View
218
Download
0
Category
Preview:
Citation preview
8/6/2019 Some Aspects of Metallurgical Assessment of Boiler Tubes-basic Principles and Case Studies
1/10
Materials Science and Engineering A 432 (2006) 9099
Some aspects of metallurgical assessment of boilertubesBasic principles and case studies
Satyabrata Chaudhuri
National Metallurgical Laboratory, Jamshedpur 831007, India
Received 1 March 2006; received in revised form 11 May 2006; accepted 12 June 2006
Abstract
Microstructural changes in boiler tubes during prolong operation at high temperature and pressure decrease load bearing capacity limiting their
useful lives. When the load bearing capacity falls below a critical level depending on operating parameters and tube geometry, failure occurs. Inorder to avoid such failures mainly from the view point of economy and safety, this paper describes some basic principles behind remaining life
assessment of service exposed components and also a few case studies related to failure of a reheatertube of 1.25Cr0.5Mo steel,a carbon steel tube
and final superheater tubes of 2.25Cr1Mo steel and remaining creeplife assessment of service exposed butunfailed platensuperheaterand reheater
tubes of 2.25Cr1Mo steel. Sticking of fly ash particlescausing reduction in effective tube wall thickness is responsible for failure of reheater tubes.
Decarburised metal containing intergranular cracks at the inner surface of the carbon steel tube exhibiting a brittle window fracture is an indicative
of hydrogen embrittlement responsible for this failure. In contrast, final superheater tube showed that the failure took place due to short-term over-
heating. The influence of prolong service revealed that unfailed reheater tubes exhibit higher tensile properties than that of platen superheater tubes.
In contrast both the tubes at 50 MPa meet the minimum creep rupture properties when compared with NRIM data. Theremaining creep life of platen
superheatertubeas estimated at50 MPa and570 C(1058 F)is morethan 10yearsandthat ofreheater tubeat 50MPaand580 C(1076 F)is9years.
2006 Elsevier B.V. All rights reserved.
Keywords: Metallurgical assessment; Platen superheater; Reheater; Final superheater; Creep rupture; Tensile; Microstructures; Remaining creep life
1. Introduction
Boiler tubes are energy conversion components where heat
energy is used to convert water into high pressure superheated
steam,whichis then delivered to a turbine for electric powergen-
eration in thermal power plants, or to run plant and machineries
in a process or manufacturing industry. Heat energy is obtained
from combustion gases produced by burning coal or oil in the
furnace. The combustion gases evaporate water into steam in
the waterwall tubes and thereafter pass over the superheater
and reheater tubes. After turning down they encounter primary
superheater and economizer tubes. The combustion gases arefed through air preheater and cleaning devices before exiting
through the stack. The boiler tubes designed for a specific period
of time operate in a complex situation involving high tem-
perature, pressure and corrosive environment. Several natural
ageing processes such as creep, corrosion, fatigue, etc. occur-
ring during prolong operation are responsible for accumulating
Tel.: +91 657 2271709x2016; fax: +91 657 2270527.
E-mail addresses: sc1@nmlindia.org , schaudhuri06@yahoo.co.in .
microstructural damages in the tubes. The effective strength,
i.e. load bearing capacity of the tubes due to microstructural
damages decreases. The failure occurs when it falls below a
critical level determined by component geometry and loading.
Such failure is the major problem concerning the availability
of boilers. Since the failure results in non-availability of electric
power,loss of industrial production, etc. life assessment exercise
performed at regular intervals is a means to ensure avoidance
of such failures. Some important remaining life assessment
methodologies are based on empirical models using creep strain
measurement, combined timetemperature parameter, tube wall
thinning, oxide scale thicknessmeasurement, hardness measure-ment and microstructural assessment, etc. The creep database
[18] of indigenously produced, service exposed and failed
boiler tubing materials is used for their metallurgical assessment
and remaining life prediction.
2. Creep resistant steels
Creep resistant steels are extensively used for large-scale
chemical, thermal and nuclear power and petroleum industries.
