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1. Case Studies in Engineering Failure Analysis 9 (2017) 17–26
Contents lists available at ScienceDirect
Case Studies in Engineering Failure Analysis
journal homepage: www.elsevier.com/locate/csefa
Failure investigation of super heater tubes of coal fired power plant
A.K. Pramanick, G. Das, S.K. Das, M. Ghosh
⁎
Materials Science & Technology, CSIR – National Metallurgical Laboratory, Jamshedpur 831007, India
A R T I C L E I N F O
Keywords:
Failure analysis
Boiler tube
Oxidation
Corrosion
Creep
A B S T R A C T
Cause of failure of two adjacent super heater tubes made of Cr-Mo steel of a coal based 60 MW
thermal power plant has been portrayed in present investigation. Oxide deposits were found on
internal surface of tubes. Deposits created significant resistance to heat transfer and resulted in
undesirable rise in component temperature. This situation, in turn, aggravated the condition of
gas side that was exposed to high temperature. Localized heating coarsened carbides as well as
propelled precipitation of new brittle phases along grain boundary resulting in embrittlement of
tube material. Continuous exposure to high temperature softened the tube material and tube wall
was thinned down with bulging toward outside. Creep void formation along grain boundary was
observed and steered intergranular cracking. All these e ectsff contributed synergistically and
tubes were failed ultimately due to overload under high Hoop stress.
1. Introduction
For any power plant, it is prime importance to generate electricity without forced outages. Failure of super heater tube of boiler is
the major concern of forced outages at coal fired thermal power plant. Flue gas passes over super heater tubes leading to damage over
the time of operation and termed as fireside damage/corrosion. Again the extent of damage is dependent on quality of coal, materials
used, operation and maintenance. Interior of these tubes are also vulnerable and primarily dependent on quality of water used for
generating high pressure steam. Continuous/steady flow of steam through these tubes is necessary to maintain tube materials under
prescribed temperature. Otherwise, there is a possibility of shoot-up of temperature causing fast detoriation of materials and sub-
sequent failure. In that case overall efficiency of the plant is dropped. Therefore, the study of tube failure and finding the solution is
needed to avoid such incident in future.
Boiler tubes of a coal fired plant faced harsh environment all the way from inside steam to outside flue gases. Tubes are exposed to
temperature in the range of 540–1000 °C, varying along length of tubes i.e. from base toward elevation. According to service con-
dition, outside of tubes are exposed to high temperature. High pressure steam flows through inside and is discharged at a temperature
of > 500 °C depending on nature and capacity of plant. Temperature shoot up above specification is most common reason of failure
for boiler tubes [1]. The reason is either scale formation on internal and/or external surfaces under prolong exposure at elevated
temperature or non-uniform steam flow through partially blocked tubes [2]. Internal scale formation reduces heat transfer rate across
tube wall. Moreover, scale formation causes non-linear (non-uniform) heating, resulting in the retardation of heat transfer further and
reduction of thermal efficiency. External oxide formation generally depends on type/quality of coal, which produces flue gas. Mostly
complex alkali sulfate scales are formed. This effect raises the temperature of tube locally and longtime exposure results in thicker
oxide formation, subsequent exclusion of the same. The later phenomenon escalates wall thinning and rupture of the tube. Material
de-generation and subsequent failure due to thermal fluctuation, have been studied by a number of investigators in recent past [3,4].
⁎
Corresponding author.
E-mail address: ghosh_mnk@yahoo.com (M. Ghosh).
http://dx.doi.org/10.1016/j.csefa.2017.06.001
Received 28 December 2016; Received in revised form 4 May 2017; Accepted 6 June 2017
Available online 09 June 2017
2213-2902/ © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

2. A.K. Pramanick et al. Case Studies in Engineering Failure Analysis 9 (2017) 17–26
18
It is to be noted, that in many cases a thin protective Fe3O4 layer is deposited on waterside of tubes. Protectiveness of this thin
layer depends on pH level and degree of contamination of water. There are many failure mechanisms that have been reported
depending upon the presence of contamination with flowing steam. These are primarily related with caustic corrosion, hydrogen
damage or pitting [5–12].
