructures (parent metal, weld metal and
HAZ) are investigated. The same tests are performed on as-received and service exposed
Grade 91 steel to provide the relevant data to be used for the NSW prediction model. In both
uniaxial and CCG experiments, the weldment (ie weld metal and HAZ) exhibits a lower creep
resistance compared to the parent material. In the CCG experiments, the
ex-service material exhibits a faster growth rate compared to the as-received material in the
similar range of C*. Based on the test results, the relevant parameters to describe creep
strength and CCG are established. The results of the analyses of short-term and long-term
creep data from the literature indicate a change in the creep exponent and failure strain at
longer creep lives. The creep failure strain also tends to diminish with a decrease in the
applied stress.To investigate the microstructure of the steel before and after the creep test and to also
understand the failure mode and the location in the uniaxial and CCG tests, metallography
was carried out. One sample from both service exposed and as-received steel before any
creep test as well as one uniaxial and one CCG specimen from each material (four in total)
after creep tests were selected for examination. Both service exposed and as-received
steels showed tempered martensite microstructure with cavities observed in the service
exposed material before the test. The analyses of creep uniaxial samples after the test
exhibit Type IV failure tendency in the as-received material compared to the service exposed
steel where failure occurred mainly at or adjacent to the weld. With regards to the CT
specimen, in a similar way the crack path in the as-received HAZ sample followed outer
region of HAZ (ie. Type IV) whereas the crack path in the service exposed sample showed
tendency to grow along the fusion line. This may be due to faster ageing process of the weld
metal compared with HAZ and parent material when exposed to high temperature.
The experimental results obtained above together with the database compiled from the
literature survey are then used, in conjunction with the NSW model, to predict the CCG of
Grade 91 steel. It is found that the appropriate failure strain under multi-axial stress state can
be estimated by extrapolating the secondary strain to the failure time rather than the final
failure strain. Using the extrapolated uniaxial failure strain data at the lower shelf (low stress)
region and a reduced creep index at long terms, conservative CCG predictions have been
made by the NSW model. For C* values of 10-5MJ/m2h and less, cracking is predicted to be
relatively faster compared to the extrapolated short-term CCG data trend. It is found that the
assumption of lower failure strains and reduced creep indices effectively produce a safe
prediction for CCG using the NSW model for long-term behaviour. However, the level of
conservatism increases depending on the use of uniaxial failure strains or the
Monkman-Grant (MMG) failure strain or in the extreme 0.2% failure strain as the lower shelf
data. The correct level of conservatism can only be determined once actual long-term CCG
data are available.
Following the analysis, a prototype model was developed based on risk-based life
assessment (RBLA) integrity management concept using data presented in this thesis for
Grade 91 piping material. Estimating remaining life based on the material limit state equation,
assessing the risk associated with creep damage and making recommendations on the most
effective mitigation plan were the main area of assessment treated in the RBLA model. In
this study, four reported Grade 91 weld failure investigations have been compared to the
assessment result from RBLA. The result was found to be similar to what the failure investigation reported, however it should be noted that when applying the risk concept, the
target risk (or target probability) plays a crucial role compared to a deterministic analysis.
The work done on failure cases and the comparison with models developed in this thesis
demonstrated that to achieve a reliable conservative result for longer-term CCG assessment,
the effect of lower creep ductility due to long-term exposure together with MMG approach to
determine a lower band failure strain should be taken into consideration. Where crack
growth data was available from the failure investigation report, a comparison with the finding
from the investigation was made with the long-term NSW model which was developed in this
work. The result indicated a good agreement with finding from the failure investigation report
and falls in the scatter band predicted from the model.
It could be concluded that the optimum approach to predict the short-term and especially
long-term creep and CCG behaviour of Grade 91 weldment is a probabilistic method as the
level of uncertainty is relatively high in material properties those contributing to failure.
1980s. However, as the 9%Cr material is relatively new, there is limited experimental data
and understanding with respect to quantifying the effect of variables, for example service
exposure, on life prediction and long-term behaviour, in particularly weldment. Many
incidents reported about premature weld failures of Grade 91 steel suggest that the design
standards and guidelines may be non-conservative for the pressure vessels and piping
made of Grade 91 steel. Furthermore, Grade 91 welds are also known to be prone to Type
IV cracking, leading to premature failure controlled by creep crack growth (CCG).