0921-5093/$ see front matter 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2006.06.026
mailto:sc1@nmlindia.orgmailto:schaudhuri06@yahoo.co.inhttp://dx.doi.org/10.1016/j.msea.2006.06.026http://dx.doi.org/10.1016/j.msea.2006.06.026mailto:schaudhuri06@yahoo.co.inmailto:sc1@nmlindia.org8/6/2019 Some Aspects of Metallurgical Assessment of Boiler Tubes-basic Principles and Case Studies
2/10
S. Chaudhuri / Materials Science and Engineering A 432 (2006) 9099 91
Table 1
Nominal composition of creep-resistant steels (wt.%)
Steel type C Cr Mo Other elements
Group 1: carbon steels
Tubes SA-192 0.060.18 Mn: 0.270.63, Si: 0.25
Pipe SA-106 0.35 Mn: 0.291.06, Si: 0.10 minimun
Group 2: low alloy steels and 9CrMo steels
1Cr0.5Mo 0.11 1.0 0.5 Mn: 0.5, Si: 0.252.25Cr1Mo 0.12 2.25 1.0 Mn: 0.5, Si: 0.25
0.5Cr0.5Mo0.25V 0.11 0.5 0.5 Mn: 0.5, Si: 0.25, V: 0.25
9Cr1Mo 0.10 9.0 1.0 Mn: 0.5, Si: 0.60
Group 3: 12Cr steels
12CrMoV 0.12 12.0 0.6 V: 0.2, Ni: 0.80
12CrMoVNb 0.11 11.0 0.6 V: 0.2, Ni: 0.8, Nb: 0.35
Group 4: austenitic steels
Type 304 0.05 18.5 Mn: 1.3, Ni: 10
Type 316 0.05 18.0 2.5 Mn: 1.4, Si: 0.4, Ni: 10
Esshete 1250 0.10 15.0 1.0 Mn: 6, Si: 0.5, Ni:10, V: 0.3, Nb: 1
Some of the important factors to be considered for selection
of such steels are resistance to creep deformation and rupture,resistance to environmental attack, creep rupture strength and
ductility of weld metal and heat affected zone, adequate duc-
tility of base material to avoid sudden failure and also to allow
the material to deform rather than fracture in the regions of high
stress concentrations.
Lack of rupture ductility has been reported as the cause of
cracking in power station steam pipes made of 0.5% Mo steel.
This cracking resulted from repeated heating and cooling to and
from service temperature of about 565C (1049 F) during two-
shift operation. The problem was eliminated by replacement of
0.5% Mo steel with more ductile 1% CrMo steel.
The normal compositions of some creep resistant steels are
given in Table 1. The steels have been classified into four groupsfor usein theincreasing order of operating temperature.They are
carbon steels, low alloy CrMo/9CrMo steels, 12Cr-steels and
austenitic steels [9]. The ASMEboiler and Pressure Vessel Code,
Paragraph A-150 of section I states the criteria for determining
allowable stresses. The allowable stresses are not to be higher
than the lowest of the following:
One-fourth of the specified minimum tensile strength at room
temperature.
One-fourth of the tensile strength at elevated temperature.
Two-third of the specified minimum yield strength at room
temperature. Two-third of the yield strength at elevated temperature.
Stress to produce 1% creep in 100,000 h.
Two-third of the average stress or four-fifth of the minimum
stress to produce creep rupture in 100,000 h, whichever is
minimum.
These criteria are employed to establish allowable stresses
for a range of steels as a function of temperature A comparison
of the allowable stresses at various temperatures for commonly
used steels is shown in Fig. 1 [10,11]. For 2.25Cr1Mo steel, it is
the creep or rupture strength that determines the allowable stress
at a temperature of beyond 482
C (900
F). Therefore, in the
Fig. 1. Allowable stressfor severalgrades of steelsas a function of temperature.
evaluation of creepbehavior of CrMo steels, estimationof long-
term rupture strength has received considerable importance.
3. Life assessment methodology
Life assessment methodology can broadly be classified into
three levels [12]. Level 1 methodology is generally employed
when service life of the components is less than 80% of their
design lives. In level 1, assessments are performed using plant
records, design stress and temperatures, and minimum values of
material properties from literature. When service life exceeds
80% of the design life, level 2 methodology is employed. It
involves actual measurements of dimensions and temperatures,
stress calculations and inspections coupled with the use of the
minimum material properties from literature. However, when
life extension begins after attainingdesign life, level3 methodol-
ogy is employed. It involves in-depth inspection, stress analysis,
8/6/2019 Some Aspects of Metallurgical Assessment of Boiler Tubes-basic Principles and Case Studies
3/10
92 S. Chaudhuri / Materials Science and Engineering A 432 (2006) 9099
plant monitoring and generation of actual material data from
samples removed from the component. The details and accu-
racy of the results increase from level 1 to level 3 but at the
same time the cost of life assessment increases. Depending on
the extent of information available and the results obtained, the
analysis may stop at any level or proceed to the next level as
necessary.