This paper presents the analysis of failure of two adjacent super heater tubes in a coal based 60 MW power plant. Inside steam
pressure was at 100 kg/cm
2
. Within short span of time in the end zone of the super heater tubes three failures were reported. The
incident happened with the component commissioned in 1988 and last overhauled in 2015. After routine maintenance, the system
was operated for nearly four months and then first failure occurred. Subsequently, second failure was reported after 10 days and third
one took place after 2 months. During entire operation period only schedule outages were made. Location of the failed region was
close vicinity of top end of the tubes and very near to boiler drum where the flu gas temperature was ∼900
°
C.
Two failed pieces of tubes made of Cr–Mo steel were chosen for this investigation. Sample-A (tube A) exhibited fish mouth
cracking at one side and bulging at opposite side. Layer wise corrosion was observed near open end of crack and over remaining wall.
Inner surface contained multiple cracks along longitudinal direction whereas the same was completely absent at outer surface.
Sample-B (Tube B), also contained fish mouth cracking at one part of the tube. Inner side of the pipe was covered with loosely
attached brownish substance. Excessive wall thinning was found close to failure. With respect to total designated service life, both the
components failed after covering nearly ¾
th
of the same. To prevent such undesirable incident in future, the investigation was taken
up to find out cause of failure and subsequently providing tentative remedial measure.
2. Experimental
Failed boiler tubes were visually examined to reveal the nature of fracture. Tubes were cut along cross section to study the
appearance of inner wall. Samples were collected from different locations for investigation as indicated in Fig. 1. The marked
locations with sample ID are collated in Table 1.
Samples- A1 to A4 and B1 to B3 were considered for metallographic examination. They were mounted, polished by conventional
technique, etched with 3% Nital and examined in optical (Leica DM 2500 M, USA) and scanning electron (JEOL JSM 840A, JAPAN)
microscopes. Fracture surfaces were cleaned using dilute EDTA solution followed by Kerosene oil and finally in Acetone by soni-
cation. The samples were studied in SEM. Bulk composition of alloy was determined in ICP and LECO using chips, obtained from
cleaned surface. Bulk hardness was evaluated near fracture and bulging zone in Brinell Scale using steel ball as indenter. Some
amount of adhered corrosion products was collected and examined by X-ray diffraction technique to identify their nature. The
investigation and corresponding inferences are described in the following sub-sections.
3. Results & discussions
3.1. Visual examination
The damaged tubes were observed in reflected light with naked eyes (Fig. 1 and 2). Two tubes were designated as Sample-A and
sample-B. Both tubes experienced temperature in range of ∼540 °C with stress level 100 kg/cm
2
during operation. Sample-A ex-
hibited fish mouth cracking at one side (Fig. 1a) and bulging at opposite side of weld (Fig. 1b). Cut length of the tube was 400 mm
and wall thickness of un-deformed region was ∼5.80 mm with ∼36.8 mm outer diameter. From weld seam the distance of cracking
was ∼40 mm and total crack length was ∼50 mm with ∼6.7 mm maximum opening. Layer wise corrosion was observed near open
end of crack and the remaining wall thickness was reduced drastically (< 1 mm). Inner surface contained multiple crack formation
along longitudinal direction (Fig. 1c) whereas the same was completely absent at outer surface. Outer surface was blackened owing to
thermal effect (Fig. 1a) and the inside surface was deep brown with numerous reddish spots (Fig. 1c).
As mentioned above, bulging was observed at lower half of the same tube. After bulging outer diameter became ∼42.0 mm with
distance of deformation ∼12 cm from weld seam. Thick scale was loosely adhered to the inner surface near bulging (Fig. 1d).
Sample-B, also contained fish mouth cracking at one part of tube and other part exhibited no such de-generation. Total length of
cut portion of tube was ∼250 mm, wall thickness was ∼6.2 mm with outer diameter ∼39.0 mm. Crack was adjacent to weld seam
with length ∼30 mm and maximum crack opening ∼2.8 mm. Both inner and outer surface contained longitudinal cracking of
variable length (Fig. 2a and b). Inner side of the pipe was loosely covered with fine brown whiskers, which were product of oxidation
corrosion (Fig. 2b). Fracture surface was dull in appearance. The area contained layered brown structure due to oxidation corrosion
owing to its exposure to air after failure (Fig. 2a). Excessive thinning was found at the close vicinity of failure.
