some aspects of metallurgical assessment of boiler tubes-basic principles and case studies

8/6/2019 Some Aspects of Metallurgical Assessment of Boiler Tubes-basic Principles and Case Studies

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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: [email protected] , [email protected] .

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:[email protected]:[email protected]://dx.doi.org/10.1016/j.msea.2006.06.026http://dx.doi.org/10.1016/j.msea.2006.06.026mailto:[email protected]:[email protected]


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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,


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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].


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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].


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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.


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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.


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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.


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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.


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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


10/10


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.

some aspects of metallurgical assessment of boiler tubes-basic principles and case studies

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