One of the crucial parameters in estimation of creep life is the
operating temperature. Although steam temperatures are occa-
sionally measured in a boiler, local metal temperatures are rarely
measured. Due to load fluctuations and steam-side oxide-scale
growth during operation, it is also unlikely that a constant metal
temperature is maintained during service. It is therefore, more
convenient to estimate mean metal temperature in service by
examination of such parameters as hardness,microstructure,and
thickness of the steam-side oxide-scale for tubes. Because the
changes in these parameters are functions of time and temper-
ature, their current values may be used to estimate mean metal
temperature for a given operating time. The estimated tem-
perature can then be used in conjunction with standard creeprupture data to estimate the remaining life. Several methods for
estimation of metal temperature have been reviewed elsewhere
[13].
3.1. Hardness-based approach
The strength of low alloy steel changes with service expo-
sure depends on time and temperature. Thus change in hardness
during service (Fig. 2) may be used to estimate mean operating
temperature for the component. This approach is particularly
suitable when strength changes in service occur primarily as a
result of carbide coarsening neglecting stress induced softening.The database on changes in hardness due to long-term service
is employed to assess remaining life [13].
Fig. 2. Changes in hardness with LarsonMiller parameter.
3.2. Microstructure-based approach
Toft and Mardsen [14] demonstrated that there are basically
six stages of spheroidisation of carbides in ferritic steels. Using
SherbyDorn parameter, they established a reasonable correla-
tion of microstructure with mean service temperature. Similar
semi quantitative and qualitative approaches involving database
on changes in microstructure as a function of service history
have been widely used [15].
3.3. Oxide scale thickness-based approach
Extensive data from literature indicate that in relatively pure
steam, the growth of oxide scales is a function of temperature
and time of exposure. Several expressions have been proposed
in the literature to describe oxide scale growth kinetics [16,17].
This approach is particularly suitable when effective operating
stress increases and strength changes due to the growth of oxide
scale in service neglecting stress induced oxidation.
4. Failure of reheater tube
A failed reheater tube (Fig. 3) in a 500 MW boiler was
of pendent type placed in the convective zone. The operat-
ing pressure was 25 kg/cm2 and steam outlet temperature was
535 C (995 F). The design temperature of flue gas in the
zone was 700720 C (12921328 F). The tube material was
1.25Cr0.5Mo steel.
The tube had suffered extensive damage on the outer surface
in the form of pits. The dimension of the pits at some places
was as big as 40 mm 10 mm with a maximum depth of 2 mm.
The pitted surface bore brownish color in contrast to the damagefree surfaces of the tube. The latter bore usual black oxide. The
failure had occurred after about 24,000 h of service life.
Theinvestigation carried out at thelaboratory showedthat the
tube had the typical microstructure consisting of ferrite and bai-
nite and the mechanical properties were also found to be normal
[18]. No appreciable diametrical expansion was observed.
Since the tube showed extensive pitted surfaces, X-raymicro-
analysis using SEM was carried out on the pitted surfaces to
ascertain the presence of corrosive elements. The presence of
highlycorrosive elements like K, Ca,Si, S andCl were observed.
The presence of these elements suggested that the attack was
Fig. 3. Extensive damage on outer surface of failed reheater tube [4].
8/6/2019 Some Aspects of Metallurgical Assessment of Boiler Tubes-basic Principles and Case Studies
4/10
S. Chaudhuri / Materials Science and Engineering A 432 (2006) 9099 93
caused by the fusion of ash particles. Such attack usually occurs
due to:
Volatilization and condensation of volatile ash constituents
containing Na2SO4 or CaSO4.
Temperature excursion beyond 650 C (1202 F) particularly
during the starting period when no steam flows through the
reheater tubes.
Ash particles of low fusion point can fuse and stick on the
tube surfaces at such temperatures. It may be noted that alkali
metals along with S and Fe can form ash with fusion temperature
as low as 620 C (1148 F). The fuel oil used for the support can
also cause this problem if the oil contains corrosive elements like
V and S. Tube wall thinning due to sticking of fly ash particles
is primarily responsible for the failure of the reheater tube.
