18th World Conference on Nondestructive Testing, 16-20 April 2012, Durban, South Africa
Evaluation of Material Degradation in Steam Pipelines
Yonka P. IVANOVA1, Todor A. PARTALIN2, Bojana M.TABAKOVA3
1Institute of Mechanics - Bulgarian Academy of Sciences, Lab. NDT, bl.4, Acad.G.Bonchev Str.
1113 Sofia, Bulgaria, e-mail: [email protected] 2Sofia University “St.Kliment Ohridski”, Faculty of Mathematics and Informatics,
James Baucher Bul. 5, Sofia, Bulgaria, Phone:+359 2 8161562, e-mail: [email protected]. 3Technical university-Sofia, Bulgaria, Phone:+359 2 9653697, e-mail: [email protected]
Abstract Evaluation of the degradation of structure state of pipelines is an important problem in materials science and
industry. The nondestructive assessment of the damages that occurs in components at high stresses and
temperature is the only approach during service time for monitoring the state and estimation the residual life.
In the paper the investigations of material degradation are carried out by classical metallographic analysis as well
as by non-destructive ultrasonic method. Various procedures are developed and applied for material
characterization. The results are compared in order to find the more suitable technique for in-service evaluation
of pipeline status. The methods of data digital processing are applied with the purpose of obtaining useful
informative parameters.
Keywords: material degradation, pipelines, ultrasonic testing
1. Introduction
The equipments in power plants are working under continuously hard conditions. Under high
temperatures and pressures the microstructure in pipelines changes as a result of creep,
corrosion, carbide phase changes and appearance of large number of micro-defects. The
exploitation of the pipelines carries risks of failure and damage because of degradation of
structure and lowering the mechanical properties of the materials. The localization of the
damages may be of use in case of repair works. That is why the state of metal is the object of
incoming and periodical testing and monitoring through service. The aim of such activities is
estimation the degree of damages and prediction the residual life of pipelines in order to avoid
the failure in power stations.
At the present time, according to Technical Conditions TU 14-3-460-2003 the assessment of
the materials is carried out by metallographic analysis and mechanical testing. The application
of non-destructive methods is limited, because of insufficient accuracy and can be expanded if
the test results became more reliable. The possibilities for structure estimation by non-
destructive testing methods are presented in [1-10]. In [10] degradation of microstructure in
pipelines is evaluated by ultrasonic spectral analysis.
This paper presents the results of ultrasonic investigations by immersion pulse echo
method of pipe elements with different degree of degradation after long service in thermo-
electric power station.
2. Test samples
The test samples are cut from the pipes used in boiler, economizer and convective steam super
heater. The pipes were subjected to the different conditions, such as pressure (P), temperature
(T), working time (D) and cycles. The working conditions are given in Table 1. The samples
1 and 5 are reference tubes that are not exploited. (See table 1). The chemical content of steels
are: steel 20 C-0.17-0.24 %, Si-0.17-0.37; Mn-0.35-0.65%, Cr<0.25%, Ni<0.25%, S<0.0125,
P<0.03%, Cu<0.3%; steel 12H1MF C-0.08-0.15 %, Si-0.17-0.37%; Mn-0.4-0.7%, Cr -0.9-1.2%, Mo-
0.25-0.35%, V-0.15-0.3%, Ni<0.25%, S<0.0125, P<0.03%, Cu<0.3%.
Table 1. Operating conditions of tube elements in thermo-electric power station
� Steel T o C P, MPa Working
Duration,
hours
Working cycles
1 20 20 0 0 0
2 20 345 16 175730 522
3 20 345 16 175 730 522
4 20 345 16 25 117 150
5 12H1MF 20 0 0 0
6 12H1MF 514 2.5 175730 522
3. Metallographic and mechanical investigations
The procedures of metallographic and mechanical testing for metal state determination of tube
elements from heated surfaces of economizer, intermediate steam over-heaters and convective
steam over-heaters were presented in details in [1,2].
Micro-structural analysis include the determination of the type and grade of non-metal
includes, the size of the grains, the composition and characteristic features of the structure, as
well as the state of external and internal surface of the tubes concerning the corrosive damage,
cracks and other defects. The estimation of the separate micro structural characteristics was
done according standards and normative documents.
It is found irregular oxidative layer with thickness of 0,4mm on the whole external surface of
all samples. It is determined a decrease of carbons along the internal and the external sides of
the heated parts of the tube samples.
On the heated parts of pipelines it is observed an irregular corrosion of the external surface of
samples from 12H1MF [1] and an intergranular corrosion on the internal surface of all tube
samples.
3.1 Type and grade of non-metal inclusions
The existence of non-metal inclusions in all samples, which in this case are dot-like oxides
(up to grade 3 – grade 2 predominates) and sulfides (up to grade 3) are within the norms (3.5
grades and 3 grades, respectively) for the examined steels, according TU 14-3-460-2003.