3.2. Chemical composition
The concentration of alloying elements in alloy has been collated in Table 2. The chemical composition of bulk specimen con-
firmed Polish composition of Steel 10H2M, which was equivalent to DIN 10CrMo9-10 or T22. There was no difference in chemical
composition of the failed tubes with respect to standard specification.
3.3. Microstructural examination
The optical microstructures of damaged components are shown in Fig. 3 and consisted of pre-dominantly polygonal ferrite. Close

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A.K. Pramanick et al. Case Studies in Engineering Failure Analysis 9 (2017) 17–26
Fig. 1. Macro image and sampling location from damaged tube A (a) cracking over surface (b) bulged region, (c) Inner surface of tube, (d) inner surface of bulged
region and (e) schematic presentation of tube assembly with marked location of failure.
to grain boundary, dark voids were observed. This was an indication of creep. Couple of globular/elliptical/near-globular diffused
shaded islands of variable size was also found. These were globular carbides occurred due to spherodisation under high temperature
exposure. With respect to un-deformed grain geometry (Figs. 3a and d), grain shape near fracture (Figs. 3b and e) exhibited slight
Table 1
Sampling location from fracture component and its nomenclature.
Tube ID Sample ID Location
Sample-A Sample-A1
Sample-A2
Sample-A3
Sample-A4
Away from fracture
Sub-surface of fracture
Near bulging
Near Fracture (Across thickness)
Sample-B Sample-B1
Sample-B2
Sample-B3
Away from fracture
Sub-surface of fracture
Near Fracture (Across thickness)

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A.K. Pramanick et al. Case Studies in Engineering Failure Analysis 9 (2017) 17–26
Fig. 2. Macro image and sampling location from damaged tube B (a) cracking over surface and (b) appearance of inner surface.
increment in aspect ratio with loss of angularity. This envisaged material flow under stress at elevated temperature leading to bulging
and subsequent wall thinning.
SEM micrographs confirmed the inferences drawn from optical imaging (Fig. 4). Along grain boundary bright islands indicated
spherodisation of carbides (indicated by arrows). Along grain boundary void coalescence resulted in crack formation (Fig. 4a and c).
Region close to bulging also revealed interconnected voids that were generated during service exploitation. Creep damage occurred at
elevated temperature under stress. Owing to local temperature rise beyond the designated/recommended maximum temperature of
use over a period of time caused grain boundary sliding. The sliding generated de-cohesion/voids at boundary. With time voids grew
in size and became inter connected. This weakened grain boundary strength with ultimate reduction in load bearing capability of
component. Severe spherodisation of carbide and void generation were the indication of local temperature rise. Therefore, tem-
perature rise at high stress level caused two phenomena i.e. carbide spherodisation and creep damage to create voids. SEM in-
vestigation also carried out from inner to outer surface to reveal pre-dominant structural changes – if any for tubes near fracture
(Fig. 5).
Practically, either inner or outer region close to fracture did not exhibit significant change with respect to Fig. 4. EDS spectrum
was obtained from one of the precipitates along grain boundary. Semi-quantitative analysis revealed that precipitates were complex
mixed alloy carbides with Cr and Mo as major constituents.
3.4. Hardness measurement
The bulk hardness at different location of failed component is given in Table 3. With respect to as received material hardness of
∼200 BHN as indicated in published literature/standards for this grade of Cr-Mo steel, the hardness was reduced drastically near
fracture indicating materials softening during service. Bulged area exhibited little higher hardness as the damage might be little less
in that region with respect to fractured location over same time span. It could be possible that with still longer exposure, the hardness
of bulged area might reach to the hardness of failure location.
3.5. X-ray diffraction study
Fig. 6 illustrated the characteristic X-ray spectrum of corrosion products obtained from different location of failed tube. X-ray
diffraction analysis revealed the presence of different Fe-oxides and hydroxides. Bulk composition of such deposits was also examined
in EDS to find out qualitatively the constituents (Fig. 7). Major peaks in illustration indicated the presence of iron and oxygen. It
referred that corrosion products were mainly iron oxide in association with minor amount of other chemical species of steel.
Table 2
Bulk chemical composition of component.