5. Failure of carbon steel tube
A failed carbon steel tube (Fig. 4a) was of the brittle windowtype without the fish mouth appearance. The operating temper-
ature of the tube was 350 C (662 F) The outer diameter and
thickness of the tube are 45 and 4.5 mm, respectively. Visual
examination showed a substantial degree of corrosion on the
water surface leaving a rough area in the vicinity of rupture.
Microstructural examination of a cross section through tube wall
revealed decarburisation and extensive discontinuous fissures
(Fig.4b).Thepresenceofoxidelayer( Fig.4b) and concentration
of chlorides at the interface between oxide and metal at the base
of corrosion pits (Fig. 4c) are also evident from microstructural
examination. The combination of water side corrosion (oxida-
tion), decarburisation and fissures, as observed in this case, ledto the conclusion that the tube had failed because of hydrogen
damage involving the formation of methane by the reaction of
dissolved hydrogen with carbon in steel.
Table 2
Material specification and operating parameters of superheater tubes
Material 2.25Cr1Mo steel as per ASTM A213 T22
Outer diameter 44.5 mm
Thickness 10 mm
Steam temperature 560580 C (10401076 F)
Steam pressure 185 kg/cm2
Steam flow rate 1700 tonnes/h
Fig. 5. Virgin, undamaged and failed final superheater tube of a 500 MW boiler.
6. Failure of final superheater tubes
Failure of final superheater tubes of a 500 M boiler occurred
during trial run following a service exposure of about 100 h [4].
Material specification and operating parameters of the tubes are
summarized in Table 2.
The tube samples selected for this investigation (Fig. 5) are apiece of tube from the zone of failure, a piece of tube adjacent to
the failed tube, termed as undamaged tube and a piece of virgin
tube.
Fig. 4. (a) Brittle window type fracture of a carbon steel tube; (b) microstructure through tube wall near rupture revealing decarburised metal and extensive
discontinuous fissures; (c) microstructure revealing chloride distribution at the interface between oxide and metal at corrosion pits [4].
8/6/2019 Some Aspects of Metallurgical Assessment of Boiler Tubes-basic Principles and Case Studies
5/10
94 S. Chaudhuri / Materials Science and Engineering A 432 (2006) 9099
Table 3
Chemical composition (in wt.%)
Element Failed tube Virgin tube Specification ASTM A213 T22
Carbon 0.12 0.1 0.060.15
Manganese 0.41 0.42 0.300.60
Silicon 0.22 0.22 0.50 maximum
Phosphorus 0.013 0.012 0.25 maximum
Sulphur 0.003 0.003 0.025 maximumChromium 2.16 2.13 1.902.50
Molybdenum 1.05 1.00 0.871.13
Table 4
Hardness values of virgin, undamaged and failed tube
Tubes Hardness, HV20
Inner Middle Outer
Virgin 145 149 151
Undamaged 143 143 145
Failed 179 178 180
The chemical composition of the failed and virgin tube,
reported in Table 3, indicates that the chemistry of both tubes
meets the ASTM specification. The outer diameter of the failed
tube was found to be 49.2 mm against the original diameter of
44.5 mm. The most significant point in this case is the gross
circumferential expansion of the failed tube up to about 19%.
Such an extensive expansion cannot be expected under normal
operating condition within a short span of service exposure of
about 100 h.
It is evident from the measurement of hardness at the inner
surface, mid section and outer surface of all tubes ( Table 4) thatthe hardness of the failed tube is significantly higher than that
of virgin and undamaged tubes.
Fig. 6. Tensileproperties of virgin, undamaged and failed final superheater tube
of a 500 MW boiler.
The tensile properties of all tubes (Fig. 6) revealed that irre-
spective of test temperature all the tubes meet the minimum
specified properties of 2.25Cr1Mo steel. The tensile strengthof the failed tube is, however, significantly higher than that of
the other tubes.
The microstructures of virgin (Fig. 7a) and undamaged
(Fig. 7b) tubes are almost similar, consisting of ferrite and
tempered bainite. In contrast the microstructure of the failed
tube (Fig. 7c) showed the presence of freshly formed bainitic
areas. Higher hardness and higher tensile strength as exhib-
ited by the failed tube based on the comparison with other
virgin and undamaged tubes confirms the presence of freshly
formed bainite in the failed tube. Besides, the oxide scale thick-
ness on the inner surface of the failed tube (Fig. 8a) was
found to be several folds more than that of the undamagedtube (Fig. 8b). In the failed tube the oxide scale thickness
was 0.25 mm.