A comprehensive literature review was carried out and extensive data is collected. This
includes data for a different range of steel materials at different creep temperatures and test
stress which in turn provided a range of test durations from a few hundred hours to several
thousand hours (ie >60,000 hours) but with the main focus of the analyses remaining on
Grade 91. The NSW based creep crack growth model is found to be capable of predicting
upper/lower bounds of creep crack growth well for Grade 91 when data is obtained from
relatively short to medium-term laboratory experiments (< 10,000 hours). It is also found that
the extrapolation of the model fails to predict the long-term creep crack propagation rates
unless the actual long-term uniaxial creep data is available.
Two types of creep testing are performed in this study to investigate the creep behaviour of
Grade 91 steel over a range of stresses on the basis of continuum damage mechanics and
fracture mechanics: Uniaxial creep and CCG. For each type of testing, creep deformation
behaviour and CCG characteristics in different microstructures (parent metal, weld metal and
HAZ) are investigated. The same tests are performed on as-received and service exposed
Grade 91 steel to provide the relevant data to be used for the NSW prediction model. In both
uniaxial and CCG experiments, the weldment (ie weld metal and HAZ) exhibits a lower creep
resistance compared to the parent material. In the CCG experiments, the
ex-service material exhibits a faster growth rate compared to the as-received material in the
similar range of C*. Based on the test results, the relevant parameters to describe creep
strength and CCG are established. The results of the analyses of short-term and long-term
creep data from the literature indicate a change in the creep exponent and failure strain at
longer creep lives. The creep failure strain also tends to diminish with a decrease in the
applied stress.To investigate the microstructure of the steel before and after the creep test and to also
understand the failure mode and the location in the uniaxial and CCG tests, metallography
was carried out. One sample from both service exposed and as-received steel before any
creep test as well as one uniaxial and one CCG specimen from each material (four in total)
after creep tests were selected for examination. Both service exposed and as-received
steels showed tempered martensite microstructure with cavities observed in the service
exposed material before the test. The analyses of creep uniaxial samples after the test
exhibit Type IV failure tendency in the as-received material compared to the service exposed
steel where failure occurred mainly at or adjacent to the weld. With regards to the CT
specimen, in a similar way the crack path in the as-received HAZ sample followed outer
region of HAZ (ie. Type IV) whereas the crack path in the service exposed sample showed
tendency to grow along the fusion line. This may be due to faster ageing process of the weld
metal compared with HAZ and parent material when exposed to high temperature.
The experimental results obtained above together with the database compiled from the
literature survey are then used, in conjunction with the NSW model, to predict the CCG of
Grade 91 steel. It is found that the appropriate failure strain under multi-axial stress state can
be estimated by extrapolating the secondary strain to the failure time rather than the final
failure strain. Using the extrapolated uniaxial failure strain data at the lower shelf (low stress)
region and a reduced creep index at long terms, conservative CCG predictions have been
made by the NSW model. For C* values of 10-5MJ/m2h and less, cracking is predicted to be
relatively faster compared to the extrapolated short-term CCG data trend. It is found that the
assumption of lower failure strains and reduced creep indices effectively produce a safe
prediction for CCG using the NSW model for long-term behaviour. However, the level of
conservatism increases depending on the use of uniaxial failure strains or the
Monkman-Grant (MMG) failure strain or in the extreme 0.2% failure strain as the lower shelf
data. The correct level of conservatism can only be determined once actual long-term CCG
data are available.
Following the analysis, a prototype model was developed based on risk-based life
assessment (RBLA) integrity management concept using data presented in this thesis for
Grade 91 piping material. Estimating remaining life based on the material limit state equation,
assessing the risk associated with creep damage and making recommendations on the most
effective mitigation plan were the main area of assessment treated in the RBLA model. In
this study, four reported Grade 91 weld failure investigations have been compared to the
assessment result from RBLA. The result was found to be similar to what the failure investigation reported, however it should be noted that when applying the risk concept, the
target risk (or target probability) plays a crucial role compared to a deterministic analysis.
The work done on failure cases and the comparison with models developed in this thesis
demonstrated that to achieve a reliable conservative result for longer-term CCG assessment,
the effect of lower creep ductility due to long-term exposure together with MMG approach to
determine a lower band failure strain should be taken into consideration. Where crack
growth data was available from the failure investigation report, a comparison with the finding
from the investigation was made with the long-term NSW model which was developed in this
work. The result indicated a good agreement with finding from the failure investigation report
and falls in the scatter band predicted from the model.
It could be concluded that the optimum approach to predict the short-term and especially
long-term creep and CCG behaviour of Grade 91 weldment is a probabilistic method as the
level of uncertainty is relatively high in material properties those contributing to failure.
Version
Open Access
Date Issued
2015-12
Date Awarded
2016-03
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Davies, Catrin
Nikbin, Kamran
Sponsor
TWI Ltd
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)