Alloy Element in wt%
C Si Mn Cr Mo S (max) P (max) Fe Ni/Cu (max)
Steel 10H2M 0.08–0.15 0.15–0.30 0.3–0.6 2.0–2.5 0.9–1.1 0.03 0.03 bal 0.30
Din 10 CrMo9-10 0.08–0.14 0.5 max 0.4–0.8 2.0–2.5 0.9–1.1 0.01 0.02 bal 0.30
T22 0.05–0.15 0.5 max 0.3–0.6 1.9–2.6 0.87–1.13 0.025 0.025 bal NI
Sample A 0.14 0.27 0.41 2.25 0.99 0.02 0.02 bal N/F
Sample B 0.13 0.32 0.47 2.10 1.03 0.02 0.02 bal N/F
*
N/F – not found, NI – not indicated.

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A.K. Pramanick et al. Case Studies in Engineering Failure Analysis 9 (2017) 17–26
Fig. 3. Optical microstructure of transverse section of damaged tubes (a) sample A1, (b) sample A2, (c) sample A3, (d) sample B1 and (e) sample B2. All samples
exhibited ferrite matrix containing large voids, spherodisation and coalescence of carbides preferably along grain boundary.
Fig. 4. SEM micrographs of transverse section of damaged tubes (a) sample A2, (b) sample A3 and (c) sample B2; g.b.voids/cracks and alloy carbides were visible
along grain boundary.

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A.K. Pramanick et al. Case Studies in Engineering Failure Analysis 9 (2017) 17–26
Fig. 5. SEM micrographs across inner to outer region of damaged tubes (a) inner – sample A4, (b) outer – sample A4, (c) inner – sample B3, (d) outer – sample B3 and
(e) EDs spectrum from precipitate as shown in Fig. 5b with quantification indicating Cr-Mo rich alloy carbide.
3.6. Fractography
SEM fractographs of both tubes are displayed in Fig. 8. Large sized micro-voids/dimples were observed envisaging softening of
tube material (Fig. 8a and c). The softening of alloy was established through reduction in bulk hardness as indicated in Table 3.
Ductile fracture signified micro-void coalescence resulting in overload failure. For sample A, fracture surface was mostly covered with
oxides and it was di cultffi to explore specific features underneath apart from ductility failure (Fig. 8a). In some regions cracked thick
skin of oxide layer was found (Fig. 8b). In case of sample B2, intergranular failure was observed (arrows in Fig. 8d) at localised
regions; however, most of the area of fracture surface indicated ductile dimple fracture (Fig. 8c). This intergranular cracking was
originated from voids adjacent to grain boundary due to creep (Fig. 4).
Thus, thin lip fish mouth opening of fracture surface of both tube A and B was the signature of mix mode failure. In that case,
creep deformation was accompanied by wall thinning through necking. This reduced strength of the alloy resulting in dimple rupture.
If creep became the only operating mechanism, then it became thick lip fish mouth failure as there would be no scope for material
flow/deformation to reduce wall thickness.
3.7. Evaluation of hoop stress
In straight tube, any force applied over circumference/cylindrical wall (i.e. a normal stress along tangential/azimuth direction) is
termed as circumferential stress or hoop stress. Hoop stress is tensile in nature. In case of thin-walled tube, it has been assumed that
wall thickness becomes no more than one-tenth of its radius. This allows for treating the wall as a surface, and subsequently using the
Young–Laplace equation [13] for estimating the hoop stress.
Fracture is governed by the hoop stress in the absence of other external loads, since it is the largest principal stress. It is note-
worthy that greatest stress is experienced inside of tube; hence cracks in tubes should theoretically start from inside the tube. Yielding
is governed by an equivalent stress that includes hoop stress and the longitudinal or radial stress when present. Hoop stress is
Table 3
Bulk hardness of failed tubes at different location.
Sample A2 Sample B2 Sample B3
Hardness (BHN) 114 ± 1.5 114 ± 0.5 126 ± 1

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A.K. Pramanick et al. Case Studies in Engineering Failure Analysis 9 (2017) 17–26
Fig. 6. X-ray characteristic spectrum from the deposits of oxidation corrosion (a) Tube – A and (b) Tube-B; Corrosion products were mostly iron oxides and hydroxides
of different grades.
expressed in following fashion,
σ = P.d. (2t)
−1
Where, P is the stress in MPa, d is the internal diameter of tube in mm and t is the wall thickness in mm. Considering inside operating
pressure of tube 100 Kgf/cm
2
, the hoop stress in un-deformed and deformed regions of tube A and B was calculated and given in
Table 4. These values were compared with room temperature tensile properties of annealed T22 alloy.