Fig. 7. Microstructures of: (a) virgin tube; (b) undamaged tubes consisting of ferrite and tempered bainite and (c) failed tube showing freshly formed bainitic regions.
8/6/2019 Some Aspects of Metallurgical Assessment of Boiler Tubes-basic Principles and Case Studies
6/10
S. Chaudhuri / Materials Science and Engineering A 432 (2006) 9099 95
Fig. 8. (a) 0.25 mm thick oxide scale at inner surface of failed tube and (b) insignificant oxide scale at inner surface of undamaged tube.
All the above observations can be reconciled in a situation
only if the temperature had exceeded the lower critical tem-
perature, which is reported to be about 800 C (1472 F) for
2.25Cr1Mo steel.
In order to predict the extent of temperature excursion beyond
the critical temperature some detailed analysis was made based
on oxide thickness as obtained on the inner surface of the
failed tube. Published kinetic data on oxide scale growth of
2.25Cr1Mo steel have been used for this purpose [11]. The
predicted timetemperature profiles in the temperature range of
500850 C (9321562 F) for a range of service exposure to
develop 0.25 mm thick oxide at the inner surface of the failed
tube are shown in Fig. 9.
Since the presence of freshly formed bainite is indicative of
temperatureexcursion beyond the lower critical temperature, the
most probable profile that the tube experienced is the one hav-
ing an exposure of 2 h, the maximum temperature being 830 C
(1526 F). Corresponding to each timetemperature profile
(Fig. 9), accumulation of strain and the diametrical expansion ofthe tube have been calculated and presented in Fig. 10. The creep
strain predicted from the timetemperature profile for 2 h expo-
sure comes to about 1%, which is indeed quite lower than the
observed value of 19%. It is mainly because theexisting material
database in the temperature range of 500600 C (9321112 F)
has been used for extrapolation. Clearly there is a need to collect
creep data at temperatures beyond 600 C (1112 F).
Fig.9. Predictedtimetemperatureprofilesto develop0.25 mmthick oxide layer
at inner surface of failed tube.
In order to ensure whether it is possible to achieve a creep
strain of about 19% within a short time at 830 C (1526 F),
a short-term creep test has been carried out in the laboratory.
Based on this it has been established that a creep strain of about
16% is achievable in less than 2 h at 830 C (1526 F) and a
stress level of 30 MPa, which represents the hoop stress corre-
sponding to the maximum operating pressure of 185 kg/cm2 for
the tube in question. This, therefore, conclusively proves that
the failure of the tube took place due to short-term overheating
to a temperature of about 830 C (1526 F). Partial chocking of
the tube by some foreign material could be responsible for such
overheating. The other tubes, however, did not suffer any heavy
temperature excursion beyond 650 C (1202 F).
The presence of freshly formed bainitic structure and higher
hardness in the damaged failed tube is responsible for higher
strength. This is mainly because the ferrite and freshly formed
bainitic structure in the damaged tube is stronger than ferrite and
partially degenerated tempered bainite as observed in virgin and
undamaged tubes. Although the high temperature mechanicalproperties of service exposed components are often found to be
better than the minimum specified level, the component dimen-
sions often change as a result of prolonged service. The most
prominent amongst these is the loss of tube wall thickness. The
growthof oxide scale at tube surfaces is primarilyresponsiblefor
this. Other external processes such as coal ash corrosion, flame
impingement and fly ash erosion also contribute to such damage
development in service. A need is thus felt to look into the life
Fig.10. Predictedcreep strain correspondingto timetemperature profile shown
in Fig. 9.
8/6/2019 Some Aspects of Metallurgical Assessment of Boiler Tubes-basic Principles and Case Studies
7/10
96 S. Chaudhuri / Materials Science and Engineering A 432 (2006) 9099
Table 5
Material specification for boiler tubes
Tube sample Identification Nominal dimension, o.d.Th (mm) Measured dimension, o.d.Th (mm) Grade of steel
PLSH coil (outlet) L-R-CKT-25 51 8.8 51.48 8.91 SA 213 T22
RH coil (outlet) L-R-CKT-1 54 3.6 54.28 5.03 SA 213 T22
Table 6
Operating parameters of service exposed tubes
Tube sample Designed steam temperature, C (F) Operating pressure (kg/cm2) Length of service (h) Zone
PLSH tube (outlet) 544 C(1011 F) 158.2 59585 Platen superheater
RH tube (outlet) 580 C(1076 F) 33.5 59585 Reheater
prediction problems considering the influence of wall thinning
due to the existence of corrosion and erosion processes during
operation of the plant. Keeping this in view the methodology
recently developed in the laboratory for creep life estimation
of boiler tubes under wall thinning condition highlighting sometypical results has been discussed elsewhere [4].