3.2 Microstructure and grain size
Under long-time operation at high pressures and temperatures, the microstructure of metals of
steam pipelines in thermal power stations is changed. Degradation of structure is expressed by
coagulation of grains and carbides, grain growth, and arrangement of pearlite in strips and
appearance of microdefects (micropores).
The results of the metallographic study of tested samples show inhomogeneity�and anisotropy
of the structure in the longitudinal and transversal directions. Microstructure consists of ferrite
and pearlite (15-20%) with spheroidization of the pearlite phases. The structure of the steel
12H1MF is evaluated as a grade 6 [1]. These results are unacceptable according normative
documents [2]. Microstructure of tube �2,3 consists of different grains from 7 to 9 scale
grade [1]. Figures 1 show typical photographs in the transversal sections (external, middle,
internal parts) of the tube elements obtained in heated and unheated parts (sample �2, �3). It
can be observed the consolidation and augmentation of the grains.
heated part �100
External side Middle Internal side
unheated part �100
External side Middle Internal side
Figure 1. Photographs of tube elements �2 in transversal sections
Some of samples have an exclusive arrangement of pearlite in strips. Micrographs of
microstructures for different points of sample �3 are showed in Figure 3. That kind of
inhomogeneities (strips of pearlite phases) can be estimated as a scale 3, line B, grade 5
according to documents [1,2].
heated part �100
External side Middle Internal side
unheated part �100
.
External side Middle Internal side
Figure 2. Photographs of tube elements �3 in longitudinal sections
heated part �100
External side Middle Internal side
unheated part �100
External side Middle Internal side
Figure 3. Photographs of tube elements �6 (steel 12H1MF) in transversal sections
Microstructure of pipe � 6 (steel 12H1MF) is shown on Figure 3 transversal and on Figure 4
longitudinal sections. Tube samples have grained structure from 6 to 8 grades with prevalent
grade 8; the consolidation of the grains in the inner and outer surface of the pipes is quite
visible. There are coagulated carbides in the border of the grains. As a result of that, some of
the margins are thickened. Some of them contain chain-bounded carbides. These features are
precondition for decreased plastic quality of the steels.
heated part unheated part
External side
Middle
Internal side
Figure 4 . Photographs of tube elements �6 (steel 12H1MF) in longitudinal sections
The investigation in [1] shows that tube samples � 2, 3, 4, 6 are in the phase of metal fragility
and low plasticity. The results of mechanical testing of samples at room temperatures (20°C)
and high temperatures (345, 345, 514 oC) show that the values of tensile and yield stress are
close to the minimal allowed values [1,2].
4. Ultrasonic Study
The experimental setup for ultrasonic study of pipelines is shown in Figure 5, where 1 is an
immersion tank full with alcohol -water solution, 2 - test object, 3 – immersion transducer, 4-
US box with pulser and receiver, 5- computer. The ultrasonic system is composed of
ultrasonic US box consisted of pulser / receiver and computer.
Figure 5.Experimental setup for ultrasonic study
Ultrasonic waves are excited in the samples by piezoelectric transducer with a central
frequency of 10 MHz. The transducer diameter is 8 mm. To obtain and record ultrasonic
signals a personal computer with LabView software is used. The typical sampling frequency
used for the Echo is 160MHz with a 12 bit resolution. During the ultrasonic investigation the
pipe samples are rotated by automatic scanning system with step of 15 degree for A-scan and
continuously for B-scan. Thus ultrasonic signals are obtained over the all perimeter of the
pipe under the same conditions. The received ultrasonic echoes are complex signals for
material structure. The registered ultrasonic echoes are stored as an A and B-scan images and
processed. The samples are investigated with corrosion layer and also after removing it.
5.Results, processing and analysis
Figure 6 presents a waveform and signal of pipe �1. The first registered signal is the
reflected pulse from interface water-sample, the second one is the first back-wall echo (from
inner side of the pipe) and all next are result of reflections between those surfaces as shown
on Figure 6a. Between those echoes emerge back-scattered (structural) noise. The total
attenuation coefficients of the pipe samples are estimated by the imposing exponential decay
of the multiple back wall echoes (Figure 6b). The first three back wall echoes are selected for
further spectrum analysis. The frequency dependent attenuation coefficient is defined by the
ratio of the spectra of two consecutive echoes [10].
127 129 131 133 135 137
A,V
1st back w all echo
2nd back w all echon th back w all
echo
reflection w ater-sample
t,µs
127 129 131 133 135 137
A,V
t,µs
a. Ultrasonic signal, pipe �1 b. A-scan and exponential decay of echoes
Figure 6.