From Table 4 it was evident, that with respect to un-deformed region, deformed region experienced excessive normal stress during
service exploitation due to oxidation corrosion assisted wall thinning, which was greater than the yield strength of tube material.
Considering T22 alloy, this category of Cr-Mo steel able to retain their strength up to ∼520 °C and then reduced drastically and may
reach ∼30 MPa (0.25 off set yield) at ∼600 °C. Therefore, apart from oxide scales, local heating beyond operating temperature
propelled material flow and softening to thin down the tube wall further. This thin wall was unable to with stand high hoop stress,
hence failed under overload.
4. Observations
i The component was made of Cr-Mo low alloy steel. Composition was close to Polish specification of ‘Steel 10H2M’. Other
equivalent specification was DIN 10CrMo910/T22. Concentration of major chemical species was within the specified limit.
ii iMicrostructure of steel tube consisted of polygonal ferrite grains containing pearlite and alloy carbide. During service exposure,
oxide scales were formed inside the tube. When the scale thickness became substantial, it started hindering heat transfer across the
tube wall. Thus localized heating took place. Scale formation, its growth and subsequent removal by loosening from material
surface were continuous process. This resulted in reduction in wall thickness of tube. However, as it becomes a non-steady state
phenomenon, any further quantification of data might lead to misleading information for high temperature deformation char-
acteristics. Moreover, Heating coarsened the carbides and propelled precipitation of new brittle phases along grain boundary. This
caused further embrittlement.
iii Under stress and temperature, grain boundary sliding promoted void formation along boundary and at latter stage they became

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A.K. Pramanick et al. Case Studies in Engineering Failure Analysis 9 (2017) 17–26
Fig. 7. Qualitative EDS analysis of scales formed inside the tube (a) scales from tube A and (b) scales from tube B.
interconnected. This further weakened the structure.
iv Dimension of the tube near fracture was changed drastically. It seemed that materials got softened. Material softening was
confirmed through evaluation of bulk hardness near failed location. The value was exorbitantly low with respect to investigated
material as indicated in literature. In addition to earlier effect of scale formation, the softening contributed in material’s flow also
and created bulging.
v Initially by creep i.e. intergranular cracking and subsequently due to overload ductile fracture under excessive hoop stress beyond
the yield point of tube material, the component failed during service exploitation. This was endorsed by ‘Thin Lip Fish Mouth’
fracture at both the failed location.
5. Conclusions
From the investigation and subsequent inferences on root cause, sequences of final failure can be illustrated as follows:
• Oxide scale formation and its subsequent spalling from inside wall of tube – occurred due to improper cleaning and/or poor water
quality over substantial period of service and resulted wall thinning.
• Creep & softening of material – oxide scale hindered smooth heat transfer across tube wall hence caused local overheating. Under
inside steam pressure i.e. stress at high temperature creep was taken place accompanied by softening of materials.
• Weakening of material – along grain boundary there was de-cohesion and at reduced wall thickness the alloy lost load bearing
ability
• Final failure – bulging occurred under excessive Hoop stress leading to failure

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A.K. Pramanick et al. Case Studies in Engineering Failure Analysis 9 (2017) 17–26
Fig. 8. SEM fractographs of damaged tubes (a) & (b) near sample A2, (c) & (d) close to sample B2.
Table 4
Hoop stress distribution in failed tubes.
Sample ID Hoop stress σ (MPa) YS (MPa) UTS (MPa)
Sample-A
Un-deformed
21.3
Deformed
224.7
Sample-B
T22 alloy
21.0 352.6
220.0 415.0
6. Preventive and corrective actions
• Cleaning of inside tube wall at regular interval might be adopted to remove all hindrance/heterogeneities.
• Checking of quality of water time to time is required to maintain its pH and oxygen content at specified level. One of the sources of
contamination is condenser tube leakage, leading to high salt concentration within water.
Acknowledgements
The authors are thankful to the Director, CSIR-NML, Jamshedpur, for his kind permission to publish the research work.
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