7. Creep life assessment of service exposed platen
superheater and reheater tubes
Service exposed platen superheater (PLSH) and reheater
(RH) tubes of a thermal power plant were identified for remain-
ing creep life assessment study [20]. The grade of steel, dimen-
sions and identification numbers of these tube samples are given
in Table 5.
The operating parameters of the service exposed tubes as
obtained from the plant engineers are given in Table 6.
The experimental work undertaken for assessing remain-
ing life of service exposed boiler tubes includes tensile tests,
hardness measurement, microstructural examination and creep
rupture tests. The test data so generated have been analyzed and
compared with National Research Institute for Metals (NRIM)
data for 2.25Cr1Mo steel to examine the influence of service
exposure on mechanical properties and remaining life of boiler
tubes.
Longitudinal section of service exposed platen superheater
and reheater tube samples Fig. 11(a and b) showed grayish oxide
layer on the inner surfaces. The measured outer diameter and
thickness of the service exposed tubes are given in Table 7.
7.1. Tensile tests and hardness measurement
Standard tensile specimens were made from the longitudinal
direction of the service exposed boiler tubes to carry out tensile
tests in air using Instron 8562 servo electric machine at a con-
Table 7
Measured dimensions of service exposed tubes
Tube nos. Outer diameter (mm) Thickness (mm)
L-R-CKT-25 51.48 8.91
L-R-CKT-1 54.28 5.03
stant displacement rate of 0.008 mm/s. The range of temperature
selected for tensile tests are from 25 to 650 C (771202 F) for
both the platen superheater and reheater tubes.
Hardness measurement at 25 C (77 F) was carried out on
specimens selected from each type of boiler tube.The results of tensile tests viz. YS/0.2% PS, UTS and %
elongation as a function of temperature as well as hardness mea-
surement for PLSH and RH tubes are shown in Tables 8a and 8b.
The temperature dependence of 0.2% PS/YS data and UTS
data of both the tubes in the range of 25650 C (771202 F)
are shown graphically in Figs. 12 and 13, respectively. For the
purpose of comparison, theminimum NRIM data are also shown
in these figures.
7.2. Microstructural examination
Specimens for microstructural examination using optical
microscope were made following standard procedure from the
transverse direction of the service exposed tubes. Microstruc-
tures of service exposed platen superheater and reheater tubes
are shown in Fig. 14 and Fig. 15, respectively.
The microstructures, in general, consist of degenerated
bainitic regions in terms of spheroidisation of carbides. Such
microstructural features are expected from the tubes, which have
undergone a prolonged service exposure. The extent of degener-
ation of bainitic regions is almost similar in PLSH and RH tubes
Fig. 11. (a andb) Service exposed platensuperheater andreheatertubesamples.
8/6/2019 Some Aspects of Metallurgical Assessment of Boiler Tubes-basic Principles and Case Studies
8/10
S. Chaudhuri / Materials Science and Engineering A 432 (2006) 9099 97
Table 8a
Tensile properties and hardness measurement of service exposed PLSH tubes
Tube no. Temperature, C (F) YS/0.2% PS (MPa) UTS (MPa) % Elongation Hardness HV30
L-R-CKT-25 25 C(77 F) 248 453 19 137
L-R-CKT-25 600 C(1112 F) 143 180 33
L-R-CKT-25 650 C(1202 F) 117 139 37
Table 8b
Tensile properties and hardness measurement of service exposed RH tubes
Tube no. Temperature, C (F) YS/0.2% PS (MPa) UTS (MPa) % Elongation Hardness HV30
L-R-CKT-1 25 C(77 F) 259 501 14 143
L-R-CKT-1 600 C(1112F) 166 227 19
L-R-CKT-1 650 C(1202 F) 129 167 21
Fig. 12. Temperature dependence of 0.2% PS/YS of PLSH and RH tubes.
(Figs.14and15). The presence of bainitic microstructures is also
confirmed from the fact that 2.25Cr1Mo steel in normalized
and tempered condition exhibits a wide range of microstruc-
tures consisting of ferrite and tempered bainite when normal-
ized at 920 C (1688 F) to fully tempered bainitic microstruc-
Fig. 13. Temperature dependence of UTS of PLSH and RH tubes.