One of the main hypotheses�in the work is that the presence of structure irregularities in the
material leads to significant scattering. The received backscatter is best observed around first
back wall echo. Figure 7 presents a B-scan of the ultrasonic signals obtained from referent
sample �1. The values of attenuation of ultrasonic waves obtained in different points of the
pipe perimeter are given in Figure 8. Some of waveform echoes are shown in the figure. The
presented results concern cleaned sample. The attenuation coefficient varies around the mean
value.
Figure 7. B-scan, pipe �1 Figure 8. Attenuation coefficients at different
positions over the perimeter
After exploitation structure degrades as we can see in photographs (Figures 1-4). The pipes
working at high temperature give different results (Figures 8, 9) compared to sample �1. The
signals are strongly deformed. �-scan shows bottom echoes, signal deformation and acoustic
noise (backscattered signals). There are areas with very high attenuation of the signal. The
distance changes between echoes indicate a change of the wall thickness or the wave velocity
or both, but it is not possible to determine the contribution of the causes. There is backscatter
that is due to high degree arrangement of pearlte in layers (strips in sections) near external and
internal surfaces of the pipe.
Figure 9.B-scan, pipe �2 Figure 10.Attenuation coefficients
Figure 11. B-scan, pipe �3 Figure 12.Attenuation coefficients
Figure13. B-scan, pipe �4 Figure 14. Attenuation coefficients
The results for the sample �4 that has worked at non stationary regime are shown on Figures
13, 14. The structural noise is high, so are the attenuation and the unevenness.
The results in Figures 13 and 14 show deformation of echoes and a lot of backscatter and
acoustic noise. Similar results are obtained for the pipes of another high-temperature resistant
steel 12H1MF, but the backscatter is smaller.
Figure15. B-scan, pipe �6 Figure16. Attenuation coefficients over the perimeter
6.Ultrasonic backscatteringThe envelope of the echo waveform inherits the scattering processes that happen in the tube
materials. There exists a relation between the shape of the signal and the type of scatterers.
However it is difficult to estimate only one changed structural feature, because the influence
of the structural parameters is more complex. The scattering the ultrasonic wave is caused by
the presence of grains with irregular and coagulated shapes, distributed in different ways
along the sections, the appearance of arranged pearlite strips or micropores.
The cumulated variance A(k) is computed for every echo signals as defined in [11]:
�
�
=
=
−
−
=iN
n
k
n
)A)n(A(
)A)n(A(
)k(A
1
2
1
2
, �=
=iN
nt
)k(AN
A
1
1 (1)
where A is the average value of the echo signal determined over N points, )k(A is the
cumulated variance of signal and is connected to the area under the echo waveform and its
envelope. Figure 17 shows the evolution of the echoes and its respective cumulated variances
related to different measurement points 3, 4, 1, 9, 11 of pipe �3. It can be seen two signals
(a) or backscatter between them and lost of second echo(b-f). Our opinion is that the pictures
are visually rich, but they are not so informative.
3
1
4
1
7 9
11
a b c d f
Figure 17.Echoes (in blue) and respective cumulated variances (in magenta)
0.01
0.1
1
1 10 100f,MHz
Np/mm
αααα
�1
�2
0.01
0.1
1
1 10 100
�5
�6
αααα
Np/mm
f,MHz
Figure 18. Attenuation versus frequency, pipes �1, �2, �5, �6
The attenuation coefficient is defined as the ratio of spectra ( 1)f(S , 2)f(S ) of ultrasonic signals.
The procedures is presented in details in [10]
d.
)f(S
)f(Sln)f(
2
1
2
1
���
����
�=αααα (2)
Figure 18 presents the experimental results for attenuation coefficients of tube elements �2
(steel 20) and �6 (12H1MF). There are obtained different dependencies of frequency
attenuation on perimeter of pipes during one scanning motion and compared to the attenuation
of reference samples �1 and �5. Obviously the inhomogeneous structure, grain coagulations
and pearlite strips influence on the attenuation values.
Conclusion
The investigations of microstructure changes in pipes were carried out. The immersion system
with automatic rotating and B-scan registration is found to be suitable approach. The
degradation of microstructure proved by metallographic study can be registered by
ultrasound. It is difficult to estimate and separate the influence of the various structural
parameters on ultrasonic backscatters and signals. Though it was confirmed that the high
degree of “banded structure” (pearlites in strips) in low carbon steel raises large backscatter
and lack of bottom echoes. The number of backscatter in high resistant steel is smaller.
Probably the higher attenuation of ultrasonic waves in 12H1MF is mostly due to the
coarsened structure and lesser to “banded structure”. Further investigation by ultrasonic shear
and surface waves will clarify and specify the criteria for structure state assessment in situ of
pipelines in power stations. Acknowledgement
The research was performed as a part of Project “Research on fatigue of ferromagnetic materielas by
ultrasound and Barkhausen noise” with the Sofia University “St. Kliment Ohridski”.
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