Fig. 14. Microstructure of service exposed platen superheater tubes.
tures when normalized at 990 C (1814 F) followed by forced
air cooling, the tempering temperature remaining the same at
730 C (1346 F) [8]. The microstructural features in PLSH and
RH tubes did not reveal presence of any significant damages
such as graphitisation, cavities, microcracks, etc. The thin oxide
scale is present at the inner surfaces of both the tubes [20].
Fig. 15. Microstructure of service exposed reheater tubes.
8/6/2019 Some Aspects of Metallurgical Assessment of Boiler Tubes-basic Principles and Case Studies
9/10
98 S. Chaudhuri / Materials Science and Engineering A 432 (2006) 9099
Table 9a
Creep rupture properties of service exposed PLSH tubes
Tube no. (type) Temperature, C (F) Stress (MPa) Rupture time (h) % Elongation
L-R-CKT-25 650 C(1202 F) 50 1073 29
L-R-CKT-25 600 C(1112 F) 100 144 31
L-R-CKT-25 600 C(1112 F) 80 851 28
Table 9b
Creep rupture properties of service exposed RH tubes
Tube no. (type) Temperature, C (F) Stress (MPa) Rupture time (h) % Elongation
L-R-CKT-1 650 C(1202 F) 50 1022 13
L-R-CKT-1 600 C(1112 F) 100 1057 21
L-R-CKT-1 600 C(1112 F) 80 2904 Test interrupted
7.3. Creep rupture tests
Standard test specimens were made from the longitudinal
direction of the service exposed boiler tubes to carry out creeprupture tests in air using Mayes creep testing machines. The
temperature levels selected for these tests are 600 and 650 C
(1112 and 1202 F). The stress levels for these tests are selected
to obtain rupture within a reasonable span of time. The results
of creep rupture tests of the specimens made from PLSH and
RH tubes are shown in Tables 9a and 9b.
LarsonMiller parameter (LMP) as a function of stress were
calculated using the formula LMP = T(20+log tr) where Tis the
temperature in (K) and tr is the rupture time in (h).
Thedependenceof LMP values on stressis shown graphically
in Fig. 16. For the purpose of comparison minimum NRIM data
are also shown in Fig. 16.
7.4. Results and discussion
Visual examination did not reveal presence of any signifi-
cant oxidation/corrosion damages on the outer surfaces of both
tubes.The inner surfaces,however,showed thepresenceof gray-
ish oxide layer Fig. 11(a and b). The reduction in tube wall
Fig. 16. Stress vs. LMP plot of PLSH and RH tubes.
thickness in reheater and platen superheater tubes as obtained
from measurement and compared with the specified thicknesswas not observed.
The results of tensile tests of service exposed PLSH and RH
tubes reveal the following features based on comparison with
NRIM data for the same grade 2.25Cr1Mo steel tube [19].
In general, deterioration in 0.2% PS (Fig. 12) and UTS
(Fig. 13) of both PLSH and RH tubes, when compared with min-
imum NRIM data, was observed. The RH tubes exhibit higher
0.2% PS and UTS than that of PLSH tubes irrespective of test
temperature. The extent of degradation is more pronounced in
case of PLSH tube. The 0.2% PS at 600 C (1112 F) and UTS
at 25 C (77 F) of RH tube meet the minimum NRIM data.
The remaining lives of service exposed tubes were estimated
using LarsonMiller parameter (LMP). The results of acceler-ated creep rupture tests of service exposed PLSH and RH tubing
steel as obtained in the laboratory are utilized for this purpose.
In absence of virgin PLSH and RH tubes, the accelerated creep
rupture data of service exposed tubes are compared with min-
imum NRIM data. In contrast to deterioration in 0.2% PS and
UTS of both PLSH and RH tubes, the applied stress dependence
of LMP for these tubes as shown in Fig. 16 clearly indicate that
the creep properties of both tubes at 50 MPa meet the minimum
properties when compared with NRIM data. At a stress level
of more than 50 MPa, the creep rupture data of PLSH tube fall
marginally below the creep rupture data of RH tube. The devia-
tion in terms of LMP is longer at higher applied stress of morethan 50 MPa and gradually increases with increasing applied
stress. Although the operating hoops stress as estimated from
the nominal tube dimension and operating pressure is 37 MPa
for PLSH tube and 16 MPa for RH tube, the remaining life
has been estimated from experimentally obtained creep rup-
ture data at the lowest stress level of 50 MPa and reported in
Table 10.
7.5. Conclusion
Based on accelerated creep rupture tests and analysis of
data, it can be said that the service exposed platen superheater
8/6/2019 Some Aspects of Metallurgical Assessment of Boiler Tubes-basic Principles and Case Studies
10/10
S. Chaudhuri / Materials Science and Engineering A 432 (2006) 9099 99
Table 10
Remaining life of PLSH and RH tube
Tube nos. Type of tube Estimated life at 50 M Pa (years)
L-R-CKT-25 Platen superheater tube 10 years at 570 C (1058 F)
L-R-CKT-1 Reheater tube 9 years at 580 C (1076 F)
and reheater tubes are in a good state of health for its continued
service provided no localized damages in the form of tube
wall thinning, circumferential expansion, excessive oxide
scale formation, microstructural degradation, etc. are present
besides adherence to the specified operating parameters in
service. The PLSH tube may be allowed to remain in service
for a period of 10 years under similar operating condition
provided tube wall temperature does not exceed 570 C
(1058 F). The RH tube may be allowed to remain in service
for a period of 9 years under similar operating condition
provided tube wall temperature does not exceed 580 C
(1076
F).A more reliable estimate of remaining life is obtained by
considering simultaneously NDT measurement at site and their
analysis,actual operatinghistory, destructive testsand theiranal-
ysis in the laboratory.
Acknowledgements
Changes in microstructures and mechanical properties of ser-
vice exposed boiler tubes presented in this paper is the outcome
of applied sponsored research for component integrity evalu-
ation program. The author wishes to thank Director, National
Metallurgical Laboratory, Jamshedpur, India for his kind per-
mission to publish this paper.
References
[1] S. Chaudhuri, R.N. Ghosh, ISIJ Int. 38 (1998) 881887.
[2] S. Chaudhuri, in: R. Singh, S.K. Sinha, S. Chaudhuri (Eds.), Power
Plant Metallurgy, NML, Jamshedpur, India, 1997, pp. 5673.
[3] S. Chaudhuri, N. Roy, R.N. Ghosh, Acta Metall. Mater 41 (1993)
273278.
[4] S. Chaudhuri, R. Singh, in: S.R. Singh, et al. (Eds.), Failure Analysis,
NML, Jamshedpur, India, 1997, pp. 107120.
[5] S. Chaudhuri, in: I. Chattoraj, et al. (Eds.), Boiler Corrosion, Proceedings
of the National Workshop (NWBC-95), NML, Jamshedpur, India, 1995,
pp. M1M22.
[6] R. Singh, S. Chaudhuri, et al., NML Report No. 11620034, December
1990.
[7] R. Singh, S. Chaudhuri, et al, NML Report No. 11620035, December
1990.
[8] S. Chaudhuri, Ph.D. Thesis, IIT, Kharagpur, 1994.
[9] S. Chaudhuri, Proceedings of a National Workshop on Component
Integrity Evaluation Program and Life Assessment, September 1996,
p. 55.
[10] ASME Boiler & Pressure Vessel Code, Section 1, ASME, New York.
[11] R. Viswanathan, Damage Mechanisms and Life Assessment of High
Temperature Components, ASM International, 1989, pp. 185229.
[12] R. Viswanathan, R.B. Dooley, in: International Conference on Creep,
ASTM, Tokyo, April 1418, JSME, J. Mech. E., ASME (1986) 349359.
[13] R. Viswanathan, J.R. Foulds, D.A. Roberts, Proceedings of the Inter-
national Conference on Life Extension and Assessment, Hague, June
1988.
[14] L.H. Toft, R.A. Mardsen, Conference on Structural Processes in Creep,
JISI/JIM, London, 1963, p. 275.
[15] R. Coade, Report No.SO/85/87, State Electricity Commission of Victo-
ria, Australia, February 1885.
[16] I.M. Rehn, W. Apblett, Report CS 1811m Electric Power Research Insti-
tute, Palo Alto, CA, 1981.
[17] S.R. Paterson, T.W. Rettig, Project RP 2253-5, Final Report, Electric
Power Research Institute, Palo Alktio, CA, 1987.
[18] S. Banerjee, R. Singh, K. Prasad, NML Report, October 1991.
[19] NRIM Creep Data Sheet No. 3A, 1976.
[20] S. Chaudhuri, Final Report, NML, Jamshedpur, June 2003.
Recommended